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Aldehydes, Ketones and Carboxylic Acids

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Chapter Notes

Overview
This chapter introduces three vital classes of carbonyl compounds: aldehydes, ketones, and carboxylic acids. It systematically covers their nomenclature, methods of preparation, physical properties, and chemical reactions, emphasizing the structure-reactivity correlation of the carbonyl group. The one big idea is that the polar nature of the carbonyl group makes it a key electrophilic centre, governing nucleophilic addition reactions in aldehydes and ketones, and nucleophilic acyl substitution and acidity in carboxylic acids.
Key Concepts (31)
Carbonyl group
A functional group composed of a carbon atom double-bonded to an oxygen atom (>C=O). The carbon atom is sp2-hybridised and forms three sigma bonds, with the pi-electron cloud lying above and below the plane.
e.g. The carbonyl group is present in acetone, a common nail polish remover.
Aldehydes
Carbonyl compounds where the carbonyl group is bonded to one carbon atom and one hydrogen atom, except for formaldehyde where it is bonded to two hydrogen atoms.
e.g. Formaldehyde is used to preserve biological specimens in school labs.
Ketones
Carbonyl compounds where the carbonyl group is bonded to two carbon atoms.
e.g. Acetone is used as a solvent in paints and adhesives.
Carboxylic acids
Carbonyl compounds in which the carbonyl carbon is bonded to a carbon or hydrogen and the oxygen of a hydroxyl moiety (-OH). The functional group is called the carboxyl group (-COOH).
e.g. Acetic acid is the main component of vinegar used in Indian kitchens.
Rosenmund reduction
A reaction where an acyl chloride is hydrogenated over a catalyst of palladium on barium sulphate to yield an aldehyde.
e.g. Benzoyl chloride is converted to benzaldehyde, which is used in almond flavouring.
Stephen reaction
A reaction where nitriles are reduced to corresponding imines with stannous chloride in the presence of hydrochloric acid, which on hydrolysis give the corresponding aldehyde.
e.g. Ethanenitrile can be converted to ethanal.
Etard reaction
A reaction where chromyl chloride (CrO2Cl2) oxidises a methyl group attached to an aromatic ring to a chromium complex, which on hydrolysis gives the corresponding benzaldehyde.
e.g. Toluene is oxidised to benzaldehyde, a flavouring agent.
Gatterman-Koch reaction
A reaction where benzene or its derivative is treated with carbon monoxide and hydrogen chloride in the presence of anhydrous aluminium chloride or cuprous chloride to give benzaldehyde or substituted benzaldehyde.
e.g. Benzene is converted to benzaldehyde.
Friedel-Crafts acylation reaction
A reaction where benzene or substituted benzene is treated with an acid chloride in the presence of anhydrous aluminium chloride to afford the corresponding ketone.
e.g. Benzene reacts with acetyl chloride to form acetophenone, used in perfumes.
Nucleophilic addition reactions
Reactions characteristic of aldehydes and ketones where a nucleophile attacks the electrophilic carbonyl carbon, changing its hybridisation from sp2 to sp3 and forming a tetrahedral alkoxide intermediate.
e.g. The reaction of acetaldehyde with HCN to form a cyanohydrin.
Cyanohydrin
A compound formed by the addition of hydrogen cyanide (HCN) to an aldehyde or ketone, containing both a hydroxyl (-OH) and a cyano (-CN) group on the same carbon.
e.g. Acetone cyanohydrin is an intermediate in making acrylic glass.
Hemiacetal
An alkoxyalcohol intermediate formed by the reaction of an aldehyde with one equivalent of a monohydric alcohol in the presence of dry hydrogen chloride.
e.g. The reaction of formaldehyde with methanol gives a hemiacetal.
Acetal
A gem-dialkoxy compound formed by the reaction of a hemiacetal with one more molecule of alcohol in the presence of dry hydrogen chloride.
e.g. Acetals are used to protect the aldehyde group during organic synthesis.
Ketal
A cyclic product formed by the reaction of a ketone with ethylene glycol under acidic conditions.
e.g. Ethylene glycol ketals are used as protecting groups for ketones in reactions.
2,4-Dinitrophenylhydrazone (2,4-DNP derivative)
A yellow, orange, or red solid derivative formed by the reaction of an aldehyde or ketone with 2,4-dinitrophenylhydrazine, useful for characterisation.
e.g. Benzaldehyde forms an orange-red precipitate with 2,4-DNP reagent.
Semicarbazone
A derivative formed by the reaction of an aldehyde or ketone with semicarbazide.
e.g. Semicarbazones are crystalline solids used to identify specific aldehydes and ketones.
Oxime
A derivative formed by the reaction of an aldehyde or ketone with hydroxylamine (NH2OH).
e.g. Cyclohexanone oxime is a key intermediate in the production of nylon-6.
Imine (Schiff's base)
A compound containing a carbon-nitrogen double bond (>C=N-), formed by the reaction of an aldehyde or ketone with ammonia or a primary amine.
e.g. Imines are intermediates in many biological reactions.
Clemmensen reduction
The reduction of the carbonyl group of aldehydes and ketones to a -CH2- group using zinc-amalgam and concentrated hydrochloric acid.
e.g. Acetophenone is reduced to ethylbenzene.
Wolff-Kishner reduction
The reduction of the carbonyl group of aldehydes and ketones to a -CH2- group using hydrazine followed by heating with sodium or potassium hydroxide in a high boiling solvent like ethylene glycol.
e.g. Propanone is reduced to propane.
Tollens' test
A test for aldehydes where warming an aldehyde with freshly prepared ammoniacal silver nitrate solution (Tollens' reagent) produces a bright silver mirror due to the formation of silver metal.
e.g. Glucose, which has an aldehyde group, gives a positive Tollens' test.
Fehling's test
A test for aldehydes where heating an aldehyde with Fehling's reagent (a mixture of Fehling's A and B solutions) gives a reddish-brown precipitate. Aromatic aldehydes do not respond.
e.g. Acetaldehyde gives a positive Fehling's test, but benzaldehyde does not.
Haloform reaction
The oxidation of methyl ketones by sodium hypohalite to give sodium salts of carboxylic acids with one less carbon atom and a haloform.
e.g. Acetone gives a yellow precipitate of iodoform with sodium hypoiodite, used as the iodoform test.
Aldol condensation
A reaction where aldehydes and ketones having at least one alpha-hydrogen undergo self-addition in the presence of dilute alkali to form beta-hydroxy aldehydes (aldol) or beta-hydroxy ketones (ketol), which readily lose water to give alpha,beta-unsaturated carbonyl compounds.
e.g. Ethanal undergoes aldol condensation to give but-2-enal.
Cross aldol condensation
An aldol condensation carried out between two different aldehydes and/or ketones, giving a mixture of four products if both reactants contain alpha-hydrogen atoms.
e.g. A mixture of ethanal and propanal gives a mixture of four aldol products.
Cannizzaro reaction
A self-oxidation and reduction (disproportionation) reaction undergone by aldehydes which do not have an alpha-hydrogen atom on heating with concentrated alkali, producing an alcohol and a carboxylic acid salt.
e.g. Benzaldehyde gives benzyl alcohol and sodium benzoate.
Esterification
The reaction of a carboxylic acid with an alcohol or phenol in the presence of a mineral acid catalyst to form an ester.
e.g. Ethanoic acid reacts with ethanol to form ethyl ethanoate, a fragrant ester.
Decarboxylation
The reaction where carboxylic acids lose carbon dioxide to form hydrocarbons when their sodium salts are heated with sodalime (NaOH and CaO).
e.g. Sodium ethanoate on decarboxylation gives methane.
Hell-Volhard-Zelinsky reaction
A reaction where carboxylic acids having an alpha-hydrogen are halogenated at the alpha-position on treatment with chlorine or bromine in the presence of a small amount of red phosphorus.
e.g. Ethanoic acid is converted to alpha-chloroethanoic acid.
Acidity of alpha-hydrogens
The acidic nature of alpha-hydrogen atoms of carbonyl compounds due to the strong electron-withdrawing effect of the carbonyl group and resonance stabilisation of the conjugate base.
e.g. The alpha-hydrogens in acetone make it undergo aldol condensation.
Effect of substituents on acidity of carboxylic acids
Electron-withdrawing groups increase the acidity of carboxylic acids by stabilising the carboxylate anion, while electron-donating groups decrease the acidity by destabilising it.
e.g. Trifluoroacetic acid is a stronger acid than acetic acid due to the electron-withdrawing fluorine atoms.
Formulas & Equations (77)
RCH2OH + [O] → RCHO + H2O
Reads as: Primary alcohol on oxidation gives aldehyde and water.
Variables: R = alkyl group; [O] = oxidising agent like PCC or K2Cr2O7
Use when: When asked to prepare an aldehyde from a primary alcohol.
⚠ Using a strong oxidising agent like KMnO4 which further oxidises the aldehyde to a carboxylic acid.
R2CHOH + [O] → R2C=O + H2O
Reads as: Secondary alcohol on oxidation gives ketone and water.
Variables: R = alkyl group; [O] = oxidising agent
Use when: When asked to prepare a ketone from a secondary alcohol.
⚠ Forgetting that tertiary alcohols resist oxidation and do not give carbonyl compounds.
RCH2OH → RCHO + H2
Reads as: Primary alcohol vapours passed over heated silver or copper catalyst give aldehyde and hydrogen gas.
Variables: R = alkyl group; catalyst = Ag or Cu
Use when: When industrial preparation of aldehydes from volatile alcohols is asked.
⚠ Confusing dehydrogenation with oxidation; no oxidising agent is used here.
R2CHOH → R2C=O + H2
Reads as: Secondary alcohol vapours passed over heated silver or copper catalyst give ketone and hydrogen gas.
Variables: R = alkyl group; catalyst = Ag or Cu
Use when: When industrial preparation of ketones from volatile alcohols is asked.
⚠ Writing the product as an aldehyde instead of a ketone for secondary alcohols.
Alkene + O3 → Ozonide; Ozonide + Zn/H2O → Carbonyl compounds
Reads as: Ozonolysis of alkenes followed by reductive workup with zinc and water gives aldehydes or ketones.
Variables: Products depend on substitution pattern of alkene
Use when: When asked to prepare aldehydes or ketones from alkenes.
⚠ Using oxidative workup (H2O2) instead of reductive workup (Zn/H2O), which would give carboxylic acids.
HC≡CH + H2O → CH3CHO
Reads as: Ethyne on hydration in presence of mercuric sulphate and sulphuric acid gives acetaldehyde.
Variables: Catalyst: H2SO4 and HgSO4
Use when: When asked to prepare acetaldehyde from ethyne.
⚠ Forgetting that alkynes other than ethyne give ketones, not aldehydes.
RC≡CH + H2O → RCOCH3
Reads as: Alkynes other than ethyne on hydration give methyl ketones.
Variables: R = alkyl group; Catalyst: H2SO4 and HgSO4
Use when: When asked to prepare a ketone from an alkyne.
⚠ Writing the product as an aldehyde; terminal alkynes other than ethyne give methyl ketones.
RCOCl + H2 → RCHO + HCl
Reads as: Acyl chloride on hydrogenation over palladium on barium sulphate gives aldehyde and hydrogen chloride.
Variables: R = alkyl or aryl group; Catalyst: Pd/BaSO4
Use when: When Rosenmund reduction is asked to prepare an aldehyde from an acid chloride.
⚠ Using ordinary Pd catalyst instead of Pd/BaSO4 poisoned catalyst, which would over-reduce to alcohol.
RCN + SnCl2 + HCl → [RCH=NH2]+Cl-; [RCH=NH2]+Cl- + H2O → RCHO + NH4Cl
Reads as: Nitrile reduced with stannous chloride and hydrochloric acid gives an imine intermediate which on hydrolysis yields aldehyde.
Variables: R = alkyl or aryl group
Use when: When Stephen reaction is asked to prepare an aldehyde from a nitrile.
⚠ Forgetting the hydrolysis step; the imine intermediate must be hydrolysed to get the aldehyde.
RCN + DIBAL-H → RCH=NH; RCH=NH + H2O → RCHO
Reads as: Nitrile selectively reduced by diisobutylaluminium hydride to an imine followed by hydrolysis gives aldehyde.
Variables: R = alkyl or aryl group; DIBAL-H = (i-Bu)2AlH
Use when: When selective reduction of nitrile to aldehyde using DIBAL-H is asked.
⚠ Writing DIBAL-H reduction as going directly to alcohol; it stops at the aldehyde stage after hydrolysis.
RCOOR' + DIBAL-H → RCHO + R'OH
Reads as: Ester reduced with diisobutylaluminium hydride gives aldehyde and alcohol.
Variables: R, R' = alkyl or aryl groups; DIBAL-H = (i-Bu)2AlH
Use when: When asked to prepare an aldehyde from an ester using DIBAL-H.
⚠ Using LiAlH4 instead of DIBAL-H, which would reduce the ester all the way to a primary alcohol.
C6H5CH3 + CrO2Cl2 → Chromium complex; Chromium complex + H2O → C6H5CHO
Reads as: Toluene treated with chromyl chloride followed by hydrolysis gives benzaldehyde.
Variables: CrO2Cl2 = chromyl chloride; Product = benzaldehyde
Use when: When Etard reaction is asked to prepare benzaldehyde from toluene.
⚠ Forgetting the hydrolysis step after formation of the chromium complex.
C6H5CH3 + CrO3 + (CH3CO)2O → C6H5CH(OCOCH3)2; C6H5CH(OCOCH3)2 + H2O/H+ → C6H5CHO + 2CH3COOH
Reads as: Toluene treated with chromic oxide in acetic anhydride gives benzylidene diacetate which on acid hydrolysis yields benzaldehyde.
Variables: CrO3 = chromic oxide; (CH3CO)2O = acetic anhydride
Use when: When preparation of benzaldehyde from toluene using CrO3 in acetic anhydride is asked.
⚠ Writing direct oxidation to benzaldehyde without the benzylidene diacetate intermediate.
C6H5CH3 + Cl2 → C6H5CHCl2; C6H5CHCl2 + H2O → C6H5CHO + 2HCl
Reads as: Side chain chlorination of toluene gives benzal chloride which on hydrolysis yields benzaldehyde.
Variables: C6H5CHCl2 = benzal chloride; Product = benzaldehyde
Use when: When commercial method of benzaldehyde preparation from toluene is asked.
⚠ Writing benzyl chloride (C6H5CH2Cl) as the intermediate instead of benzal chloride (C6H5CHCl2).
C6H6 + CO + HCl → C6H5CHO
Reads as: Benzene treated with carbon monoxide and hydrogen chloride in presence of anhydrous aluminium chloride gives benzaldehyde.
Variables: Catalyst: anhydrous AlCl3 or CuCl
Use when: When Gatterman-Koch reaction is asked to prepare benzaldehyde from benzene.
⚠ Confusing with Friedel-Crafts acylation; Gatterman-Koch uses CO and HCl, not an acyl chloride.
2RMgX + CdCl2 → R2Cd + 2MgXCl; R2Cd + 2R'COCl → 2R'COR + CdCl2
Reads as: Dialkylcadmium prepared from Grignard reagent and cadmium chloride reacts with acyl chloride to give ketone.
Variables: R, R' = alkyl or aryl groups; X = halogen
Use when: When preparation of ketone from acyl chloride using dialkylcadmium is asked.
⚠ Using Grignard reagent directly with acyl chloride, which would give tertiary alcohol instead of ketone.
RCN + R'MgX → RC(R')=NMgX; RC(R')=NMgX + H2O/H+ → RCOR' + NH3 + Mg(OH)X
Reads as: Nitrile treated with Grignard reagent followed by hydrolysis yields a ketone.
Variables: R, R' = alkyl or aryl groups; X = halogen
Use when: When preparation of ketone from nitrile using Grignard reagent is asked.
⚠ Forgetting the hydrolysis step; the imine salt intermediate must be hydrolysed to get the ketone.
C6H6 + RCOCl → C6H5COR + HCl
Reads as: Benzene treated with acyl chloride in presence of anhydrous aluminium chloride gives aromatic ketone.
Variables: R = alkyl or aryl group; Catalyst: anhydrous AlCl3
Use when: When Friedel-Crafts acylation is asked to prepare aromatic ketones.
⚠ Using alkyl chloride instead of acyl chloride, which would give alkylbenzene (Friedel-Crafts alkylation).
>C=O + HCN → >C(OH)CN
Reads as: Carbonyl compound reacts with hydrogen cyanide to form cyanohydrin.
Variables: >C=O is aldehyde or ketone; HCN is hydrogen cyanide; product is cyanohydrin.
Use when: When asked about nucleophilic addition of HCN to carbonyl compounds.
⚠ Forgetting that the reaction is base-catalysed and slow with pure HCN.
>C=O + NaHSO₃ → >C(OH)SO₃Na
Reads as: Carbonyl compound reacts with sodium hydrogensulphite to form addition product.
Variables: >C=O is aldehyde or methyl ketone; NaHSO₃ is sodium hydrogensulphite.
Use when: When asked about separation or purification of aldehydes.
⚠ Assuming all ketones give this reaction; most ketones have equilibrium to the left.
RCHO + R'OH (dry HCl) → RCH(OH)OR' (hemiacetal) → RCH(OR')₂ (acetal)
Reads as: Aldehyde reacts with alcohol in presence of dry hydrogen chloride to form hemiacetal then acetal.
Variables: RCHO is aldehyde; R'OH is monohydric alcohol; dry HCl is catalyst.
Use when: When asked about protection of aldehyde group or acetal formation.
⚠ Not writing the intermediate hemiacetal step.
>C=O + HOCH₂CH₂OH (dry HCl) → cyclic ethylene ketal
Reads as: Ketone reacts with ethylene glycol in presence of dry hydrogen chloride to form cyclic ketal.
Variables: >C=O is ketone; HOCH₂CH₂OH is ethylene glycol.
Use when: When asked about protection of ketone carbonyl group.
⚠ Confusing ketal formation with acetal formation.
>C=O + H₂N-Z (H⁺) → >C=N-Z + H₂O
Reads as: Carbonyl compound reacts with ammonia derivative to form imine derivative and water.
Variables: H₂N-Z is ammonia derivative; Z = alkyl, aryl, OH, NH₂, C₆H₅NH, NHCONH₂.
Use when: When asked about formation of oximes, hydrazones, phenylhydrazones, semicarbazones, 2,4-DNP derivatives.
⚠ Forgetting that the reaction is acid-catalysed and reversible.
>C=O + H₂N-OH → >C=N-OH + H₂O
Reads as: Carbonyl compound reacts with hydroxylamine to form oxime.
Variables: H₂N-OH is hydroxylamine; product is oxime.
Use when: When asked about identification or characterisation of carbonyl compounds.
⚠ Writing the product without removing water molecule.
>C=O + H₂N-NH₂ → >C=N-NH₂ + H₂O
Reads as: Carbonyl compound reacts with hydrazine to form hydrazone.
Variables: H₂N-NH₂ is hydrazine; product is hydrazone.
Use when: When asked about derivatives of aldehydes and ketones.
⚠ Confusing hydrazone with hydrazine starting material.
>C=O + H₂N-NHC₆H₅ → >C=N-NHC₆H₅ + H₂O
Reads as: Carbonyl compound reacts with phenylhydrazine to form phenylhydrazone.
Variables: H₂N-NHC₆H₅ is phenylhydrazine; product is phenylhydrazone.
Use when: When asked about characterisation of aldehydes and ketones.
⚠ Writing incorrect structure of phenylhydrazine.
>C=O + H₂N-NHCONH₂ → >C=N-NHCONH₂ + H₂O
Reads as: Carbonyl compound reacts with semicarbazide to form semicarbazone.
Variables: H₂N-NHCONH₂ is semicarbazide; product is semicarbazone.
Use when: When asked about identification of carbonyl compounds.
⚠ Using both NH₂ groups of semicarbazide; only one is involved.
>C=O + H₂N-NH-C₆H₃(NO₂)₂ → >C=N-NH-C₆H₃(NO₂)₂ + H₂O
Reads as: Carbonyl compound reacts with 2,4-dinitrophenylhydrazine to form 2,4-DNP derivative.
Variables: H₂N-NH-C₆H₃(NO₂)₂ is 2,4-DNP reagent; product is yellow/orange/red solid.
Use when: When asked about test for aldehydes and ketones or characterisation.
⚠ Forgetting that 2,4-DNP test is positive for both aldehydes and ketones.
>C=O + NaBH₄ or LiAlH₄ → >CH-OH (1° or 2° alcohol)
Reads as: Carbonyl compound reduced by sodium borohydride or lithium aluminium hydride to alcohol.
Variables: NaBH₄ reduces aldehydes to 1° alcohols, ketones to 2° alcohols; LiAlH₄ is stronger.
Use when: When asked about reduction of carbonyl compounds to alcohols.
⚠ Using NaBH₄ for carboxylic acid reduction; NaBH₄ does not reduce carboxyl group.
>C=O + Zn-Hg/HCl → >CH₂ (Clemmensen reduction)
Reads as: Carbonyl group reduced to methylene group using zinc amalgam and concentrated hydrochloric acid.
Variables: Zn-Hg is zinc amalgam; HCl is concentrated hydrochloric acid.
Use when: When asked about reduction of carbonyl to methylene group under acidic conditions.
⚠ Using Clemmensen reduction for acid-sensitive compounds.
>C=O + NH₂NH₂/KOH/heat → >CH₂ (Wolff-Kishner reduction)
Reads as: Carbonyl group reduced to methylene group using hydrazine and potassium hydroxide in ethylene glycol.
Variables: NH₂NH₂ is hydrazine; KOH is base; ethylene glycol is high boiling solvent.
Use when: When asked about reduction of carbonyl to methylene group under basic conditions.
⚠ Confusing Wolff-Kishner with Clemmensen reduction conditions.
RCHO + [Ag(NH₃)₂]⁺ + OH⁻ → RCOO⁻ + Ag (silver mirror)
Reads as: Aldehyde reduces Tollens' reagent to form carboxylate ion and silver mirror.
Variables: [Ag(NH₃)₂]⁺ is Tollens' reagent (ammoniacal silver nitrate); Ag is silver metal.
Use when: When asked to distinguish aldehydes from ketones; Tollens' test.
⚠ Thinking ketones also give positive Tollens' test.
RCHO + 2Cu²⁺ + 5OH⁻ → RCOO⁻ + Cu₂O (red ppt) + 3H₂O
Reads as: Aldehyde reduces Fehling's reagent to form carboxylate ion and red precipitate of cuprous oxide.
Variables: Cu²⁺ is from Fehling solution A (CuSO₄); tartarate is from Fehling solution B; Cu₂O is red precipitate.
Use when: When asked to distinguish aliphatic aldehydes from ketones and aromatic aldehydes.
⚠ Expecting aromatic aldehydes like benzaldehyde to give positive Fehling's test.
CH₃CO-R + 3NaOX → CHX₃ + RCOONa + 2NaOH (Haloform reaction)
Reads as: Methyl ketone reacts with sodium hypohalite to give haloform and sodium salt of carboxylic acid.
Variables: CH₃CO-R is methyl ketone; NaOX is sodium hypohalite; CHX₃ is haloform.
Use when: When asked about oxidation of methyl ketones or haloform reaction.
⚠ Applying haloform reaction to aldehydes without methyl ketone group.
CH₃CO-R + 3NaOI → CHI₃ (yellow ppt) + RCOONa + 2NaOH (Iodoform test)
Reads as: Methyl ketone or ethanol derivative reacts with sodium hypoiodite to give yellow precipitate of iodoform.
Variables: NaOI is sodium hypoiodite (I₂/NaOH); CHI₃ is iodoform (yellow precipitate).
Use when: When asked to detect CH₃CO- group or CH₃CH(OH)- group.
⚠ Forgetting that ethanol and secondary alcohols with CH₃CH(OH)- group also give positive test.
2CH₃CHO (dil NaOH) → CH₃CH(OH)CH₂CHO (aldol) → CH₃CH=CHCHO + H₂O
Reads as: Two molecules of acetaldehyde undergo aldol condensation in dilute alkali to form 3-hydroxybutanal then crotonaldehyde.
Variables: CH₃CHO is ethanal; dil NaOH is dilute alkali; product is β-hydroxy aldehyde (aldol) then α,β-unsaturated aldehyde.
Use when: When asked about aldol condensation of aldehydes with α-hydrogen.
⚠ Not showing dehydration step to form α,β-unsaturated carbonyl compound.
2CH₃COCH₃ (dil Ba(OH)₂) → (CH₃)₂C(OH)CH₂COCH₃ (ketol) → (CH₃)₂C=CHCOCH₃ + H₂O
Reads as: Two molecules of acetone undergo aldol condensation to form diacetone alcohol then mesityl oxide.
Variables: CH₃COCH₃ is acetone; product is β-hydroxy ketone (ketol) then α,β-unsaturated ketone.
Use when: When asked about aldol condensation of ketones with α-hydrogen.
⚠ Calling the product aldol instead of ketol for ketone reactions.
CH₃CHO + CH₃CH₂CHO (dil NaOH) → mixture of four products (Cross aldol condensation)
Reads as: Mixture of ethanal and propanal in dilute alkali gives four different aldol condensation products.
Variables: CH₃CHO is ethanal; CH₃CH₂CHO is propanal; four products from self and cross condensation.
Use when: When asked about cross aldol condensation between two different aldehydes or ketones.
⚠ Forgetting that both aldehydes can act as nucleophile and electrophile giving four products.
2HCHO (conc NaOH) → CH₃OH + HCOONa (Cannizzaro reaction)
Reads as: Two molecules of formaldehyde undergo disproportionation in concentrated alkali to methanol and sodium formate.
Variables: HCHO is formaldehyde (no α-hydrogen); conc NaOH is concentrated alkali; products are alcohol and carboxylate salt.
Use when: When asked about reaction of aldehydes without α-hydrogen with concentrated alkali.
⚠ Applying Cannizzaro reaction to aldehydes having α-hydrogen.
2C₆H₅CHO (conc NaOH) → C₆H₅CH₂OH + C₆H₅COONa
Reads as: Two molecules of benzaldehyde undergo Cannizzaro reaction to benzyl alcohol and sodium benzoate.
Variables: C₆H₅CHO is benzaldehyde (no α-hydrogen); products are benzyl alcohol and sodium benzoate.
Use when: When asked about Cannizzaro reaction of aromatic aldehydes without α-hydrogen.
⚠ Writing benzoic acid instead of sodium benzoate as product in alkaline medium.
RCHO + 2[Ag(NH₃)₂]⁺ + 3OH⁻ → RCOO⁻ + 2Ag↓ + 4NH₃ + 2H₂O
Reads as: Aldehyde is oxidised by Tollens' reagent to carboxylate ion with deposition of silver mirror.
Variables: [Ag(NH₃)₂]⁺ is Tollens' reagent; Ag is metallic silver forming bright mirror.
Use when: When writing balanced equation for Tollens' test in exam.
⚠ Writing Ag⁺ instead of [Ag(NH₃)₂]⁺ complex ion.
RCHO + 2Cu²⁺ + 5OH⁻ → RCOO⁻ + Cu₂O↓ + 3H₂O
Reads as: Aliphatic aldehyde reduces Fehling's reagent to give red precipitate of cuprous oxide.
Variables: Cu²⁺ from Fehling A; OH⁻ from alkaline tartarate (Fehling B); Cu₂O is reddish brown precipitate.
Use when: When writing balanced equation for Fehling's test.
⚠ Writing CuO or Cu instead of Cu₂O as the precipitate.
CH₃COCH₃ + 3I₂ + 4NaOH → CHI₃↓ + CH₃COONa + 3NaI + 3H₂O
Reads as: Acetone reacts with iodine and sodium hydroxide to give yellow precipitate of iodoform.
Variables: CH₃COCH₃ is acetone; I₂/NaOH generates NaOI; CHI₃ is iodoform (yellow ppt).
Use when: When writing balanced equation for iodoform test of methyl ketones.
⚠ Balancing iodine atoms incorrectly in the equation.
C₆H₅COCH₃ + 3NaOI → CHI₃ + C₆H₅COONa + 2NaOH
Reads as: Acetophenone undergoes iodoform reaction to give iodoform and sodium benzoate.
Variables: C₆H₅COCH₃ is acetophenone (methyl ketone); NaOI is sodium hypoiodite.
Use when: When asked about iodoform test for aromatic methyl ketones.
⚠ Thinking benzophenone (C₆H₅COC₆H₅) will give iodoform test.
Rosenmund reduction: RCOCl + H₂ (Pd/BaSO₄) → RCHO + HCl
Reads as: Acyl chloride is reduced to aldehyde using hydrogen and palladium on barium sulphate catalyst.
Variables: RCOCl is acyl chloride; Pd/BaSO₄ is poisoned catalyst; RCHO is aldehyde.
Use when: When asked about preparation of aldehydes from acyl chlorides.
⚠ Using ordinary Pd catalyst which reduces aldehyde further to alcohol.
Stephen reaction: RCN + SnCl₂/HCl → RCH=NH·HCl → RCHO + NH₄Cl
Reads as: Nitrile reduced with stannous chloride and HCl to imine hydrochloride then hydrolysed to aldehyde.
Variables: RCN is nitrile; SnCl₂/HCl is reducing agent; RCHO is aldehyde.
Use when: When asked about Stephen reaction for preparation of aldehydes.
⚠ Forgetting the hydrolysis step after imine formation.
Etard reaction: C₆H₅CH₃ + CrO₂Cl₂ → complex → C₆H₅CHO
Reads as: Toluene oxidised by chromyl chloride to a chromium complex which on hydrolysis gives benzaldehyde.
Variables: C₆H₅CH₃ is toluene; CrO₂Cl₂ is chromyl chloride; C₆H₅CHO is benzaldehyde.
Use when: When asked about Etard reaction for preparation of aromatic aldehydes.
⚠ Writing CrO₃ or KMnO₄ instead of CrO₂Cl₂ for Etard reaction.
Gatterman-Koch reaction: C₆H₆ + CO + HCl (anhyd AlCl₃/CuCl) → C₆H₅CHO
Reads as: Benzene reacts with carbon monoxide and hydrogen chloride in presence of anhydrous aluminium chloride to give benzaldehyde.
Variables: C₆H₆ is benzene; CO is carbon monoxide; HCl is hydrogen chloride; AlCl₃/CuCl is catalyst.
Use when: When asked about Gatterman-Koch reaction for preparation of benzaldehyde.
⚠ Confusing with Friedel-Crafts acylation which uses RCOCl instead of CO+HCl.
Friedel-Crafts acylation: C₆H₆ + RCOCl (anhyd AlCl₃) → C₆H₅COR + HCl
Reads as: Benzene reacts with acyl chloride in presence of anhydrous aluminium chloride to give ketone.
Variables: C₆H₆ is benzene; RCOCl is acyl chloride; AlCl₃ is Lewis acid catalyst; C₆H₅COR is ketone.
Use when: When asked about preparation of aromatic ketones.
⚠ Using more than one equivalent of AlCl₃; catalyst is required in stoichiometric amount.
C₆H₅CH₃ + CrO₃/(CH₃CO)₂O → C₆H₅CH(OCOCH₃)₂ → C₆H₅CHO
Reads as: Toluene treated with chromic oxide in acetic anhydride gives benzylidene diacetate which hydrolyses to benzaldehyde.
Variables: CrO₃ is chromic oxide; (CH₃CO)₂O is acetic anhydride; C₆H₅CHO is benzaldehyde.
Use when: When asked about oxidation of methylbenzene to benzaldehyde using CrO₃.
⚠ Skipping the benzylidene diacetate intermediate step.
C₆H₅CH₃ + Cl₂ (hv) → C₆H₅CHCl₂ → C₆H₅CHO
Reads as: Side chain chlorination of toluene gives benzal chloride which on hydrolysis gives benzaldehyde.
Variables: C₆H₅CH₃ is toluene; C₆H₅CHCl₂ is benzal chloride; C₆H₅CHO is benzaldehyde.
Use when: When asked about commercial preparation of benzaldehyde.
⚠ Writing benzyl chloride (C₆H₅CH₂Cl) instead of benzal chloride (C₆H₅CHCl₂).
RCH2OH + [O] → RCOOH
Reads as: Primary alcohol plus oxidising agent gives carboxylic acid.
Variables: R = alkyl group; [O] = oxidising agent like KMnO4, K2Cr2O7, or CrO3 in acidic media (Jones reagent).
Use when: Used when asked to prepare a carboxylic acid from a primary alcohol.
⚠ Forgetting that the oxidising agent must be strong (e.g., KMnO4) and that aldehydes are intermediates but are further oxidised.
RCHO + [O] → RCOOH
Reads as: Aldehyde plus mild oxidising agent gives carboxylic acid.
Variables: R = alkyl or aryl group; [O] = mild oxidising agent like Tollens' reagent or Fehling's reagent.
Use when: Used when preparing carboxylic acids from aldehydes or distinguishing aldehydes from ketones.
⚠ Using strong oxidising agents is also correct, but students often forget that mild oxidising agents like Tollens' reagent specifically oxidise aldehydes, not ketones.
ArCH3 + [O] → ArCOOH
Reads as: Alkylbenzene plus strong oxidising agent gives aromatic carboxylic acid.
Variables: Ar = aryl group; [O] = chromic acid or acidic/alkaline KMnO4.
Use when: Used for vigorous oxidation of alkyl benzenes where the entire side chain is converted to a carboxyl group.
⚠ Thinking that the length of the alkyl side chain affects the product; the entire side chain is oxidised to -COOH regardless of length, except for tertiary alkyl groups which are not affected.
RCN + 2H2O → RCONH2 → RCOOH + NH3
Reads as: Nitrile plus water in the presence of acid or base catalyst gives amide, then carboxylic acid and ammonia.
Variables: R = alkyl group; H+ or OH- as catalyst.
Use when: Used to prepare carboxylic acids from nitriles via hydrolysis.
⚠ Forgetting that mild reaction conditions can stop the reaction at the amide stage; vigorous conditions are needed to get the carboxylic acid.
RMgX + CO2 → RCOOMgX → RCOOH
Reads as: Grignard reagent plus carbon dioxide gives magnesium salt of carboxylic acid, which on acidification gives carboxylic acid.
Variables: R = alkyl or aryl group; X = halogen (Cl, Br, I).
Use when: Used to prepare carboxylic acids with one carbon atom more than the parent alkyl halide (ascending the series).
⚠ Forgetting the acid hydrolysis step (H3O+) to convert the carboxylate salt to the free carboxylic acid.
RCOCl + H2O → RCOOH + HCl
Reads as: Acyl chloride plus water gives carboxylic acid and hydrogen chloride.
Variables: R = alkyl or aryl group.
Use when: Used for hydrolysis of acyl chlorides to prepare carboxylic acids.
⚠ Not recognising that hydrolysis with aqueous base gives carboxylate ions first, which then require acidification to give the free acid.
(RCO)2O + H2O → 2RCOOH
Reads as: Acid anhydride plus water gives two molecules of carboxylic acid.
Variables: R = alkyl or aryl group.
Use when: Used for hydrolysis of anhydrides to prepare carboxylic acids.
⚠ Forgetting that symmetrical anhydrides yield two identical acid molecules, while mixed anhydrides yield two different acids.
RCOOR' + H2O → RCOOH + R'OH
Reads as: Ester plus water in acidic medium gives carboxylic acid and alcohol.
Variables: R, R' = alkyl or aryl groups; H+ catalyst.
Use when: Used for acidic hydrolysis of esters to prepare carboxylic acids.
⚠ Confusing acidic hydrolysis (gives free acid) with basic hydrolysis (saponification, gives carboxylate salt which needs acidification).
2RCOOH + 2Na → 2RCOONa + H2
Reads as: Carboxylic acid plus sodium metal gives sodium carboxylate and hydrogen gas.
Variables: R = alkyl or aryl group.
Use when: Used to show the acidic nature of carboxylic acids; reaction with electropositive metals.
⚠ Writing the product as RCOONa2 or forgetting to balance the hydrogen gas evolved.
RCOOH + NaOH → RCOONa + H2O
Reads as: Carboxylic acid plus sodium hydroxide gives sodium carboxylate and water.
Variables: R = alkyl or aryl group.
Use when: Used to show neutralisation of carboxylic acids with strong bases.
⚠ Forgetting that this is a simple acid-base neutralisation and the salt formed is water-soluble.
RCOOH + NaHCO3 → RCOONa + H2O + CO2
Reads as: Carboxylic acid plus sodium hydrogencarbonate gives sodium carboxylate, water, and carbon dioxide.
Variables: R = alkyl or aryl group.
Use when: Used as a test to detect the presence of a carboxyl group; effervescence of CO2 indicates carboxylic acid.
⚠ Thinking that phenols also give this reaction; only carboxylic acids (stronger acids) react with weak bases like NaHCO3 to evolve CO2.
RCOOH + Na2CO3 → 2RCOONa + H2O + CO2
Reads as: Carboxylic acid plus sodium carbonate gives sodium carboxylate, water, and carbon dioxide.
Variables: R = alkyl or aryl group.
Use when: Used to distinguish carboxylic acids from phenols and alcohols.
⚠ Incorrectly balancing the equation; two moles of acid react with one mole of carbonate.
RCOOH ⇌ RCOO- + H+
Reads as: Carboxylic acid dissociates reversibly in water to give carboxylate anion and hydronium ion.
Variables: R = alkyl or aryl group.
Use when: Used to explain the acidity of carboxylic acids and calculate Ka or pKa values.
⚠ Forgetting that the equilibrium lies to the left for weak acids like carboxylic acids; they do not dissociate completely.
pKa = -log Ka
Reads as: pKa equals negative logarithm of the acid dissociation constant Ka.
Variables: Ka = acid dissociation constant; pKa = measure of acid strength.
Use when: Used to compare the strengths of different carboxylic acids; smaller pKa means stronger acid.
⚠ Confusing the relationship; thinking larger pKa means stronger acid.
2RCOOH → (RCO)2O + H2O
Reads as: Two molecules of carboxylic acid on heating with mineral acid or P2O5 give acid anhydride and water.
Variables: R = alkyl or aryl group; catalyst = H2SO4 or P2O5.
Use when: Used for the preparation of acid anhydrides from carboxylic acids.
⚠ Forgetting that this is a dehydration reaction requiring a strong dehydrating agent like P2O5 or H2SO4.
RCOOH + R'OH ⇌ RCOOR' + H2O
Reads as: Carboxylic acid plus alcohol in the presence of mineral acid catalyst gives ester and water.
Variables: R, R' = alkyl or aryl groups; catalyst = conc. H2SO4 or HCl gas.
Use when: Used for esterification (Fischer esterification) of carboxylic acids with alcohols or phenols.
⚠ Forgetting that this is a reversible reaction and that the equilibrium can be shifted by removing water or using excess alcohol.
RCOOH + PCl5 → RCOCl + POCl3 + HCl
Reads as: Carboxylic acid plus phosphorus pentachloride gives acyl chloride, phosphoryl chloride, and hydrogen chloride.
Variables: R = alkyl or aryl group.
Use when: Used to replace the hydroxyl group of carboxylic acid with chlorine to form acyl chlorides.
⚠ Writing incorrect by-products; with PCl5 the by-products are POCl3 and HCl, not H3PO3.
3RCOOH + PCl3 → 3RCOCl + H3PO3
Reads as: Three molecules of carboxylic acid plus phosphorus trichloride give three molecules of acyl chloride and phosphorous acid.
Variables: R = alkyl or aryl group.
Use when: Used to prepare acyl chlorides from carboxylic acids using PCl3.
⚠ Forgetting the stoichiometry; three moles of acid react with one mole of PCl3.
RCOOH + SOCl2 → RCOCl + SO2 + HCl
Reads as: Carboxylic acid plus thionyl chloride gives acyl chloride, sulfur dioxide, and hydrogen chloride.
Variables: R = alkyl or aryl group.
Use when: Preferred method for preparing acyl chlorides because the gaseous by-products (SO2 and HCl) escape, easing purification.
⚠ Forgetting that SOCl2 is preferred over PCl5 or PCl3 because the by-products are gaseous and easily removed.
RCOOH + NH3 → RCOONH4 → RCONH2 + H2O
Reads as: Carboxylic acid plus ammonia gives ammonium carboxylate, which on heating gives amide and water.
Variables: R = alkyl or aryl group.
Use when: Used to prepare amides from carboxylic acids via ammonium salt intermediate.
⚠ Forgetting that heating is required to dehydrate the ammonium salt to the amide.
RCOOH + LiAlH4 → RCH2OH
Reads as: Carboxylic acid plus lithium aluminium hydride gives primary alcohol.
Variables: R = alkyl or aryl group; LiAlH4 = lithium aluminium hydride.
Use when: Used for reduction of carboxylic acids to primary alcohols.
⚠ Using NaBH4 instead of LiAlH4; sodium borohydride does not reduce the carboxyl group.
RCOOH + B2H6 → RCH2OH
Reads as: Carboxylic acid plus diborane gives primary alcohol.
Variables: R = alkyl or aryl group; B2H6 = diborane.
Use when: Used for selective reduction of carboxylic acids to primary alcohols without affecting ester, nitro, or halo groups.
⚠ Forgetting that diborane is preferred when other reducible functional groups are present, as it does not easily reduce esters, nitro, or halo groups.
RCOONa + NaOH (CaO) → RH + Na2CO3
Reads as: Sodium salt of carboxylic acid heated with sodalime gives hydrocarbon and sodium carbonate.
Variables: R = alkyl or aryl group; sodalime = NaOH + CaO in 3:1 ratio.
Use when: Used for decarboxylation of carboxylic acids to prepare hydrocarbons with one less carbon atom.
⚠ Forgetting that the free acid does not undergo decarboxylation; the sodium salt must be used with sodalime.
2RCOONa + 2H2O → R-R + 2CO2 + 2NaOH + H2
Reads as: Aqueous solution of sodium carboxylate on electrolysis gives hydrocarbon with twice the number of carbon atoms, carbon dioxide, sodium hydroxide, and hydrogen.
Variables: R = alkyl group.
Use when: Used in Kolbe electrolysis to prepare symmetrical hydrocarbons from carboxylic acid salts.
⚠ Forgetting that the hydrocarbon product has twice the number of carbon atoms as the alkyl group of the starting acid.
RCH2COOH + X2 → RCHXCOOH + HX
Reads as: Carboxylic acid with alpha-hydrogen plus halogen in the presence of red phosphorus gives alpha-halocarboxylic acid and hydrogen halide.
Variables: R = H or alkyl group; X = Cl or Br; catalyst = red phosphorus.
Use when: Used in Hell-Volhard-Zelinsky (HVZ) reaction for alpha-halogenation of carboxylic acids.
⚠ Forgetting that red phosphorus is essential as a catalyst; direct halogenation without it does not occur at the alpha position.
ArCOOH + HNO3 → m-NO2ArCOOH + H2O
Reads as: Aromatic carboxylic acid plus nitric acid gives meta-nitro aromatic carboxylic acid and water.
Variables: Ar = aryl group; electrophilic substitution with nitration.
Use when: Used for electrophilic substitution on aromatic ring of carboxylic acids; -COOH is meta-directing.
⚠ Forgetting that the carboxyl group is deactivating and meta-directing, so electrophilic substitution occurs at the meta position.
How It Works
Mechanism of Nucleophilic Addition to Carbonyl Group
  1. A nucleophile attacks the electrophilic carbon atom of the polar carbonyl group from a direction approximately perpendicular to the plane of sp2 hybridised orbitals of carbonyl carbon.
  2. The hybridisation of carbon changes from sp2 to sp3, producing a tetrahedral alkoxide intermediate.
  3. This intermediate captures a proton from the reaction medium to give the electrically neutral product.
💡 The net result is addition of Nu- and H+ across the carbon-oxygen double bond. Students often forget that the hybridisation of carbonyl carbon changes from sp2 to sp3.
Mechanism of Esterification of Carboxylic Acids
  1. Protonation of the carbonyl oxygen activates the carbonyl group towards nucleophilic addition of the alcohol.
  2. Nucleophilic addition of alcohol to the activated carbonyl group forms a tetrahedral intermediate.
  3. Proton transfer in the tetrahedral intermediate converts the hydroxyl group into -+OH2 group.
  4. The -+OH2 group, being a better leaving group, is eliminated as a neutral water molecule.
  5. The protonated ester so formed finally loses a proton to give the ester.
💡 Esterification is a nucleophilic acyl substitution, not a simple addition. Water is eliminated, and the acid catalyst is regenerated.
Aldol Condensation
  1. Aldehydes and ketones having at least one α-hydrogen undergo reaction in the presence of dilute alkali as catalyst.
  2. The α-hydrogen is abstracted by base to form an enolate ion.
  3. The enolate ion acts as a nucleophile and attacks the carbonyl carbon of another molecule.
  4. This forms β-hydroxy aldehydes (aldol) or β-hydroxy ketones (ketol).
  5. The aldol or ketol readily loses water to give α,β-unsaturated carbonyl compounds.
💡 The name 'aldol' is derived from aldehyde and alcohol. Ketones also undergo this reaction, and the general name aldol condensation still applies.
Cross Aldol Condensation
  1. Aldol condensation is carried out between two different aldehydes and/or ketones.
  2. If both components contain α-hydrogen atoms, a mixture of four products is obtained.
  3. Ketones can also be used as one component in the cross aldol reactions.
💡 Cross aldol condensation between two different carbonyl compounds both having α-hydrogens gives a mixture of four products, making it synthetically less useful unless one reactant lacks α-hydrogens.
Cannizzaro Reaction
  1. Aldehydes which do not have an α-hydrogen atom undergo self oxidation and reduction (disproportionation) on heating with concentrated alkali.
  2. One molecule of the aldehyde is reduced to alcohol.
  3. Another molecule of the aldehyde is oxidised to carboxylic acid salt.
💡 This reaction is specific to aldehydes with no α-hydrogen atom. It is a disproportionation reaction, not an addition or condensation.
Hell-Volhard-Zelinsky Reaction (α-Halogenation of Carboxylic Acids)
  1. Carboxylic acids having an α-hydrogen are treated with chlorine or bromine in the presence of a small amount of red phosphorus.
  2. The halogen replaces the α-hydrogen atom.
  3. The product is an α-halocarboxylic acid.
💡 Red phosphorus is essential as a catalyst. The reaction specifically halogenates the α-position of carboxylic acids.
Decarboxylation of Carboxylic Acids
  1. Sodium salts of carboxylic acids are heated with sodalime (NaOH and CaO in the ratio of 3:1).
  2. The carboxylate group loses carbon dioxide.
  3. The corresponding hydrocarbon is formed.
💡 The reaction is called decarboxylation. Sodalime is a mixture of NaOH and CaO in 3:1 ratio.
Kolbe Electrolysis
  1. Alkali metal salts of carboxylic acids undergo decarboxylation on electrolysis of their aqueous solutions.
  2. The carboxylate ion loses CO2 at the anode.
  3. Two alkyl radicals combine to form hydrocarbons having twice the number of carbon atoms present in the alkyl group of the acid.
💡 This reaction forms hydrocarbons with twice the number of carbon atoms as the original alkyl group.
Clemmensen Reduction
  1. The carbonyl group of aldehydes and ketones is treated with zinc-amalgam and concentrated hydrochloric acid.
  2. The carbonyl group is reduced to a CH2 group.
💡 Clemmensen reduction uses zinc-amalgam and concentrated HCl. It reduces >C=O to >CH2.
Wolff-Kishner Reduction
  1. The carbonyl group of aldehydes and ketones is treated with hydrazine.
  2. This is followed by heating with sodium or potassium hydroxide in a high boiling solvent such as ethylene glycol.
  3. The carbonyl group is reduced to a CH2 group.
💡 Wolff-Kishner reduction uses hydrazine and strong base (KOH/NaOH) in ethylene glycol. It is used for compounds sensitive to acid.
Etard Reaction
  1. Chromyl chloride (CrO2Cl2) oxidises the methyl group of toluene or its derivatives.
  2. A chromium complex is formed as an intermediate.
  3. Hydrolysis of the chromium complex gives the corresponding benzaldehyde.
💡 Etard reaction uses chromyl chloride (CrO2Cl2) followed by hydrolysis to stop oxidation at the aldehyde stage.
Gatterman-Koch Reaction
  1. Benzene or its derivative is treated with carbon monoxide and hydrogen chloride.
  2. The reaction occurs in the presence of anhydrous aluminium chloride or cuprous chloride.
  3. Benzaldehyde or substituted benzaldehyde is formed.
💡 This reaction introduces an aldehyde group (-CHO) directly onto the benzene ring using CO and HCl.
Rosenmund Reduction
  1. Acyl chloride (acid chloride) is hydrogenated over a catalyst of palladium on barium sulphate.
  2. The acyl chloride is reduced to the corresponding aldehyde.
💡 Rosenmund reduction selectively reduces acyl chlorides to aldehydes without further reduction to alcohols. The catalyst is Pd/BaSO4.
Stephen Reaction
  1. Nitriles are reduced with stannous chloride (SnCl2) in the presence of hydrochloric acid.
  2. The corresponding imine is formed.
  3. Hydrolysis of the imine gives the corresponding aldehyde.
💡 Stephen reaction uses SnCl2/HCl to reduce nitriles to aldehydes via imine intermediate.
Friedel-Crafts Acylation
  1. Benzene or substituted benzene is treated with acid chloride in the presence of anhydrous aluminium chloride.
  2. The acyl group attaches to the benzene ring.
  3. The corresponding ketone is formed.
💡 Friedel-Crafts acylation introduces an acyl group (RCO-) onto the benzene ring. Anhydrous AlCl3 is the catalyst.
Tollens' Test
  1. An aldehyde is warmed with freshly prepared ammoniacal silver nitrate solution (Tollens' reagent).
  2. The aldehyde is oxidised to the corresponding carboxylate anion.
  3. Silver metal is deposited, producing a bright silver mirror.
💡 Tollens' reagent is specific for aldehydes. Ketones do not give this test. The reaction occurs in alkaline medium.
Fehling's Test
  1. Fehling solution A (aqueous copper sulphate) and Fehling solution B (alkaline sodium potassium tartarate) are mixed in equal amounts.
  2. An aldehyde is heated with Fehling's reagent.
  3. The aldehyde is oxidised to the corresponding carboxylate anion.
  4. A reddish brown precipitate of cuprous oxide is obtained.
💡 Aromatic aldehydes do not respond to Fehling's test. Only aliphatic aldehydes give a positive result.
Iodoform Reaction (Haloform Reaction)
  1. Aldehydes and ketones having at least one methyl group linked to the carbonyl carbon (methyl ketones) are oxidised by sodium hypohalite.
  2. The methyl group is converted to haloform (e.g., iodoform).
  3. The remaining part is converted to sodium salts of corresponding carboxylic acids having one carbon atom less.
💡 This test is specific for methyl ketones (CH3CO- group) or compounds with CH3CH(OH)- group that oxidise to CH3CO- group. A yellow precipitate of iodoform indicates a positive test.
Side Chain Chlorination Followed by Hydrolysis (Commercial Preparation of Benzaldehyde)
  1. Side chain chlorination of toluene gives benzal chloride.
  2. Benzal chloride is hydrolysed.
  3. Benzaldehyde is formed.
💡 This is a commercial method for manufacturing benzaldehyde from toluene.
Preparation of Aldehydes from Hydrocarbons using Chromic Oxide
  1. Toluene or substituted toluene is treated with chromic oxide (CrO3) in acetic anhydride.
  2. Benzylidene diacetate is formed.
  3. Benzylidene diacetate is hydrolysed with aqueous acid to give the corresponding benzaldehyde.
💡 This method uses CrO3 in acetic anhydride to stop oxidation at the aldehyde stage by forming benzylidene diacetate intermediate.
Preparation of Ketones from Acyl Chlorides using Dialkylcadmium
  1. Dialkylcadmium is prepared by the reaction of cadmium chloride with Grignard reagent.
  2. Acyl chloride is treated with dialkylcadmium.
  3. The corresponding ketone is formed.
💡 Dialkylcadmium reagents are used to prepare ketones from acyl chlorides. Grignard reagents themselves would over-react.
Preparation of Ketones from Nitriles using Grignard Reagent
  1. A nitrile is treated with Grignard reagent.
  2. The addition product is formed.
  3. Hydrolysis yields a ketone.
💡 Grignard reagents add to nitriles to form ketones after hydrolysis.
Preparation of Carboxylic Acids from Alkylbenzenes
  1. Aromatic carboxylic acids are prepared by vigorous oxidation of alkyl benzenes with chromic acid or acidic or alkaline potassium permanganate.
  2. The entire side chain is oxidised to the carboxyl group irrespective of the length of the side chain.
  3. Primary and secondary alkyl groups are oxidised; tertiary groups are not affected.
💡 Any alkyl side chain on benzene (except tertiary) is completely oxidised to -COOH group. The length of the chain does not matter.
Exam Traps — Don't Make These Mistakes
⚠ IUPAC numbering in aldehydes
✗ CH3CH2CH2CHO as butan-4-al or 4-butanal
→ The aldehyde carbon is always numbered 1 in IUPAC nomenclature; the suffix '-al' itself indicates position 1, so the number is not written before '-al'.
✓ Butanal
⚠ Tollens' vs Fehling's test for aromatic aldehydes
✗ Benzaldehyde gives a positive Fehling's test
→ Aromatic aldehydes do not respond to Fehling's test because they are less reactive towards oxidation; only aliphatic aldehydes reduce Fehling's reagent.
✓ Benzaldehyde gives a negative Fehling's test but a positive Tollens' test (silver mirror).
⚠ Cannizzaro reaction requirement
✗ Propanal undergoes Cannizzaro reaction with concentrated NaOH
→ The Cannizzaro reaction occurs only with aldehydes that have NO α-hydrogen atom. Propanal has α-hydrogens and will undergo aldol condensation instead.
✓ Only aldehydes without α-hydrogen (e.g., methanal, benzaldehyde, trimethylacetaldehyde) undergo Cannizzaro reaction.
⚠ Acidity comparison: carboxylic acid vs phenol
✗ Phenol is more acidic than carboxylic acid because phenoxide ion has more resonating structures
→ Although phenoxide has more resonating structures, the negative charge is delocalised over one oxygen and less electronegative carbon atoms. In carboxylate ion, the negative charge is delocalised over two equivalent electronegative oxygen atoms, making it more stabilised.
✓ Carboxylic acids are stronger acids than phenols because the carboxylate ion is stabilised by two equivalent resonance structures with negative charge on electronegative oxygen atoms.
⚠ Reduction of carboxylic acids
✗ Carboxylic acids are reduced to primary alcohols by NaBH4
→ Sodium borohydride (NaBH4) does not reduce the carboxyl group. It is a mild reducing agent that reduces only aldehydes and ketones.
✓ Carboxylic acids are reduced to primary alcohols by lithium aluminium hydride (LiAlH4) or diborane (B2H6).
⚠ Boiling point order with ethers
✗ Ethers have higher boiling points than aldehydes of comparable molecular mass
→ Aldehydes and ketones have higher boiling points than ethers of comparable molecular masses due to stronger dipole-dipole interactions arising from the polar carbonyl group.
✓ Boiling point order: Alcohols > Aldehydes/Ketones > Ethers > Hydrocarbons (for comparable molecular masses).
Compare & Contrast
Reactivity towards nucleophilic addition
Aldehydes
Ketones
Aldehydes are more reactive than ketones due to (i) steric reason: only one alkyl group in aldehydes vs two in ketones hinders nucleophile approach less, (ii) electronic reason: two alkyl groups in ketones reduce electrophilicity of carbonyl carbon more effectively than one in aldehydes.
Reagent and conditions
Clemmensen reduction
Wolff-Kishner reduction
Clemmensen reduction uses zinc-amalgam and concentrated HCl (acidic medium), while Wolff-Kishner reduction uses hydrazine (NH2NH2) followed by heating with NaOH/KOH in ethylene glycol (basic medium). Both reduce >C=O to >CH2.
Response with aromatic aldehydes
Tollens' reagent
Fehling's reagent
Tollens' reagent (ammoniacal AgNO3) oxidises both aliphatic and aromatic aldehydes giving a silver mirror. Fehling's reagent (alkaline CuSO4 + Rochelle salt) oxidises only aliphatic aldehydes, giving a reddish-brown precipitate; aromatic aldehydes do NOT respond.
Structural requirement of the aldehyde
Aldol condensation
Cannizzaro reaction
Aldol condensation requires the presence of at least one α-hydrogen atom in the aldehyde/ketone and occurs in dilute alkali. Cannizzaro reaction occurs with aldehydes having NO α-hydrogen atom and requires concentrated alkali.
Exam Focus
Marks pattern: 1 MCQ (1 mark), 1 Very Short Answer (2 marks), 1 Short Answer (3 marks), 1 Long Answer (5 marks) — total approximately 11 marks
Most asked: Nucleophilic addition reactions and their mechanism · Aldol condensation and cross aldol condensation · Distinguishing tests (Tollens', Fehling's, iodoform test) · Acidity of carboxylic acids and effect of substituents · Name reactions (Rosenmund, Stephen, Etard, Gatterman-Koch, Clemmensen, Wolff-Kishner, Cannizzaro, HVZ)
Don't skip: Mechanism of nucleophilic addition reactions of aldehydes and ketones; Aldol and cross aldol condensation with products; Effect of substituents on acidity of carboxylic acids (explain with reasoning); Distinguishing between pairs using chemical tests
Safe to skip: Uses of aldehydes, ketones and carboxylic acids (Section 8.5 and 8.10) — these are rarely asked for more than 1 mark and can be glanced through quickly.
One-Line Revision (read aloud before the exam)
  1. The carbonyl carbon is sp2-hybridised, forming three sigma bonds, with the p-orbital forming a π-bond with oxygen; bond angles are approximately 120°.
  2. The carbon-oxygen double bond is polarised due to higher electronegativity of oxygen, making carbonyl carbon electrophilic and carbonyl oxygen nucleophilic.
  3. IUPAC names of aldehydes end with '-al' and ketones end with '-one'; the aldehyde carbon is always numbered 1.
  4. Rosenmund reduction: Acyl chloride is reduced to aldehyde using H2 over Pd/BaSO4 catalyst.
  5. Stephen reaction: Nitriles are reduced to aldehydes using SnCl2/HCl followed by hydrolysis.
  6. Etard reaction: Oxidation of methylbenzene using chromyl chloride (CrO2Cl2) followed by hydrolysis gives benzaldehyde.
  7. Gatterman-Koch reaction: Benzene + CO + HCl in presence of anhydrous AlCl3/CuCl gives benzaldehyde.
  8. Friedel-Crafts acylation: Benzene + acyl chloride in presence of anhydrous AlCl3 gives aromatic ketone.
  9. Boiling points: Alcohols > Aldehydes/Ketones > Ethers > Hydrocarbons (for comparable molecular masses) due to hydrogen bonding and dipole-dipole interactions.
  10. Lower aldehydes and ketones are miscible with water due to hydrogen bond formation between carbonyl oxygen and water.
  11. Aldehydes are more reactive than ketones in nucleophilic addition due to steric (less crowding) and electronic (greater electrophilicity) reasons.
  12. Cyanohydrin formation: Aldehydes/ketones + HCN (catalysed by base) gives cyanohydrins; the cyanide ion (CN-) is the nucleophile.
  13. Sodium hydrogensulphite addition product is water-soluble and can be used for separation and purification of aldehydes.
  14. Acetal formation: Aldehyde + 2 equivalents of monohydric alcohol in dry HCl gives gem-dialkoxy compound (acetal).
  15. 2,4-DNP (2,4-dinitrophenylhydrazine) forms yellow, orange or red precipitates with aldehydes and ketones, useful for characterisation.
  16. Tollens' test: Aldehydes reduce ammoniacal AgNO3 to metallic silver (silver mirror); ketones do not respond.
  17. Fehling's test: Aliphatic aldehydes reduce Cu2+ to Cu+ giving reddish-brown precipitate; aromatic aldehydes and ketones do not respond.
  18. Iodoform test: Methyl ketones (CH3CO-) or compounds with CH3CH(OH)- group give yellow precipitate of CHI3 with I2/NaOH.
  19. Clemmensen reduction: >C=O reduced to >CH2 using Zn-Hg/conc. HCl.
  20. Wolff-Kishner reduction: >C=O reduced to >CH2 using NH2NH2 followed by KOH/NaOH in ethylene glycol.
  21. Aldol condensation: Aldehydes/ketones with α-hydrogen react in dilute alkali to form β-hydroxy aldehydes/ketones, which on heating give α,β-unsaturated carbonyl compounds.
  22. Cross aldol condensation: Aldol reaction between two different aldehydes/ketones, both containing α-hydrogens, gives a mixture of four products.
  23. Cannizzaro reaction: Aldehydes without α-hydrogen undergo disproportionation in concentrated alkali — one molecule oxidised to acid salt, another reduced to alcohol.
  24. Carboxylic acids are named with suffix '-oic acid' in IUPAC; the carboxyl carbon is numbered 1.
  25. Carboxylic acids have higher boiling points than alcohols of comparable mass due to extensive intermolecular hydrogen bonding, existing as dimers.
  26. Carboxylic acids are stronger acids than phenols because carboxylate ion has two equivalent resonance structures with negative charge on electronegative oxygen atoms.
  27. Electron-withdrawing groups (e.g., -NO2, -Cl, -F) increase acidity of carboxylic acids by stabilising the conjugate base; electron-donating groups decrease acidity.
  28. Hell-Volhard-Zelinsky (HVZ) reaction: Carboxylic acids with α-hydrogen are halogenated at α-position using Cl2/Br2 and red phosphorus.
  29. Decarboxylation: Sodium salt of carboxylic acid heated with sodalime (NaOH + CaO) loses CO2 to form hydrocarbon.
  30. Kolbe electrolysis: Electrolysis of aqueous solution of alkali metal salt of carboxylic acid gives hydrocarbon with twice the number of carbon atoms.
  31. Esterification mechanism: Protonation of carbonyl oxygen activates the carbonyl group, followed by nucleophilic addition of alcohol, proton transfer, loss of water, and deprotonation.
  32. DIBAL-H (diisobutylaluminium hydride) selectively reduces nitriles and esters to aldehydes.

MCQs — 10 Questions with Answers

Q1. The IUPAC name of CH₃CH₂CH(Cl)CHO is:

  • A. 3-Chloropropanal
  • B. 3-Chlorobutanal ✓
  • C. 2-Chlorobutanal
  • D. 4-Chlorobutanal

Answer: B — The aldehyde carbon is C1; the chain has 4 carbons (butanal); Cl is on C3, so the name is 3-chlorobutanal.

Q2. Which statement about the carbonyl group is INCORRECT?

  • A. The carbonyl carbon is sp² hybridised with trigonal planar geometry.
  • B. The oxygen atom in a carbonyl is more electronegative and acts as a nucleophilic centre. ✓
  • C. The carbonyl carbon is an electrophilic centre due to polarisation.
  • D. The π-bond in C=O is formed by overlap of p-orbitals.

Answer: B — The oxygen is the nucleophilic centre (has lone pairs and electron density), not an electrophilic centre; the statement reverses their roles.

Q3. Formaldehyde undergoes the Cannizzaro reaction while acetaldehyde does not. The key difference is:

  • A. Formaldehyde is more volatile.
  • B. Formaldehyde has no α-hydrogen; acetaldehyde has α-hydrogens. ✓
  • C. Acetaldehyde is a stronger acid.
  • D. Formaldehyde is a gas at room temperature.

Answer: B — Cannizzaro is a disproportionation occurring only with aldehydes lacking α-H (like HCHO); acetaldehyde with α-H undergoes aldol condensation instead.

Q4. In the aldol condensation, the new C–C bond forms between:

  • A. The α-carbon of one aldehyde and the carbonyl carbon of another. ✓
  • B. Two carbonyl carbons directly.
  • C. The α-hydrogen of one aldehyde and the carbonyl carbon of another.
  • D. The hydroxyl group and the carbonyl carbon.

Answer: A — The nucleophilic enolate ion (from α-carbon of one aldehyde) attacks the electrophilic carbonyl carbon of the second aldehyde, forming a C–C bond.

Q5. Which aldehyde is more reactive toward nucleophilic addition and why?

  • A. Acetaldehyde is more reactive because it has methyl groups to stabilise the intermediate.
  • B. Formaldehyde is more reactive because H is less bulky than alkyl, making C more accessible. ✓
  • C. Acetaldehyde is more reactive because it has more α-hydrogens.
  • D. Both are equally reactive; reactivity depends only on solvent polarity.

Answer: B — Formaldehyde's lack of bulky alkyl groups allows easier access to the carbonyl carbon; acetaldehyde is hindered by the methyl group, reducing nucleophilic approach.

Q6. The pKa of benzoic acid (C₆H₅COOH) is ~4.2. If a nitro group (–NO₂) is attached to the benzene ring, the pKa of the resulting compound is likely to be:

  • A. Greater than 4.2 (more weakly acidic).
  • B. Less than 4.2 (more strongly acidic). ✓
  • C. Equal to 4.2 (no change).
  • D. Cannot be determined without additional data.

Answer: B — –NO₂ is an electron-withdrawing group that stabilises the conjugate base (COO⁻) via resonance, increasing acidity and lowering pKa.

Q7. The structure of 4-oxopentanal (CH₃COCH₂CH₂CHO) contains:

  • A. One aldehyde and one ketone functional group. ✓
  • B. Two aldehyde functional groups.
  • C. Two ketone functional groups.
  • D. One aldehyde and one carboxylic acid.

Answer: A — The compound has an aldehyde (–CHO) at one end and a ketone (CH₃CO–) at position 4, making it both an aldehyde and a ketone.

Q8. When 50 g of ethanol (C₂H₅OH) is oxidised to ethanal (CH₃CHO), approximately how many grams of ethanal can be produced (assuming 100% yield)? (Molar mass: C = 12, H = 1, O = 16; M(ethanol) = 46 g/mol, M(ethanal) = 44 g/mol)

  • A. 47.8 g ✓
  • B. 44.0 g
  • C. 50.0 g
  • D. 52.2 g

Answer: A — Moles of ethanol = 50/46 ≈ 1.087 mol; 1:1 molar ratio in oxidation; mass of ethanal = 1.087 × 44 ≈ 47.8 g.

Q9. Both assertion and reason are given. Assertion: Aldehydes are more reactive than ketones toward nucleophilic addition. Reason: Aldehydes have a less bulky hydrogen atom attached to the carbonyl carbon, making the carbon more accessible.

  • A. Both assertion and reason are correct, and reason is the correct explanation. ✓
  • B. Both assertion and reason are correct, but reason is not the correct explanation.
  • C. Assertion is correct, but reason is incorrect.
  • D. Assertion is incorrect, but reason is correct.

Answer: A — Aldehydes are indeed more reactive; the reason (reduced steric hindrance from H vs. alkyl) correctly explains why this reactivity difference exists.

Q10. In nucleophilic addition to a carbonyl (R₂C=O + :Nu⁻), the first step forms a tetrahedral intermediate. This intermediate is stabilised by:

  • A. The π-bond in C=O.
  • B. Resonance delocalisation of the negative charge from the newly formed C–O⁻ bond. ✓
  • C. Breaking of the C=O σ-bond.
  • D. Electrostatic attraction between the negatively charged oxygen and the positively charged carbon.

Answer: B — The negative charge on oxygen in the tetrahedral intermediate is stabilised by resonance, allowing the negative charge to delocalise via the C–O bond.

Flashcards

What is the hybridisation of carbonyl carbon and its bond geometry?

Carbonyl carbon is sp² hybridised with trigonal planar geometry (~120° bond angles).

Why is the carbonyl group polarised?

Oxygen is more electronegative than carbon, pulling electron density toward itself and making the carbonyl carbon electrophilic.

What is the IUPAC naming rule for open-chain aldehydes?

Replace the terminal '-e' of the alkane with '-al' and number the chain starting from the aldehyde carbon (C1).

How do IUPAC names of ketones differ from aldehydes in numbering?

In ketones, number the chain from the end nearest to the carbonyl group to give it the lowest number.

What does nucleophilic addition to a carbonyl mean?

A nucleophile attacks the electrophilic carbonyl carbon, forming a tetrahedral intermediate with a new C–Nu bond.

What is the aldol condensation reaction?

Two carbonyl compounds with α-hydrogens condense to form a β-hydroxy carbonyl (aldol), followed by dehydration to an α,β-unsaturated carbonyl.

Why can formaldehyde undergo Cannizzaro reaction but acetaldehyde cannot?

Formaldehyde has no α-hydrogen and undergoes disproportionation (one molecule oxidised to formic acid, one reduced to methanol); acetaldehyde has α-H and undergoes aldol instead.

What makes aldehydes more reactive than ketones toward nucleophilic addition?

Aldehydes have one less bulky group (H instead of alkyl) around the carbonyl, making the carbon more accessible and transition states less sterically hindered.

What is the carboxyl group structure and why are carboxylic acids acidic?

Carboxyl group is –COOH; carboxylic acids are acidic because the conjugate base (COO⁻) is stabilised by resonance between two C–O bonds.

How do electron-withdrawing groups affect carboxylic acid acidity?

Electron-withdrawing groups (e.g., Cl, NO₂) attached to the α-carbon stabilise the negative charge on COO⁻, increasing acidity (lower pKa).

Important Board Questions

Define the carbonyl group and write the structure of an aldehyde and a ketone. Which one is more reactive toward nucleophilic addition and why? [2 marks]

Carbonyl is C=O; aldehydes have RCHO structure (less bulky H around C), ketones have R₂CO (more bulky alkyl). Aldehydes are more reactive due to reduced steric hindrance.

Explain the aldol condensation mechanism for two molecules of ethanal (CH₃CHO). Include the formation of the enolate ion, the nucleophilic attack, and the final product after dehydration. Show all steps. [5 marks]

Step 1: α-H deprotonation forms enolate (CH₂=CHO⁻). Step 2: Enolate attacks second ethanal's C=O (nucleophilic addition). Step 3: Tetrahedral intermediate formed. Step 4: Dehydration removes water, giving CH₃CH=C(OH)CH₃ (but-3-en-2-ol) → dehydration to α,β-unsaturated ketone. Show resonance stabilisation of conjugate base.

Carboxylic acids are weakly acidic (pKa ≈ 4.75) and their acidity is affected by substituents. (a) Using resonance structures, explain why the carboxyl group (–COOH) is acidic. (b) Compare the acidity of benzoic acid (C₆H₅COOH), 4-nitrobenzoic acid (O₂N–C₆H₄–COOH), and 4-methoxybenzoic acid (CH₃O–C₆H₄–COOH). Justify the order using electronic effects. Show all resonance forms of the conjugate base. [6 marks]

Part (a): COOH ⇌ COO⁻ + H⁺; resonance in COO⁻ shows two equivalent C–O bonds, delocalising negative charge (stabilising conjugate base, favouring ionisation). Part (b): –NO₂ withdraws electrons (EWG) → stabilises COO⁻ → increases acidity (pKa < benzoic); –OCH₃ donates electrons (EDG) → destabilises COO⁻ → decreases acidity (pKa > benzoic). Order: 4-nitrobenzoic acid > benzoic acid > 4-methoxybenzoic acid. Draw two resonance forms of COO⁻ showing C=O and C–O⁻ structures.

Assertion-Reason Questions for Aldehydes, Ketones and Carboxylic Acids

Q1. Assertion (A):

The carbonyl carbon in aldehydes and ketones is electrophilic in nature.

Reason (R):

The carbon-oxygen double bond is polarised due to the higher electronegativity of oxygen, making the carbon atom electron-deficient and thus electrophilic.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both statements are true. The high electronegativity of oxygen withdraws electron density from carbon via the C=O bond, making carbon susceptible to nucleophilic attack. This explains why aldehydes and ketones undergo nucleophilic addition reactions. The reason correctly explains the assertion.

Q2. Assertion (A):

In the IUPAC nomenclature of aldehydes, the carbonyl carbon is always given priority and numbered as position 1.

Reason (R):

This numbering convention ensures the aldehyde group is positioned at the end of the parent chain, making the structure unambiguous.

Answer: B — Both A and R are true, but R is NOT the correct explanation of A — The assertion is true—the aldehyde carbon is numbered 1. However, the reason is partially incorrect. The aldehyde is inherently terminal (at the chain end), so numbering convention ensures consistency and minimises numbers for other substituents. The reason does not fully capture why this rule exists; it exists for standardisation, not to position the group at the end.

Q3. Assertion (A):

Ketones are more resistant to oxidation than aldehydes under mild oxidising conditions.

Reason (R):

Aldehydes have an α-hydrogen atom bonded to the carbonyl carbon, which is easily oxidised, whereas ketones lack this α-hydrogen.

Answer: C — A is true but R is false — The assertion is correct. Aldehydes are readily oxidised to carboxylic acids by mild oxidising agents, while ketones resist oxidation unless strong conditions are applied. However, the reason is incorrect. Both aldehydes and ketones have α-hydrogens (hydrogens on carbons adjacent to the carbonyl). The true reason is that aldehydes have a C−H bond on the carbonyl carbon itself, which is easily oxidised to C=O (forming RCOOH), whereas ketones lack this H on the carbonyl carbon.

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Case-Based Questions (CBQ) for Aldehydes, Ketones and Carboxylic Acids

Case Study:

A perfume manufacturer in India is developing a new fragrance line using natural aldehydes and ketones. They extract salicylaldehyde (2-hydroxybenzaldehyde) from meadow sweet and cinnamaldehyde (3-phenylprop-2-enal) from cinnamon. These compounds are then blended with synthetic acetone (dimethyl ketone) as a solvent. The manufacturer needs to label these compounds with their IUPAC names on the product packaging for regulatory compliance. During quality control, the chemist must verify the purity and structural integrity of each compound before blending.

Q1. What is the IUPAC name of salicylaldehyde mentioned in the passage?

  • A. 2-Hydroxybenzaldehyde ✓
  • B. Hydroxybenzene-1-carbaldehyde
  • C. 2-Hydroxybenzene carbaldehyde
  • D. Salicyl methanol

Answer: A

Q2. Cinnamaldehyde has the structure C6H5CH=CHCHO. Its IUPAC name is 3-phenylprop-2-enal. Why is the position of the double bond numbered as '2' rather than '1'?

  • A. Because IUPAC prioritises the aldehyde group at position 1, and the double bond is numbered from that reference point ✓
  • B. Because the phenyl group occupies position 1
  • C. Because double bonds are always secondary to aldehyde groups in numbering
  • D. Because the molecular weight requires position 2 designation

Answer: A

Q3. Acetone (dimethyl ketone) used as a solvent has the formula CH3COCH3. What is its IUPAC name?

  • A. Dimethyl ketone
  • B. Propanone ✓
  • C. 2-Propanone
  • D. Methyl ethyl ketone

Answer: B

Q4. During quality control, the chemist observes that salicylaldehyde solution in acetone gradually turns yellowish over time. This is likely due to:

  • A. Reduction of acetone by salicylaldehyde
  • B. Oxidation of the aldehyde group in salicylaldehyde by dissolved oxygen ✓
  • C. Polymerisation of acetone molecules
  • D. Evaporation of acetone solvent

Answer: B

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Previous Year Questions (PYQ) for Aldehydes, Ketones and Carboxylic Acids

Real CBSE board exam questions from past papers — free to attempt.

CBSE 2026 · Set 56(B) · 3 marks

How do you convert the following ? (a) Propanone to Propan-2-ol (b) Ethanal to Ethane (c) Sodium benzoate to Benzene

CBSE 2026 · Set 56(B) · 5 marks

Give reasons for the following : (i) Oxidation of propanal is easier than propanone. (ii) There are two − NH2 groups in semicarbazide. However, only one − NH2 groups is involved in the formation of semicarbazones. (iii) HCHO does not undergo aldol condensation reaction. (iv) Boiling points of carboxylic acids are higher than those of alcohols of comparable molecular masses. (v) Aldehydes are more reactive towards nucleophilic addition reactions than ketones.

CBSE 2026 · Set 56/1/1 · 1 mark

Which of the following is 'not' true about enantiomers?

  • They have the same chemical reactivity.
  • They have the same specific rotation.
  • They have the same melting or boiling point.
  • They have the same refractive index.

CBSE 2026 · Set 56/1/1 · 3 marks

Give reasons for the following: (a) Carboxylic acids have higher boiling point than alcohols of comparable molecular masses. (b) Alpha (α) hydrogens of aldehydes and ketones are acidic in nature. (c) Nucleophilic addition of ammonia and its derivatives does not occur with carbonyl group in strongly acidic medium.

CBSE 2026 · Set 56/1/1 · 5 marks

An organic compound (X) has the molecular formula C5H10O. Draw structures for (X) if it: (I) does not give Tollen's test but gives a positive iodoform test. (II) does not give Tollen's test and iodoform test but undergoes Aldol condensation. (III) undergoes Cannizzaro's reaction.

CBSE 2026 · Set 56/1/2 · 1 mark

Which of the following Grignard Reagent will be used to prepare [structure] when treated with methanal?

  • CH3MgBr
  • [Option B - diagram]
  • [Option C - diagram]
  • [Option D - diagram]

CBSE 2026 · Set 56/1/2 · 5 marks

(a) (i) An organic compound (X) has the molecular formula C5H10O. Draw structures for (X) if it: (I) does not give Tollen's test but gives a positive iodoform test. (II) does not give Tollen's test and iodoform test but undergoes Aldol condensation. (III) undergoes Cannizzaro's reaction. (ii) Show how each of the following compounds can be converted to benzoic acid: (I) Acetophenone (II) Ethyl benzene

CBSE 2026 · Set 56/1/3 · 1 mark

Identify 'X' in the following reaction : CH 3 – CO – CH2 – COO – CH3 + NaBH4 → 'X'

  • CH 3 – CHOH – CH2 – COO – CH3
  • CH3 – CO – CH2 – CH2 – OH
  • CH 3 – CHOH – CH2 – CH2 – OH
  • CH3 – CHOH – CH2 – COOH

CBSE 2026 · Set 56/1/3 · 3 marks

Give reasons for the following : (a) Carboxylic acids have higher boiling point than alcohols of comparable molecular masses. (b) Alpha ( α ) hydrogens of aldehydes and ketones are acidic in nature. (c) Nucleophilic addition of ammonia and its derivatives does not occur with carbonyl group in strongly acidic medium.

CBSE 2026 · Set 56/1/3 · 5 marks

(a) (i) An organic compound (X) has the molecular formula C5H10O. Draw structures for (X) if it : (I) does not give Tollen's test but gives a positive iodoform test. (II) does not give Tollen's test and iodoform test but undergoes Aldol condensation. (III) undergoes Cannizzaro's reaction.

CBSE 2026 · Set 56/2/1 · 3 marks

Write the reaction involved in (a) Rosenmund's reduction (b) Cannizzaro's reaction (c) Hell – Volhard – Zelinsky reaction

CBSE 2026 · Set 56/2/1 · 5 marks

How will you convert the following: (i) Propanone to Propene (ii) Benzoic acid to Benzaldehyde (iii) Benzene to m-Nitroacetophenone. Arrange the following compounds in increasing order of their reactivity towards HCN. Identify the compounds, which would undergo Aldol condensation.

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Class 12 Chemistry Formula Sheet

Every formula and named reaction from all 10 chapters on one page, checked against the NCERT textbook and the CBSE 2026-27 syllabus. Free PDF download.

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Class 12 Chemistry Previous Year Questions, Chapter-wise

All 1241 CBSE board questions from 2022–2026, sorted by chapter and question type. Free PDF download.

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