Aldehydes, Ketones and Carboxylic Acids

ALDEHYDES, KETONES AND CARBOXYLIC ACIDS

8.1 NOMENCLATURE AND STRUCTURE OF CARBONYL GROUP

8.1.1 Nomenclature of Aldehydes and Ketones

**Aldehydes** contain the **carbonyl group (-CHO)** bonded to one carbon and one hydrogen, while **ketones** contain the carbonyl group bonded to two carbon atoms. Both are crucial in organic chemistry with widespread applications in medicines, perfumes, solvents, and food products.

**Common Names of Aldehydes:**

Derived from the common names of corresponding carboxylic acids by replacing the ending "-ic acid" with "-aldehyde"

Location of substituents indicated by Greek letters (α, β, γ, δ) where α-carbon is directly attached to the aldehyde group

Examples: Formaldehyde (HCHO), Acetaldehyde (CH₃CHO), Valeraldehyde (CH₃CH₂CH₂CH₂CHO)

**Common Names of Ketones:**

Named by identifying the two alkyl or aryl groups bonded to the carbonyl group

Substituent positions indicated as αα', ββ', etc.

Simplest ketone (CH₃COCH₃) is called **acetone**

Alkyl phenyl ketones named by adding the acyl group name as prefix to "phenone"

Examples: Methyl n-propyl ketone, Diisopropyl ketone

**IUPAC Names of Aldehydes:**

Replace the terminal "-e" of the corresponding alkane with "-**al**"

For open-chain aldehydes, numbering starts from the carbonyl carbon

When aldehyde group is attached to a ring, use suffix "**-carbaldehyde**" (e.g., benzene**carbaldehyde**)

Aromatic aldehydes: **Benzaldehyde** (IUPAC accepts common name); derivatives named as substituted benzaldehydes

Examples: Methanal (HCHO), Ethanal (CH₃CHO), 2-Methylpropanal [(CH₃)₂CHCHO]

**IUPAC Names of Ketones:**

Replace terminal "-e" of corresponding alkane with "-**one**"

Numbering begins from the end nearer to the carbonyl carbon

For cyclic ketones, carbonyl carbon is numbered as position 1

Substituents prefixed in alphabetical order with position numerals

Examples: Pentan-2-one (CH₃COCH₂CH₂CH₃), 2,4-Dimethylpentan-3-one

8.1.2 Structure of Carbonyl Group

**Hybridization and Bonding:**

Carbonyl carbon is **sp² hybridized**, forming three sigma (σ) bonds in the same plane

Fourth valence electron remains in p-orbital and forms a **π-bond** with oxygen's p-orbital

Oxygen atom possesses two non-bonding electron pairs

Three atoms attached to carbonyl carbon, carbonyl carbon, and oxygen lie in the same plane

Bond angles approximately **120°** (trigonal planar geometry)

**Polarity and Resonance:**

**Carbon-oxygen double bond is polar** due to higher electronegativity of oxygen (3.44) vs. carbon (2.55)

Carbonyl carbon acts as **electrophilic (Lewis acid) center**

Carbonyl oxygen acts as **nucleophilic (Lewis base) center**

Carbonyl compounds have **substantial dipole moments**, making them more polar than ethers

Polarity explained by resonance between neutral structure (A) and dipolar structure (B):

Structure A: Neutral with C=O double bond

Structure B: Polar with C⁺-O⁻ charge separation

This resonance explains both the electrophilicity of carbon and nucleophilicity of oxygen

**Key Exam Point:** The planar sp² geometry of carbonyl carbon and its polarity are fundamental to understanding all subsequent nucleophilic addition reactions.

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8.2 PREPARATION OF ALDEHYDES AND KETONES

8.2.1 General Methods for Both Aldehydes and Ketones

**1. Oxidation of Alcohols:**

Primary alcohols → Aldehydes (oxidized with mild oxidizing agents like PCC, DMP, or Jones reagent)

Secondary alcohols → Ketones (oxidized with same agents)

Refers back to Unit 7 mechanisms; key reagent is **pyridinium chlorochromate (PCC)**

Example: CH₃CH₂OH → CH₃CHO (ethanol to ethanal)

**2. Dehydrogenation of Alcohols:**

Suitable for **volatile alcohols**; industrially important

Alcohol vapors passed over heavy metal catalysts: **Silver (Ag) or Copper (Cu)**

Primary alcohols → Aldehydes; Secondary alcohols → Ketones

Reaction: R-CH₂OH → R-CHO + H₂

Example: CH₃CH₂OH → CH₃CHO at high temperature over Cu or Ag catalyst

**3. From Hydrocarbons - Ozonolysis of Alkenes:**

Alkene treated with ozone (O₃), followed by zinc dust and water

**Zn breaks the ozonide** (1,2,4-trioxolane intermediate) to yield carbonyl compounds

Product depends on alkene substitution:

Terminal alkenes give aldehydes

Internal alkenes give ketones

Mixed substitution gives mixture of both

Example: CH₃CH=CHCH₃ → CH₃CHO + CH₃CHO

**4. From Hydrocarbons - Hydration of Alkynes:**

Addition of water to alkynes in presence of **H₂SO₄ and HgSO₄** catalyst

Ethyne (HC≡CH) → Acetaldehyde (CH₃CHO)

All other alkynes → Ketones (follows Markovnikov's rule; OH adds to more substituted carbon)

Reaction: RC≡CR' + H₂O → RCOCH₂R' or RCOCR' depending on substitution

8.2.2 Specific Methods for Aldehydes

**1. Rosenmund Reduction (From Acyl Chlorides):**

Acid chloride (acyl chloride) reduced with **H₂ over Pd/BaSO₄** catalyst

**Pd/BaSO₄** is a special catalyst (Lindlar-type) that reduces C=O without further reduction to CH₂

**Reaction:** R-COCl + H₂ → R-CHO + HCl

Example: CH₃COCl + H₂ → CH₃CHO

BaSO₄ acts as "**poison**" to prevent over-reduction of aldehyde to alcohol

**2. Stephen Reaction (From Nitriles):**

Nitrile reduced with **SnCl₂ in HCl** to form **imine intermediate** [R-CH=NH⁺]

Imine hydrolyzed with water to give **aldehyde**

**Reaction:** R-CN + SnCl₂/HCl → R-CH=NH (imine) → R-CHO + NH₃

Example: CH₃CH₂CN → CH₃CH₂CHO

Advantage: Stops at aldehyde stage, doesn't over-reduce to alcohol

**3. DIBAL-H Reduction (From Nitriles and Esters):**

**Diisobutylaluminium hydride (DIBAL-H)** at **low temperature (-78°C)**

Selective reduction:

Nitrile → Imine intermediate → Aldehyde (upon hydrolysis)

Ester → Aldehyde (upon hydrolysis)

**Reaction:** R-CN + DIBAL-H → R-CH=N-Al(iBu)₂ → R-CHO (after aqueous workup)

Example: HOCH₂CH₂CHO can be prepared from EOCRCH₂CH₂OEt

**4. Etard Reaction (From Methylbenzene/Toluene):**

Toluene or substituted toluene treated with **CrO₂Cl₂** (chromyl chloride)

Methyl group oxidized to chromium complex intermediate

Complex hydrolyzed with **water** to give **benzaldehyde or substituted benzaldehyde**

**Reaction:** C₆H₅CH₃ + CrO₂Cl₂ → C₆H₅CHO + Cr³⁺ salts

Example: p-CH₃-C₆H₄-CH₃ → p-CH₃-C₆H₄-CHO

Advantage: Stops at aldehyde stage; strong oxidants like KMnO₄ would give carboxylic acid

**5. Chromic Oxide Method (From Methylbenzene):**

Toluene treated with **CrO₃ in acetic anhydride**

Forms **benzylidene diacetate intermediate** [C₆H₅CH(OAc)₂]

Diacetate hydrolyzed with **dilute aqueous acid** to give **benzaldehyde**

**Reaction:** C₆H₅CH₃ + CrO₃/(CH₃CO)₂O → C₆H₅CH(OAc)₂ + H⁺/H₂O → C₆H₅CHO + CH₃COOH

Example: Used for m-bromobenzaldehyde preparation from m-bromotoluene

**6. Side-Chain Chlorination Followed by Hydrolysis (From Toluene):**

**Commercial method** for benzaldehyde manufacture

Toluene treated with **Cl₂ in presence of light (photochemical reaction)**

Forms **benzal chloride** [C₆H₅CHCl₂] after substitution of two H atoms

Benzal chloride hydrolyzed with **aqueous base or acid** to yield **benzaldehyde**

**Reaction:** C₆H₅CH₃ + 2Cl₂ (light) → C₆H₅CHCl₂ + 2HCl; then C₆H₅CHCl₂ + H₂O → C₆H₅CHO + 2HCl

Advantage: Simple, economical, scalable

**7. Gatterman-Koch Reaction (From Benzene/Substituted Benzenes):**

Benzene or substituted benzene treated with **CO and HCl** in presence of **AlCl₃ (anhydrous) or CuCl (cuprous chloride)**

Directly inserts -CHO group on benzene ring without side chain first

**Reaction:** C₆H₆ + CO + HCl → C₆H₅CHO

Example: p-CH₃-C₆H₄ + CO/HCl/AlCl₃ → p-CH₃-C₆H₄-CHO

Advantage: Direct formylation without need for toluene intermediate

8.2.3 Specific Methods for Ketones

**1. Friedel-Crafts Acylation (From Benzene/Substituted Benzenes):**

Benzene or substituted benzene treated with **acid chloride (R-COCl) in presence of AlCl₃ (anhydrous)**

Forms **ketone (Ar-CO-R)**

**Reaction:** C₆H₆ + CH₃COCl → C₆H₅COCH₃ + HCl (requires AlCl₃)

Example: C₆H₆ + n-C₄H₉COCl → C₆H₅CO-C₄H₉ (butyrophenone)

Mechanism: Acylium ion [R-CO]⁺ attacks benzene ring as electrophile

Limitations: Cannot use on benzene rings with electron-withdrawing groups (-CN, -NO₂) as they deactivate ring

**2. From Acyl Chlorides with Dialkylcadmium (Cadmium Method):**

Acid chloride treated with **dialkylcadmium** reagent

Dialkylcadmium prepared by reacting **CdCl₂ with Grignard reagent (R-MgBr)**

**Reaction:** CdCl₂ + 2R-MgBr → (R)₂Cd + 2MgBrCl; then (R)₂Cd + R'-COCl → R-CO-R' + CdClR

Example: (CH₃)₂Cd + CH₃CH₂COCl → CH₃COCH₂CH₃ (pentan-2-one)

Advantage: **Doesn't over-reduce** to alcohols (unlike Grignard directly with acid chloride)

Cadmium reagents more selective than Grignard

**3. From Nitriles with Grignard Reagents:**

Nitrile treated with **excess Grignard reagent (R-MgBr)**, followed by **aqueous acid hydrolysis**

Forms **imine intermediate [R-C≡N + R'-MgBr]**, which is hydrolyzed to **ketone**

**Reaction:** R-CN + R'-MgBr (excess) → R-C(=NMgBr)-R' → R-CO-R' (after H₂O workup)

Example: CH₃CN + C₂H₅MgBr → CH₃COCH₂CH₃ (pentan-2-one)

Note: Excess Grignard ensures complete reaction; aqueous acid workup neutralizes MgBr

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8.3 PHYSICAL PROPERTIES OF ALDEHYDES AND KETONES

**State of Matter at Room Temperature:**

**Methanal (HCHO):** Gas at room temperature (bp -19°C)

**Ethanal (CH₃CHO):** Volatile liquid

**Higher aldehydes and ketones:** Liquids or solids at room temperature

**Boiling Points:**

**Higher than hydrocarbons and ethers** of comparable molecular mass due to **polar C=O dipole and dipole-dipole interactions**

**Lower than alcohols** of similar molecular mass due to **absence of intermolecular hydrogen bonding** (oxygen has non-bonding pairs but cannot form H-bonds as donor; carbon-hydrogen not sufficiently polar)

**Comparative Boiling Point Table:**

| Compound | Molecular Mass | B.P. (K) |

|----------|---|---|

| n-Butane | 58 | 273 |

| Methoxyethane (diethyl ether) | 60 | 281 |

| Propanal | 58 | 322 |

| Acetone (propanone) | 58 | 329 |

| Propan-1-ol | 60 | 370 |

**Interpretation:** n-Butane < Methoxyethane < Propanal < Acetone < Propan-1-ol

n-Butane (weakest: only van der Waals forces)

Ether (slightly stronger: weak dipole interactions)

Aldehyde (stronger: dipole-dipole from C=O)

Ketone (similar to aldehyde: strong dipole-dipole)

Alcohol (strongest: hydrogen bonding)

**Solubility in Water:**

**Lower members (methanal, ethanal, propanone):** Miscible with water in all proportions

Reason: **Form hydrogen bonds with water molecules** (C=O oxygen acts as H-bond acceptor)

**Higher homologs:** Solubility decreases rapidly with increasing chain length

Reason: Hydrophobic alkyl chains dominate; lipophilic character increases

All aldehydes and ketones: **Fairly soluble in organic solvents** (benzene, ether, methanol, chloroform, acetone)

**Odor:**

**Lower aldehydes:** Sharp, pungent odors (unpleasant)

Methanal: Pungent, irritating

Ethanal: Pungent

**Higher aldehydes:** Odor becomes less pungent, more fragrant

**Many natural aldehydes and ketones:** Used in perfumes and flavoring agents

Vanillin (from vanilla): Pleasant vanilla fragrance

Cinnamaldehyde (from cinnamon): Spicy-sweet fragrance

Salicylaldehyde (from meadowsweet): Floral fragrance

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8.4 CHEMICAL REACTIONS OF ALDEHYDES AND KETONES

Both aldehydes and ketones possess the same functional group (carbonyl) and therefore undergo **very similar chemical reactions**. Key differences arise from steric and electronic effects.

8.4.1 Nucleophilic Addition Reactions

**Contrast with Alkenes:**

**Alkenes undergo electrophilic addition** (electrophile attacks π-electron-rich C=C)

**Aldehydes and ketones undergo nucleophilic addition** (nucleophile attacks electrophilic C)

**Mechanism of Nucleophilic Addition:**

**Step 1: Nucleophilic Attack**

Nucleophile (Nu⁻) approaches the electrophilic carbonyl carbon from a direction **approximately perpendicular to the plane of sp² hybridized orbitals**

Attack occurs from above or below the plane of the C=O bond

Electrons from Nu⁻ form new C-Nu σ-bond

**Step 2: Hybridization Change and Intermediate Formation**

Carbonyl carbon hybridization changes from **sp² to sp³** during attack

π-electrons of C=O move onto oxygen, forming **alkoxide intermediate** (R₂C-O⁻)

Intermediate is tetrahedral with negative charge localized on oxygen

**Step 3: Protonation**

Alkoxide intermediate **captures proton (H⁺)** from reaction medium (water, alcohol, acid)

Forms electrically neutral final product: R₂C(OH)-Nu

**Overall Reaction:** C=O + Nu⁻ + H⁺ → CH(OH)-Nu

**Reactivity Differences: Aldehydes vs. Ketones**

**Aldehydes are more reactive than ketones** in nucleophilic addition reactions due to:

1. **Steric Reasons:**

Aldehydes: Only ONE bulky R group attached to carbonyl carbon (R-CHO)

Ketones: TWO bulky R groups attached to carbonyl carbon (R-CO-R')

Two large substituents in ketones create **steric hindrance** preventing nucleophile approach

Nucleophile can access aldehyde carbon more easily

2. **Electronic Reasons:**

Two alkyl groups in ketones **reduce electrophilicity of carbonyl carbon** more effectively than one group in aldehydes

Alkyl groups are electron-donating (through σ-bonding), increasing electron density on carbon

Increased electron density reduces carbon's ability to accept electrons from nucleophile

One alkyl group (in aldehyde) has weaker electron-donating effect

**Reactivity Order:** HCHO > RCHO > (R)₂CO (Formaldehyde most reactive, then aldehydes, then ketones)

**Aromatic vs. Aliphatic Aldehydes:**

**Benzaldehyde is less reactive than aliphatic aldehydes (e.g., propanal)** because:

Benzene ring carbon (to which CHO is attached) is **sp² hybridized**

Lone pairs on benzene ring oxygen can **resonate** through the ring, **delocalize negative charge**, and **reduce electrophilicity** of carbonyl carbon

Resonance structure: C₆H₅-CHO ↔ C₆H₅⁺-CHO⁻ (dipolar form reduces electrophilicity)

In aliphatic aldehydes, no such resonance stabilization occurs

8.4.2 Important Nucleophilic Addition Reactions

**(a) Addition of Hydrogen Cyanide (HCN) - Cyanohydrin Formation:**

**Reaction:** RCOR' + HCN → RCH(OH)CN (aldehyde) or R₂C(OH)CN (ketone)

**Specific Example:** CH₃CHO + HCN → CH₃CH(OH)CN (acetaldehyde cyanohydrin)

**Mechanism:**

Pure HCN reacts very slowly (not a good nucleophile)

**Reaction is catalyzed by weak base (NaOH, NH₃, or KCN)** which generates **cyanide ion (CN⁻)**, a **strong nucleophile**

CN⁻ attacks carbonyl carbon forming C-CN bond

Intermediate accepts H⁺ from solution or regenerated HCN

**Overall: RR'C=O + CN⁻ → RR'C(-CN)O⁻ → RR'C(OH)CN**

**Importance:**

**Cyanohydrins are useful synthetic intermediates**

Can be reduced (with LiAlH₄) to corresponding **primary amines** [RCH(OH)CN → RCH(OH)CH₂NH₂]

Can be hydrolyzed to **hydroxy acids** [RCH(OH)CN + H₂O → RCH(OH)COOH]

Adds two carbon chain (one from CN, one becomes carboxylic acid or amine group)

Allows **chain elongation** in synthesis

**Real-life Example:** Acetaldehyde cyanohydrin (acetone cyanohydrin) is intermediate in industrial synthesis of methacrylic acid.

**(b) Addition of Sodium Hydrogensulfite (NaHSO₃) - Bisulfite Addition:**

**Reaction:** RR'C=O + NaHSO₃ → RR'C(OSO₃⁻Na⁺)(H) or RR'C(OH)(SO₃⁻)

**Structural representation:** Formation of **addition product** (sulfonated adduct)

**Mechanism:**

Hydrogen sulfite ion (HSO₃⁻) acts as nucleophile

Attacks carbonyl carbon, forms C-OSO₃⁻ linkage

Oxygen of carbonyl receives proton

**Equilibrium Position:**

**For most aldehydes:** Equilibrium lies **largely to the right** (products favored)

Reason: Less steric hindrance in aldehydes

Addition products are stable

**For most ketones:** Equilibrium lies **largely to the left** (reactants favored)

Reason: Steric hindrance from two alkyl groups

Addition products are less stable or don't form readily

**Properties of Addition Products:**

**Water-soluble** (unlike original carbonyl compounds)

**Can be reconverted** to original aldehyde/ketone by:

Treatment with **dilute mineral acid (HCl, H₂SO₄)** which regenerates free aldehyde/ketone

Treatment with **alkali (NaOH)** which also regenerates carbonyl compound

**Application:** **Separation and purification of aldehydes** from ketones

Aldehyde-rich mixture treated with NaHSO₃

Aldehydes selectively removed as water-soluble bisulfite adducts

Ketones remain in organic layer (not affected)

Aldehydes recovered by adding acid back to bisulfite adducts

Used industrially for aldehyde purification

**(c) Addition of Grignard Reagents:**

**Reaction:** RR'C=O + R''MgX → RR'C(OMgX)R'' → RR'C(OH)R'' (after aqueous workup)

**Mechanism:**

Grignard reagent [R''⁻...⁺MgX] acts as source of carbanion (R''⁻)

Carbanion attacks electrophilic carbonyl carbon

Forms C-R'' bond and C-O⁻ bond (alkoxide intermediate with Mg²⁺ coordinated to O⁻)

Aqueous acid workup protonates alkoxide: RR'C(OMgX)R'' + H⁺ → RR'C(OH)R'' + Mg²⁺ salts

**Product Formation:**

**Aldehyde + Grignard → Secondary alcohol:** RCHO + R''MgX → RCH(OH)R''

**Ketone + Grignard → Tertiary alcohol:** R-CO-R' + R''MgX → R-C(OH)(R'')-R'

**Ester + Grignard (2 equiv.) → Tertiary alcohol:** RC(=O)OR' + 2R''MgX → RC(OH)(R'')₂ + R'OMgX

**Example:** CH₃CHO + C₂H₅MgBr → CH₃CH(OH)C₂H₅ (sec-butyl alcohol, secondary alcohol)

**Refer to Unit 7 for detailed mechanism and applications.**

**(d) Addition of Alcohols - Acetal and Ketal Formation:**

**Reaction with Aldehydes (Acetal Formation):**

**First equivalent:** Aldehyde + Monohydric alcohol (in presence of dry HCl) → **Hemiacetal** (intermediate)

RCH=O + R'OH → RCH(OR')(OH) [hemiacetal]

**Second equivalent:** Hemiacetal + Another alcohol molecule → **Acetal**

RCH(OR')(OH) + R'OH → RCH(OR')(OR'') + H₂O [acetal or gem-dialkoxy compound]

**Overall Reaction:** RCH=O + 2R'OH ⇌ RCH(OR')(OR'') + H₂O

**Specific Example:** CH₃CHO + 2CH₃OH ⇌ CH₃CH(OCH₃)₂ + H₂O (acetaldehyde dimethyl acetal)

**Mechanism:**

Dry HCl **protonates oxygen** of carbonyl: RCH=O + H⁺ → RCH=O⁺H (acylium ion)

Protonation **increases electrophilicity** of carbonyl carbon

Alcohol nucleophile attacks: RCH(O⁺H) + R'OH → RCH(OH)(OR') + H⁺ (hemiacetal forms)

Second equivalent of alcohol attacks hemiacetal: RCH(OH)(OR') + H⁺ → RCH(O⁺H)(OR') → RCH(OR')(OR'') (loss of H₂O)

Product is acetal (fully protected form of aldehyde)

**Reaction with Ketones (Ketal Formation):**

Ketones react with **ethylene glycol** under similar conditions (dry HCl catalyst)

Forms **cyclic ketal** (five-membered ring containing two oxygen atoms)

**Overall Reaction:** R₂C=O + HOCH₂CH₂OH → [cyclic structure with R, R, and two oxygens] + H₂O

**Specific Example:** Acetone + ethylene glycol → **Acetone ethylene glycol ketal** (2,2-dimethyl-1,3-dioxolane)

**Mechanism:**

Similar to acetal formation

First alcohol OH attacks protonated ketone

Second OH (from same molecule, i.e., ethylene glycol) attacks intermediate

Forms five-membered ring (1,3-dioxolane) with two C-O bonds and two oxygens

**Importance of Acetals and Ketals:**

**Protecting groups in synthesis:** Aldehyde/ketone can be masked as acetal/ketal during reactions that might affect the carbonyl

**Hydrolysis regenerates carbonyl:** Treat with aqueous mineral acid (HCl, H₂SO₄, dilute) to regenerate free aldehyde/ketone

RCH(OR')(OR'') + H₂O (acidic) → RCHO + 2R'OH

**Real-life Application:** In steroid synthesis and complex organic molecules, aldehydes are converted to acetals to protect them during subsequent transformations (e.g., Grignard reactions on other parts of molecule), then regenerated when needed.

**(e) Addition of Ammonia and Its Derivatives:**

**General Reaction:** RR'C=O + H₂N-Z → RR'C=N-Z + H₂O (loss of water)

**Where Z = Alkyl group (R), aryl group (Ar), OH (hydroxyl), NH₂ (amino), C₆H₅NH (phenyl), NHCONH₂ (semicarbazide), etc.**

**Mechanism:**

Ammonia or substituted ammonia (R'NH, R'OH, etc.) acts as nucleophile

Attacks carbonyl carbon forming tetrahedral intermediate: RR'C(OH)(NH-Z)

Intermediate loses water (dehydration occurs readily, especially under acidic conditions)

Forms **C=N double bond (imine or substituted imine): RR'C=N-Z**

Reaction is **reversible and acid-catalyzed**

**Equilibrium favors product** due to rapid dehydration of intermediate

**Products and Nomenclature:**

| Nucleophile (H₂N-Z) | Reagent Name | Product | Product Name |

|---|---|---|---|

| NH₃ | Ammonia | RR'C=NH | Imine or Schiff's base |

| R-NH₂ | Amine | RR'C=N-R | Substituted imine (Schiff's base) |

| HO-NH₂ | Hydroxylamine | RR'C=N-OH | Oxime |

| H₂N-NH₂ | Hydrazine | RR'C=N-NH₂ | Hydrazone |

| C₆H₅-NH-NH₂ | Phenylhydrazine | RR'C=N-NH-C₆H₅ | Phenylhydrazone |

| O₂N-C₆H₄-NH-NH₂ | 2,4-Dinitrophenylhydrazine | RR'C=N-NH-C₆H₃(NO₂)₂ | 2,4-DNP derivative* |

| H₂N-CO-NH-NH₂ | Semicarbazide | RR'C=N-NH-CO-NH₂ | Semicarbazone |

*2,4-DNP derivatives are **yellow, orange, or red solids**, useful for **characterization of aldehydes and ketones** (melting point values are unique and help identify specific compounds). These derivatives are insoluble in aqueous solution, facilitating isolation and purification.

**Exam-Important Reactions:**

**Oxime formation:** Used to identify aldehydes and ketones; oximes have characteristic melting points

**Phenylhydrazone formation:** Phenylhydrazones form as orange-red or yellow solids; useful for compound identification

**2,4-DNP formation:** Yellow/red solids; rapid test for aldehyde/ketone functional group

**Semicarbazone formation:** White crystalline solids; also used for identification and purification

**Real-life Example:** Brady's reagent (2,4-dinitrophenylhydrazine in ethanol + HCl) is used in lab to detect aldehydes and ketones within seconds — a purple/red/orange precipitate confirms presence of C=O functional group.

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8.5 REDUCTION OF ALDEHYDES AND KETONES

8.5.1 Reduction to Primary and Secondary Alcohols

**Reagents:** Sodium borohydride (NaBH₄), Lithium aluminium hydride (LiAlH₄), Catalytic hydrogenation (H₂/Pd, Ni, Pt)

**Reactions:**

**Aldehyde → Primary alcohol:** RCHO + NaBH₄ (or LiAlH₄) → RCH₂OH

Example: CH₃CHO + NaBH₄ → CH₃CH₂OH

Example: HCHO + LiAlH₄ → CH₃OH (methanol)

**Ketone → Secondary