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Haloalkanes and Haloarenes

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Chapter Notes

Overview
This chapter covers the classification, nomenclature, preparation, physical and chemical properties, and reactions of haloalkanes and haloarenes. It is essential in the syllabus because halogenated compounds are widely used as solvents, anaesthetics, pesticides, and in pharmaceuticals. The one big idea a student must walk away with is that the reactivity and reaction mechanisms (SN1, SN2, elimination) of these compounds are governed by the hybridisation of the carbon-halogen bond, steric hindrance, and the stability of carbocation intermediates.
Key Concepts (23)
Haloalkanes
Compounds containing halogen atom(s) attached to the sp3 hybridised carbon atom of an alkyl group.
e.g. CH3CH2Cl (Chloroethane) is a haloalkane.
Haloarenes
Compounds containing halogen atom(s) attached to sp2 hybridised carbon atom(s) of an aryl group.
e.g. C6H5Cl (Chlorobenzene) is a haloarene.
Allylic halides
Compounds in which the halogen atom is bonded to an sp3-hybridised carbon atom adjacent to a carbon-carbon double bond (C=C), i.e., to an allylic carbon.
e.g. CH2=CHCH2Br (Allyl bromide) is an allylic halide.
Benzylic halides
Compounds in which the halogen atom is bonded to an sp3-hybridised carbon atom attached to an aromatic ring.
e.g. C6H5CH2Cl (Benzyl chloride) is a benzylic halide.
Vinylic halides
Compounds in which the halogen atom is bonded to an sp2-hybridised carbon atom of a carbon-carbon double bond (C=C).
e.g. CH2=CHCl (Vinyl chloride) is a vinylic halide.
Aryl halides
Compounds in which the halogen atom is directly bonded to the sp2-hybridised carbon atom of an aromatic ring.
e.g. C6H5Br (Bromobenzene) is an aryl halide.
Geminal dihalides (gem-dihalides)
Dihalo compounds in which both halogen atoms are present on the same carbon atom of the chain.
e.g. CH3CHCl2 (Ethylidene chloride) is a geminal dihalide.
Vicinal dihalides (vic-dihalides)
Dihalo compounds in which halogen atoms are present on adjacent carbon atoms.
e.g. CH2ClCH2Cl (Ethylene dichloride) is a vicinal dihalide.
Finkelstein reaction
The reaction of alkyl chlorides/bromides with NaI in dry acetone to form alkyl iodides, driven by the precipitation of NaCl or NaBr.
e.g. CH3CH2Cl + NaI → CH3CH2I + NaCl (in dry acetone).
Swarts reaction
The synthesis of alkyl fluorides by heating an alkyl chloride/bromide in the presence of a metallic fluoride such as AgF, Hg2F2, CoF2, or SbF3.
e.g. CH3Cl + AgF → CH3F + AgCl.
Sandmeyer's reaction
A reaction in which a diazonium salt is treated with cuprous chloride or cuprous bromide to replace the diazonium group by –Cl or –Br.
e.g. C6H5N2+Cl- + CuCl → C6H5Cl + N2 + CuCl.
Grignard reagent
An organometallic compound of the type RMgX (alkyl magnesium halide), obtained by the reaction of haloalkanes with magnesium metal in dry ether.
e.g. CH3CH2Br + Mg → CH3CH2MgBr (in dry ether).
Wurtz reaction
A reaction in which alkyl halides react with sodium in dry ether to give hydrocarbons containing double the number of carbon atoms present in the halide.
e.g. 2CH3CH2Br + 2Na → CH3CH2CH2CH3 + 2NaBr.
Wurtz-Fittig reaction
A reaction in which a mixture of an alkyl halide and an aryl halide gives an alkylarene when treated with sodium in dry ether.
e.g. C6H5Br + CH3Br + 2Na → C6H5CH3 + 2NaBr.
Fittig reaction
A reaction in which aryl halides are treated with sodium in dry ether to join two aryl groups together.
e.g. 2C6H5Br + 2Na → C6H5C6H5 + 2NaBr.
Racemic mixture
A mixture containing two enantiomers in equal proportions, having zero optical rotation, represented by prefixing dl or (±) before the name.
e.g. (±)-butan-2-ol is a racemic mixture.
Chirality
The property of an object or molecule that is non-superimposable on its mirror image; chiral molecules are optically active.
e.g. Butan-2-ol is chiral because it has an asymmetric carbon atom.
Enantiomers
Stereoisomers related to each other as non-superimposable mirror images; they possess identical physical properties but differ in the direction of rotation of plane polarised light.
e.g. (+)-2-chlorobutane and (–)-2-chlorobutane are enantiomers.
Ambident nucleophile
A nucleophile that possesses two nucleophilic centres and can attack through either of them, e.g., cyanide ion (CN-) and nitrite ion (NO2-).
e.g. KCN gives alkyl cyanides (RCN) while AgCN gives isocyanides (RNC).
Markovnikov's rule
In the addition of hydrogen halides to an unsymmetrical alkene, the hydrogen atom attaches to the carbon with more hydrogen atoms, and the halogen attaches to the carbon with fewer hydrogen atoms.
e.g. Propene + HBr → 2-bromopropane (major product).
Zaitsev's rule (Saytzeff rule)
In dehydrohalogenation reactions, the preferred product is that alkene which has the greater number of alkyl groups attached to the doubly bonded carbon atoms.
e.g. 2-bromopentane gives pent-2-ene as the major product.
Freons
Chlorofluorocarbon compounds of methane and ethane that are extremely stable, unreactive, non-toxic, non-corrosive, and easily liquefiable gases, used as aerosol propellants and refrigerants.
e.g. Freon 12 (CCl2F2) is a common freon.
DDT (p,p'-Dichlorodiphenyltrichloroethane)
A chlorinated organic insecticide that is chemically stable and fat-soluble, leading to bioaccumulation in fatty tissues; its use is banned in many countries due to environmental and health concerns.
e.g. DDT was used to control malaria-carrying mosquitoes.
Formulas & Equations (12)
R–OH + HX → R–X + H2O
Reads as: Alcohol plus hydrogen halide gives alkyl halide plus water
Variables: R = alkyl group; X = Cl, Br, I
Use when: Preparing alkyl halides from alcohols using concentrated halogen acids
⚠ Students forget that tertiary alcohols react at room temperature, while primary and secondary alcohols require ZnCl2 catalyst with HCl.
R–OH + PCl5 → R–Cl + POCl3 + HCl
Reads as: Alcohol plus phosphorus pentachloride gives alkyl chloride, phosphoryl chloride, and hydrogen chloride
Variables: R = alkyl group
Use when: Preparing alkyl chlorides from alcohols using phosphorus halides
⚠ Students confuse PCl5 with PCl3; PCl5 gives POCl3, while PCl3 gives H3PO3.
R–OH + SOCl2 → R–Cl + SO2 + HCl
Reads as: Alcohol plus thionyl chloride gives alkyl chloride, sulfur dioxide, and hydrogen chloride
Variables: R = alkyl group
Use when: Preferred method for preparing pure alkyl chlorides because by-products are gases
⚠ Students forget that this reaction is preferred because SO2 and HCl are escapable gases, giving pure product.
R–X + NaI → R–I + NaX (in dry acetone)
Reads as: Alkyl halide plus sodium iodide in dry acetone gives alkyl iodide and sodium halide
Variables: X = Cl or Br
Use when: Finkelstein reaction for preparing alkyl iodides
⚠ Students forget that dry acetone is essential to precipitate NaX and drive the reaction forward.
R–X + AgF → R–F + AgX
Reads as: Alkyl halide plus silver fluoride gives alkyl fluoride and silver halide
Variables: X = Cl or Br
Use when: Swarts reaction for preparing alkyl fluorides
⚠ Students think any fluoride salt works; only metallic fluorides like AgF, Hg2F2, CoF2, or SbF3 are used.
C6H5N2+Cl- + CuCl → C6H5Cl + N2 + CuCl
Reads as: Benzenediazonium chloride plus cuprous chloride gives chlorobenzene, nitrogen gas, and cuprous chloride
Variables: C6H5 = phenyl group
Use when: Sandmeyer's reaction for preparing aryl chlorides from primary aromatic amines
⚠ Students forget that for iodination, cuprous halide is not required; simply shaking with KI works.
R–X + Mg → RMgX (in dry ether)
Reads as: Alkyl halide plus magnesium in dry ether gives Grignard reagent
Variables: R = alkyl or aryl group; X = Cl, Br, I
Use when: Preparing Grignard reagents for further synthesis
⚠ Students forget that absolutely dry ether is required because Grignard reagents react with water, alcohols, and amines.
2R–X + 2Na → R–R + 2NaX (in dry ether)
Reads as: Two alkyl halide molecules plus two sodium atoms in dry ether gives a hydrocarbon with double the carbon atoms and two sodium halide molecules
Variables: R = alkyl group; X = Cl, Br, I
Use when: Wurtz reaction for preparing higher alkanes
⚠ Students forget that this reaction only works with symmetrical alkyl halides; mixed halides give a mixture of products.
C6H5X + R–X + 2Na → C6H5R + 2NaX (in dry ether)
Reads as: Aryl halide plus alkyl halide plus two sodium atoms in dry ether gives alkylarene and two sodium halide molecules
Variables: X = Cl, Br, I; R = alkyl group
Use when: Wurtz-Fittig reaction for preparing alkylarenes
⚠ Students confuse this with the simple Wurtz reaction; remember it involves both an aryl and an alkyl halide.
2C6H5X + 2Na → C6H5C6H5 + 2NaX (in dry ether)
Reads as: Two aryl halide molecules plus two sodium atoms in dry ether gives biphenyl and two sodium halide molecules
Variables: X = Cl, Br, I
Use when: Fittig reaction for preparing biaryl compounds
⚠ Students forget that this is a specific case of Wurtz-Fittig using only aryl halides.
Rate ∝ [R–X][Nu-] (for SN2)
Reads as: Rate of SN2 reaction is proportional to the concentration of alkyl halide and the concentration of nucleophile
Variables: R–X = alkyl halide; Nu- = nucleophile
Use when: Determining reaction order for SN2 mechanisms
⚠ Students think SN2 is first order; it is second order overall (first order in each reactant).
Rate ∝ [R–X] (for SN1)
Reads as: Rate of SN1 reaction is proportional only to the concentration of alkyl halide
Variables: R–X = alkyl halide
Use when: Determining reaction order for SN1 mechanisms
⚠ Students think SN1 is second order; it is first order overall because the slow step is carbocation formation.
How It Works
SN2 Reaction Mechanism (Bimolecular Nucleophilic Substitution)
  1. Step 1: The nucleophile approaches the carbon bearing the leaving group from the side opposite to the leaving group (backside attack).
  2. Step 2: As the nucleophile forms a bond with the carbon, the carbon-halogen bond weakens simultaneously in a single concerted step.
  3. Step 3: A transition state forms where the carbon is bonded to five atoms (the incoming nucleophile, the leaving group, and three other groups).
  4. Step 4: The leaving group departs, and the nucleophile is fully bonded, resulting in inversion of configuration at the carbon centre.
💡 SN2 reactions are favoured by primary alkyl halides and strong nucleophiles; steric hindrance around the carbon slows the reaction dramatically.
SN1 Reaction Mechanism (Unimolecular Nucleophilic Substitution)
  1. Step 1: The C–X bond undergoes slow, reversible cleavage to form a carbocation and a halide ion (rate-determining step).
  2. Step 2: The carbocation is planar (sp2 hybridised) and achiral.
  3. Step 3: The nucleophile attacks the carbocation from either side of the plane, leading to a racemic mixture of products.
  4. Step 4: The product is formed with both retention and inversion of configuration, resulting in racemisation.
💡 SN1 reactions are favoured by tertiary alkyl halides and polar protic solvents; the stability of the carbocation determines the reaction rate.
Elimination Reaction (Dehydrohalogenation)
  1. Step 1: A haloalkane with a β-hydrogen atom is treated with alcoholic KOH.
  2. Step 2: The base (OH-) abstracts a β-hydrogen atom, forming a water molecule.
  3. Step 3: Simultaneously, the halogen atom leaves as a halide ion, and a double bond forms between the α and β carbon atoms.
  4. Step 4: If multiple β-hydrogens are available, the major product follows Zaitsev's rule: the more substituted alkene is formed.
💡 Alcoholic KOH favours elimination, while aqueous KOH favours substitution; bulky bases also favour elimination over substitution.
Preparation of Grignard Reagent
  1. Step 1: A haloalkane (R–X) is dissolved in dry ether (anhydrous diethyl ether).
  2. Step 2: Magnesium metal is added to the solution.
  3. Step 3: The reaction proceeds to form alkyl magnesium halide (RMgX), where the carbon-magnesium bond is covalent but highly polar.
  4. Step 4: The reagent is highly reactive and must be protected from moisture, as it reacts with any proton source (water, alcohols, amines) to form hydrocarbons.
💡 The reaction must be carried out in absolutely dry ether because even traces of moisture will destroy the Grignard reagent.
Electrophilic Substitution in Haloarenes
  1. Step 1: The halogen atom on the benzene ring is ortho- and para-directing due to resonance, which increases electron density at ortho and para positions.
  2. Step 2: However, the halogen also exerts a strong -I effect, making the ring slightly deactivated compared to benzene.
  3. Step 3: Electrophilic substitution (e.g., nitration, halogenation, sulphonation, Friedel-Crafts) occurs at ortho and para positions, but requires more drastic conditions than benzene.
  4. Step 4: The resonance effect controls orientation, while the inductive effect controls reactivity (net deactivation).
💡 Chlorine is deactivating but ortho-para directing; students often think deactivating groups are meta-directing, but halogens are an exception.
Nucleophilic Substitution in Haloarenes (via Benzyne or Addition-Elimination)
  1. Step 1: Haloarenes are extremely less reactive towards nucleophilic substitution due to resonance (partial double bond character of C–X bond), sp2 hybridisation (shorter, stronger bond), and instability of phenyl cation.
  2. Step 2: Under drastic conditions (e.g., chlorobenzene + NaOH at 623K, 300 atm), substitution occurs via an addition-elimination mechanism.
  3. Step 3: The presence of electron-withdrawing groups (e.g., –NO2) at ortho and para positions greatly increases reactivity by stabilising the intermediate carbanion.
  4. Step 4: A nitro group at meta position has no effect on reactivity because the negative charge in the intermediate cannot be delocalised onto the meta-nitro group.
💡 The –NO2 group activates the ring for nucleophilic substitution only when at ortho or para positions, not meta.
Exam Traps — Don't Make These Mistakes
⚠ Confusing SN1 and SN2 reactivity order
✗ Tertiary alkyl halides react fastest in SN2 reactions because they have more alkyl groups.
→ Tertiary halides are the least reactive in SN2 due to steric hindrance; bulky groups hinder the approach of the nucleophile. The correct order for SN2 is primary > secondary > tertiary.
✓ For SN2 reactions, the order of reactivity is: primary > secondary > tertiary. For SN1 reactions, the order is: tertiary > secondary > primary.
⚠ Forgetting that halogens are deactivating but ortho-para directing
✗ Chlorine is an electron-withdrawing group, so it is meta-directing in electrophilic substitution.
→ Although chlorine withdraws electrons by inductive effect, it also donates electrons by resonance, which increases electron density at ortho and para positions. The resonance effect controls orientation, making chlorine ortho-para directing despite being deactivating.
✓ Chlorine is ortho-para directing in electrophilic substitution because its resonance effect dominates orientation, even though its inductive effect deactivates the ring.
⚠ Mistaking the product of KCN vs AgCN with alkyl halides
✗ Both KCN and AgCN give the same product, alkyl cyanide (RCN), when reacted with alkyl halides.
→ KCN is ionic and provides CN- ions, which attack through carbon to give RCN (alkyl cyanide). AgCN is covalent, and the nucleophile attacks through nitrogen, giving RNC (alkyl isocyanide).
✓ KCN gives alkyl cyanide (RCN) as the main product, while AgCN gives alkyl isocyanide (RNC) as the chief product.
⚠ Confusing inversion and retention in SN2 reactions
✗ SN2 reactions proceed with retention of configuration because the nucleophile attacks from the same side as the leaving group.
→ In SN2, the nucleophile attacks from the side opposite to the leaving group (backside attack), leading to inversion of configuration, like an umbrella turning inside out.
✓ SN2 reactions are accompanied by inversion of configuration because the nucleophile attacks from the side opposite to the leaving group.
⚠ Forgetting that SN1 leads to racemisation
✗ SN1 reactions give a product with the same configuration as the starting material because the carbocation is attacked from one side.
→ The carbocation formed in SN1 is planar (sp2 hybridised) and achiral. The nucleophile can attack from either side with equal probability, resulting in a racemic mixture (50:50 mixture of enantiomers).
✓ SN1 reactions of optically active alkyl halides are accompanied by racemisation because the planar carbocation can be attacked from either side.
⚠ Thinking that all dihalides are named the same way
✗ Both CH3CHCl2 and CH2ClCH2Cl are called ethylene dichloride.
→ CH3CHCl2 is a geminal dihalide (both halogens on same carbon) and is named ethylidene chloride in common system. CH2ClCH2Cl is a vicinal dihalide (halogens on adjacent carbons) and is named ethylene dichloride.
✓ Geminal dihalides are named as alkylidene halides (e.g., ethylidene chloride for CH3CHCl2), while vicinal dihalides are named as alkylene dihalides (e.g., ethylene dichloride for CH2ClCH2Cl).
Compare & Contrast
Reaction kinetics and mechanism
SN1 Reaction
SN2 Reaction
SN1 is a two-step, first-order reaction (rate depends only on [R-X]), involving a carbocation intermediate, and leads to racemisation. SN2 is a one-step, second-order reaction (rate depends on [R-X] and [Nu-]), with no intermediate, and leads to inversion of configuration.
Reactivity towards nucleophilic substitution
Haloalkanes
Haloarenes
Haloalkanes are much more reactive than haloarenes because in haloarenes, the C–X bond has partial double bond character due to resonance, the carbon is sp2 hybridised (shorter, stronger bond), and the phenyl cation is unstable. Haloalkanes have sp3 hybridised carbon and a weaker, more polarised C–X bond.
Product formed with alkyl halides
KCN (Potassium cyanide)
AgCN (Silver cyanide)
KCN is ionic and gives alkyl cyanides (RCN) because the nucleophile attacks through carbon. AgCN is covalent and gives alkyl isocyanides (RNC) because the nucleophile attacks through nitrogen.
Type of reaction favoured
Alcoholic KOH
Aqueous KOH
Alcoholic KOH favours elimination reactions (dehydrohalogenation) to form alkenes. Aqueous KOH favours nucleophilic substitution reactions to form alcohols.
Exam Focus
Marks pattern: 2 MCQs (1 mark each), 1 SA-I (2 marks), 1 SA-II (3 marks), 1 LA (5 marks) — total approximately 11-12 marks in the board exam.
Most asked: Nomenclature of haloalkanes and haloarenes (IUPAC and common names) · SN1 and SN2 reaction mechanisms and stereochemistry (inversion, racemisation) · Preparation of alkyl halides from alcohols (using HX, PCl5, SOCl2) · Finkelstein and Swarts reactions · Sandmeyer's reaction for aryl halides · Grignard reagents and their reactions · Wurtz, Wurtz-Fittig, and Fittig reactions · Electrophilic substitution in haloarenes (ortho-para directing, deactivating nature) · Nucleophilic substitution in haloarenes (effect of –NO2 group at ortho/para positions) · Elimination vs substitution (Zaitsev's rule)
Don't skip: Long answer question (5 marks) on SN1 vs SN2 mechanisms with stereochemistry (inversion, racemisation, retention); Numerical/descriptive question on preparation of haloalkanes from alcohols (3 marks); Reasoning question on why haloarenes are less reactive than haloalkanes (3 marks); Question on Wurtz, Wurtz-Fittig, and Fittig reactions (2-3 marks)
One-Line Revision (read aloud before the exam)
  1. Haloalkanes have halogen attached to sp3 hybridised carbon; haloarenes have halogen attached to sp2 hybridised carbon.
  2. Allylic halides have halogen on a carbon adjacent to a C=C bond; benzylic halides have halogen on a carbon attached to an aromatic ring.
  3. Vinylic halides have halogen directly on a C=C bond; aryl halides have halogen directly on an aromatic ring.
  4. Geminal dihalides have both halogens on the same carbon; vicinal dihalides have halogens on adjacent carbons.
  5. Finkelstein reaction: R–X + NaI → R–I + NaX (in dry acetone) for preparing alkyl iodides.
  6. Swarts reaction: R–X + AgF → R–F + AgX for preparing alkyl fluorides.
  7. Sandmeyer's reaction uses CuCl or CuBr to replace the diazonium group with Cl or Br in aryl halides.
  8. Grignard reagents (RMgX) are prepared in dry ether and react with any proton source to give hydrocarbons.
  9. Wurtz reaction: 2R–X + 2Na → R–R + 2NaX (in dry ether) doubles the carbon chain.
  10. Wurtz-Fittig reaction: C6H5X + R–X + 2Na → C6H5R + 2NaX (in dry ether) gives alkylarenes.
  11. SN2 reactions are second-order, involve backside attack, and cause inversion of configuration.
  12. SN1 reactions are first-order, involve a carbocation intermediate, and cause racemisation.
  13. Alcoholic KOH favours elimination (dehydrohalogenation); aqueous KOH favours substitution.
  14. Zaitsev's rule: In elimination, the more substituted alkene is the major product.
  15. Haloarenes are less reactive than haloalkanes due to resonance, sp2 hybridisation, and instability of phenyl cation.
  16. In haloarenes, the halogen is ortho-para directing but deactivating due to resonance and inductive effects.
  17. Electron-withdrawing groups like –NO2 at ortho/para positions increase reactivity of haloarenes in nucleophilic substitution.
  18. KCN gives alkyl cyanides (RCN); AgCN gives alkyl isocyanides (RNC) due to different bonding nature.
  19. Chloroform (CHCl3) is stored in dark bottles to prevent oxidation to phosgene (COCl2).
  20. Freons are CFCs used as refrigerants and aerosol propellants that deplete the ozone layer.

MCQs — 10 Questions with Answers

Q1. Which of the following is a secondary alkyl halide?

  • A. CH₃CH₂CH₂Br
  • B. (CH₃)₃CBr
  • C. CH₃CH(Br)CH₂CH₃ ✓
  • D. C₆H₅CH₂Cl

Answer: C — In option C, bromine is attached to a carbon bonded to two alkyl groups, making it secondary (2°).

Q2. The IUPAC name of CHCl₃ is:

  • A. Chloroform
  • B. Trichloromethane ✓
  • C. Trichloroethane
  • D. Methyl trichloride

Answer: B — CHCl₃ has three chlorine atoms on one carbon (CH); the IUPAC name is trichloromethane.

Q3. Which compound is a vic-dihalide?

  • A. CH₂ClCHCl₂
  • B. CHCl₂CHCl₂
  • C. CH₃CHClCHClCH₃ ✓
  • D. CCl₂(CH₃)₂

Answer: C — Vic-dihalides have halogen atoms on adjacent carbons; option C has Cl on positions 2 and 3 of the chain.

Q4. The C-F bond is stronger than C-I bond because:

  • A. Fluorine has higher atomic number
  • B. Fluorine is smaller, resulting in shorter and stronger bond ✓
  • C. Iodine is radioactive
  • D. Fluorine forms ionic bonds with carbon

Answer: B — Bond strength increases with bond length decrease; fluorine's smaller size creates a shorter, stronger C-F bond.

Q5. How many structural isomers have the molecular formula C₄H₉Br?

  • A. 2
  • B. 3
  • C. 4 ✓
  • D. 5

Answer: C — Four isomers exist: 1-bromobutane (1°), 2-bromobutane (2°), 1-bromo-2-methylpropane (1°), and 2-bromo-2-methylpropane (3°).

Q6. Which of the following statements about allylic halides is correct? A: Halogen is bonded to sp³ carbon adjacent to C=C B: Halogen is bonded directly to sp² carbon of C=C Choose:

  • A. Only A is true ✓
  • B. Only B is true
  • C. Both A and B are true
  • D. Neither A nor B is true

Answer: A — Allylic halides have halogen on sp³ carbon next to the double bond (A is correct); vinylic halides have halogen on the sp² carbon of C=C (B describes vinylic halides, not allylic).

Q7. The common name 'allyl bromide' corresponds to IUPAC name:

  • A. 1-Bromopropane
  • B. 2-Bromopropene
  • C. 3-Bromopropene ✓
  • D. 1-Bromopropene

Answer: C — Allyl bromide is CH₂=CHCH₂Br; numbering from the double bond gives 3-bromopropene.

Q8. Which is NOT a correct statement about haloalkanes?

  • A. Primary alkyl halides have halogen on carbon bonded to one alkyl group
  • B. Benzylic halides have halogen directly bonded to aromatic ring carbon ✓
  • C. Bond length increases as C-F < C-Cl < C-Br < C-I
  • D. Haloalkanes are used as solvents and synthetic intermediates

Answer: B — Benzylic halides have halogen bonded to sp³ carbon attached to the aromatic ring, not directly to the ring carbon (which would be aryl halide).

Q9. If a haloalkane contains 4 carbons in the main chain with Br at position 2 and a methyl group at position 3, the IUPAC name is: A: 2-Bromo-3-methylbutane B: 3-Bromo-2-methylbutane C: 2-Bromo-3-methylpentane D: Impossible structure Choose the correct option:

  • A. 2-Bromo-3-methylbutane ✓
  • B. 3-Bromo-2-methylbutane
  • C. 2-Bromo-3-methylpentane
  • D. Impossible structure

Answer: A — 4 carbons in main chain = butane; Br at position 2 and CH₃ at position 3 gives 2-bromo-3-methylbutane; numbering prioritizes the halogen.

Q10. Compared to alkyl halides, aryl halides are less reactive in nucleophilic substitution because: (i) sp² carbon is less polarized (ii) aryl carbocations are too unstable (iii) C-Ar bond is shorter and stronger Which statements are correct?

  • A. Only (i) and (ii)
  • B. Only (ii) and (iii) ✓
  • C. Only (i) and (iii)
  • D. All three are correct

Answer: B — Aryl carbocations are highly unstable (ii is true); C-Ar bond is shorter due to sp² hybridisation (iii is true); sp² carbon is actually more polarized, not less (i is false).

Flashcards

What is the key difference between a primary and tertiary alkyl halide?

Primary (1°) has halogen on carbon bonded to 1 alkyl group; tertiary (3°) has halogen on carbon bonded to 3 alkyl groups.

Why is the C-F bond stronger than C-I bond?

Fluorine is smaller and more electronegative, forming a shorter and stronger bond with carbon.

Distinguish between gem-dihalide and vic-dihalide.

Gem-dihalide has both halogens on the same carbon atom; vic-dihalide has halogens on adjacent carbon atoms.

What is an allylic halide? Give one structural feature.

An allylic halide has halogen bonded to sp³ carbon adjacent to a C=C double bond.

Name the compound: CH3CH2CH(Br)CH3 using IUPAC rules.

2-Bromobutane.

Define benzylic halide with an example.

A benzylic halide has halogen bonded to sp³ carbon directly attached to an aromatic ring; example: C6H5CH2Cl (benzyl chloride).

Why does the carbon-halogen bond length increase from F to I?

Because halogen atomic size increases down the group (F < Cl < Br < I), increasing bond length.

What hybridisation must carbon have for a vinylic halide?

sp² hybridisation because the halogen is bonded to a carbon atom of a C=C double bond.

Convert common name 'tert-butyl bromide' to IUPAC name.

2-Bromo-2-methylpropane.

Why are aryl halides unreactive towards nucleophilic substitution compared to alkyl halides?

The C-Ar bond is shorter and stronger due to sp² hybridisation, and aryl carbocations are too unstable to form easily.

Important Board Questions

Define primary, secondary and tertiary alkyl halides. Give one structural example of each. [2 marks]

Define based on number of alkyl groups bonded to the carbon carrying halogen (1, 2, or 3 respectively). Use simple structures: CH₃CH₂CH₂Br (1°), CH₃CH(Br)CH₃ (2°), (CH₃)₃CBr (3°).

Write the IUPAC names of the following compounds and classify each as primary, secondary or tertiary halide: (a) CH₃CH₂CH(Br)CH₂CH₃ (b) (CH₃)₃CCl (c) CH₂=CHCH₂I [5 marks]

For (a): 5-carbon chain, Br at position 3 → 3-bromopentane (2°, check: carbon 3 has 2 alkyl groups). For (b): 4 carbons (2,2-dimethylpropane), Cl at quaternary carbon → 2-chloro-2-methylpropane (3°). For (c): 3-carbon chain with double bond from position 1, I at position 3 (allylic) → 3-iodopropene (1°). Show numbering priority: halogens get lowest numbers.

Explain why the C-F bond is much stronger than the C-I bond. Also explain why aryl halides are unreactive towards nucleophilic substitution compared to alkyl halides. Justify your answers with relevant structural and bonding arguments. [6 marks]

Part 1: Use periodic trends—fluorine is smallest, shortest C-F bond, strong overlap, lowest bond dissociation energy (recall Table 6.2 bond enthalpies). Iodine is largest, longest C-I bond, weak overlap, high bond dissociation energy. Show: bond strength ∝ 1/bond length. Part 2: Aryl halides have sp² C-X (shorter, stronger than sp³ C-X), aryl carbocations are unstable (no resonance stabilisation in this context), sp² carbon less electron-rich than sp³ (less susceptible to nucleophilic attack). Contrast with alkyl halides: sp³ C-X is longer/weaker, alkyl carbocations form readily (3° > 2° > 1°). Use examples: C₆H₅Cl vs CH₃CH₂Cl.

Assertion-Reason Questions for Haloalkanes and Haloarenes

Q1. Assertion (A):

Alkyl halides are classified as primary, secondary, or tertiary based on the nature of the carbon atom bearing the halogen.

Reason (R):

The carbon atom's environment (number of other carbons attached) determines the reactivity and nucleophilic substitution mechanism.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both assertion and reason are true, and the reason correctly explains the assertion. The classification depends on the carbon environment, which directly influences reactivity and reaction pathways.

Q2. Assertion (A):

Carbon-halogen bond length increases from C-F to C-I.

Reason (R):

Halogen atoms become increasingly electronegative as we move down the halogen group.

Answer: C — A is true but R is false — The assertion is true (bond length does increase F to I), but the reason is false. Electronegativity actually decreases down the group (F > Cl > Br > I). The correct reason is that atomic radius increases down the group.

Q3. Assertion (A):

Vinylic halides and aryl halides are structurally different, with halogen bonded to sp² hybridized carbons in both cases.

Reason (R):

Vinylic halides contain halogen on a carbon of the C=C double bond, while aryl halides contain halogen on aromatic ring carbons.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both statements are true and the reason correctly explains the distinction. Although both involve sp² carbons, the chemical environments (double bond vs aromatic ring) make them fundamentally different compounds with different properties.

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Case-Based Questions (CBQ) for Haloalkanes and Haloarenes

Case Study:

A pharmaceutical company is developing new antimalarial drugs and is studying the structural features of existing chloroquine (containing aromatic chlorine) and comparing it with a newly synthesized compound that contains a bromine atom on an allylic carbon. The research team needs to understand how these structural differences affect the compounds' chemical reactivity and stability. The compounds will be used as starting materials for further chemical modifications. Based on their understanding of halogenated compounds, they want to predict which compound will be more reactive in nucleophilic substitution reactions.

Q1. In chloroquine (with Cl on the aromatic ring), the halogen is bonded to which type of hybridized carbon?

  • A. sp hybridized carbon
  • B. sp² hybridized carbon ✓
  • C. sp³ hybridized carbon
  • D. sp³d hybridized carbon

Answer: B

Q2. The newly synthesized compound has bromine on an allylic carbon. This means the halogen is bonded to:

  • A. An sp³ hybridized carbon adjacent to a C=C double bond ✓
  • B. An sp² hybridized carbon of a C=C double bond
  • C. An sp² hybridized aromatic carbon
  • D. An sp³ hybridized carbon two positions away from a C=C

Answer: A

Q3. Comparing the two compounds, which would likely show greater reactivity in nucleophilic substitution reactions?

  • A. Chloroquine, because chlorine is more electronegative
  • B. The allylic bromide, because allylic carbocation is stabilized by resonance from the nearby C=C ✓
  • C. Both would show equal reactivity because both contain halogens
  • D. Neither would react because aromatic and allylic systems are unreactive

Answer: B

Q4. If the research team wanted to name the new compound using IUPAC nomenclature, and it has the structure CH2=CHCH2Br, what would be the correct name?

  • A. Allyl bromide
  • B. 1-Bromopropene
  • C. 3-Bromopropene ✓
  • D. Vinyl bromide

Answer: C

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Previous Year Questions (PYQ) for Haloalkanes and Haloarenes

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

CBSE 2026 · Set 56(B) · 1 mark

Chloroform, in the presence of air and sunlight, produces a poisonous gas known as :

  • Phosgene
  • Phosphine
  • Tear gas
  • Mustard gas

CBSE 2026 · Set 56(B) · 2 marks

What happens when : (a) n-butyl chloride is treated with alcoholic KOH ? (b) Chlorobenzene is treated with CH3Cl in the presence of anhydrous AlCl3 ?

CBSE 2026 · Set 56(B) · 3 marks

Write two differences between SN1 and SN2 reactions. Out of chlorobenzene and 2,4,6-trinitrochlorobenzene, which one is more reactive towards nucleophilic substitution reactions ?

CBSE 2026 · Set 56/1/1 · 2 marks

Why are haloarenes less reactive towards nucleophilic substitution reaction? Give two reasons.

CBSE 2026 · Set 56/1/1 · 3 marks

Compound 'X' with molecular formula C4H9Br reacts with aqueous KOH to give an alcohol. The rate of this reaction depends only on the concentration of the compound 'X'. When an optically active isomer 'Y' of the compound 'X' was treated with aqueous KOH solution, the rate of reaction was found to be dependent on concentration of compound 'Y' and aqueous KOH both. (a) Write down the structural formula of both 'X' and 'Y'. (b) Out of 'X' and 'Y', which one will undergo racemisation and why? (c) Out of 'X' and 'Y', which one will form product with inversion of configuration and why?

CBSE 2026 · Set 56/1/2 · 1 mark

(CH3)3C–OC2H5 on reaction with HI gives

  • (CH3)3C–I and C2H5–I
  • (CH3)3C–OH and C2H5–I
  • (CH3)3C–I and C2H5–OH
  • (CH3)3C–OH and C2H5–OH

CBSE 2026 · Set 56/1/3 · 2 marks

Why are haloarenes less reactive towards nucleophilic substitution reaction ? Give two reasons.

CBSE 2026 · Set 56/1/3 · 3 marks

Compound 'X' with molecular formula C4H9Br reacts with aqueous KOH to give an alcohol. The rate of this reaction depends only on the concentration of the compound 'X'. When an optically active isomer 'Y' of the compound 'X' was treated with aqueous KOH solution, the rate of reaction was found to be dependent on concentration of compound 'Y' and aqueous KOH both. (a) Write down the structural formula of both 'X' and 'Y'. (b) Out of 'X' and 'Y', which one will undergo racemisation and why ? (c) Out of 'X' and 'Y', which one will form product with inversion of configuration and why ?

CBSE 2026 · Set 56/2/1 · 2 marks

Draw the structure of the given compound: 4–Bromo–3–methylpent–2–ene. What happens when chloroethane is treated with aqueous potassium hydroxide?

CBSE 2026 · Set 56/2/1 · 3 marks

Write the major product in the following reactions: + Br2 (heat) → ?. Define Racemic mixture. p-dichlorobenzene has a higher melting point as compared to its ortho and meta isomers. Why?

CBSE 2026 · Set 56/2/2 · 2 marks

(a) Draw the structure of the given compound : 4–Bromo–3–methylpent–2–ene (b) What happens when chloroethane is treated with aqueous potassium hydroxide ?

CBSE 2026 · Set 56/2/2 · 3 marks

(a) Write the major product in the following reaction : + Br2 heat → ? (b) Define Chirality. (c) Why racemisation occurs in SN1 reaction ?

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Class 12 Chemistry Formula Sheet

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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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