JEE Main Chemistry · Chapter 15

Haloalkanes and Haloarenes

SN1 and SN2 mechanisms, stereochemistry, elimination, named reactions and the low reactivity of aryl halides.

NCERT Class 12JEE Main: 1 Q per shift 7 min read

1. Chapter at a Glance

Haloalkanes (alkyl halides, R–X) have a halogen bonded to an sp³ carbon; haloarenes (aryl halides, Ar–X) have it bonded to an sp² carbon of a benzene ring. The C–X bond is polar, so the carbon is slightly positive and is attacked by nucleophiles. This chapter is really about reaction mechanisms: SN1, SN2 and elimination. JEE Main often asks which substrate reacts faster, what the stereochemistry of the product is, and why aryl halides resist substitution.

What JEE asks most
  • Comparing SN1 and SN2 rates for different alkyl halides.
  • Stereochemistry: inversion (SN2) and racemisation (SN1).
  • Elimination with alcoholic KOH and Saytzeff's rule.
  • Low reactivity of haloarenes; named reactions (Finkelstein, Swarts, Wurtz–Fittig).

2. Preparation

  • From alcohols: R–OH + HX (ZnCl₂ for HCl), PCl₃, PCl₅ or SOCl₂. Thionyl chloride is best because the by-products SO₂ and HCl are gases.
  • From alkenes: addition of HX (Markovnikov, or anti-Markovnikov for HBr with peroxide).
  • Halogen exchange: Finkelstein — R–Cl/Br + NaI in dry acetone → R–I (NaCl/NaBr precipitates and drives the reaction). Swarts — R–Br + AgF (or Hg₂F₂, CoF₂) → R–F.
  • Haloarenes: electrophilic halogenation of benzene (Cl₂/FeCl₃), or from benzenediazonium salts (Sandmeyer: CuCl/HCl or CuBr/HBr; Gattermann: Cu powder).

3. Physical Properties

  • Boiling point increases with molecular mass: RI > RBr > RCl > RF; and decreases with branching.
  • Density: iodides, bromides and polyhalides are denser than water.
  • Para-dihalobenzene melts higher than ortho and meta because it packs better in the crystal.
  • Bond strength: C–F > C–Cl > C–Br > C–I, so reactivity towards substitution is R–I > R–Br > R–Cl > R–F (I⁻ is the best leaving group).

4. Nucleophilic Substitution: SN2 and SN1

FeatureSN2SN1
StepsOne step, concerted (backside attack)Two steps via a carbocation
Rate lawRate = k[RX][Nu⁻] (second order)Rate = k[RX] (first order)
Substrate orderCH₃X > 1° > 2° > 3°3° > 2° > 1° > CH₃X (benzyl, allyl very fast)
StereochemistryComplete inversion (Walden inversion)Racemisation (with some inversion)
Favoured byStrong nucleophile, polar aprotic solvent (acetone, DMSO)Weak nucleophile, polar protic solvent (water, alcohol)
RearrangementNoPossible (carbocation)
TS 1 (slow)TS 2R⁺ intermediate R–XR–Nu reaction progressE
SN1: the first step (ionisation to a carbocation) has the higher barrier and controls the rate. SN2 shows a single hump.

Ambident nucleophiles

  • KCN (ionic) attacks through C → alkyl cyanide (nitrile) R–CN. AgCN (covalent) attacks through N → isocyanide R–NC.
  • KNO₂ attacks through O → alkyl nitrite R–ONO. AgNO₂ attacks through N → nitroalkane R–NO₂.

5. Elimination and Other Reactions

  • β-Elimination (E2): alcoholic KOH removes HX. Saytzeff's rule: the more substituted alkene is the major product. 2-Bromobutane → but-2-ene (major) + but-1-ene.
  • Substitution vs elimination: aqueous KOH favours substitution (alcohol); alcoholic KOH and higher temperature favour elimination. 3° halides mostly eliminate.
  • Wurtz: 2RX + 2Na → R–R. Wurtz–Fittig: ArX + RX + 2Na → Ar–R. Fittig: 2ArX + 2Na → Ar–Ar.
  • Grignard reagent: R–X + Mg (dry ether) → RMgX. It reacts with any compound having acidic H (water, alcohol) to give an alkane, so it must be kept dry.

6. Haloarenes

Aryl halides are much less reactive towards nucleophilic substitution than alkyl halides because:

  1. Resonance gives the C–X bond partial double-bond character.
  2. The carbon is sp² (more electronegative), so the C–X bond is shorter and stronger.
  3. A phenyl cation is very unstable, so SN1 is not possible.
  4. The electron-rich ring repels an approaching nucleophile.
  • Chlorobenzene needs 623 K and 300 atm with NaOH to give phenol (Dow process). Electron-withdrawing –NO₂ groups at ortho/para positions make substitution much easier: 2,4,6-trinitrochlorobenzene reacts with warm water.
  • Halogens are ortho/para directing but deactivating in electrophilic substitution.

Polyhalogen compounds

  • Chloroform (CHCl₃): slowly oxidised by air to poisonous phosgene (COCl₂); stored in dark bottles with a little ethanol.
  • Iodoform (CHI₃): yellow antiseptic.
  • CCl₄: solvent; damages the ozone layer. Freons (CFCs): refrigerants that deplete ozone.
  • DDT: insecticide; not biodegradable, accumulates in food chains.

7. Master Table

Reaction / ruleSummary
FinkelsteinRCl + NaI (acetone) → RI + NaCl↓
SwartsRBr + AgF → RF + AgBr
SN2 ratek[RX][Nu⁻]; CH₃ > 1° > 2° > 3°
SN1 ratek[RX]; 3° > 2° > 1°
Leaving group abilityI⁻ > Br⁻ > Cl⁻ > F⁻
Eliminationalc. KOH, Saytzeff (more substituted alkene)
KCN / AgCNR–CN / R–NC

8. Mind Map

Haloalkanes & Haloarenes
Preparation
  • From alcohols (SOCl₂)
  • Finkelstein, Swarts
  • Sandmeyer
Substitution
  • SN2: inversion
  • SN1: racemisation
  • Ambident nucleophiles
Elimination & metals
  • Saytzeff
  • Wurtz, Fittig
  • Grignard
Haloarenes
  • Low reactivity
  • o/p directing
  • CHCl₃, DDT, freons

9. Tips and Tricks

Shortcuts
  • "Crowded carbon → SN1; open carbon → SN2." Benzyl and allyl halides are fast in both.
  • If the question mentions optical activity of the product, think about the mechanism: SN2 keeps activity (inverted); SN1 loses most of it.
  • Silver salts (AgCN, AgNO₂) are covalent, so the more electronegative atom is not free to attack — the other end attacks.
  • Neopentyl bromide is very slow in SN2 despite being primary, because of the bulky t-butyl group next door.
Mistakes students make
  • Mixing up aqueous KOH (substitution) and alcoholic KOH (elimination).
  • Assigning R/S wrongly after inversion — inversion of configuration does not always flip the R/S label if priorities change.
  • Thinking chlorobenzene undergoes SN1 like benzyl chloride.

10. Important One-liners

  • Thionyl chloride gives pure alkyl chloride because its by-products are gases.
  • SN2 reactions proceed with Walden inversion.
  • Tertiary butyl bromide reacts by SN1 in water.
  • Grignard reagents are destroyed by traces of water.
  • Chloroform is stored in dark bottles to prevent phosgene formation.
  • –NO₂ groups at ortho/para positions activate haloarenes towards nucleophilic substitution.

11. PYQ-pattern Questions with Solutions

These questions are original and modelled on concepts frequently tested in JEE Main.

Q1Easy

Which alkyl halide reacts fastest by the SN2 mechanism?

  1. (CH₃)₃CBr
  2. (CH₃)₂CHBr
  3. CH₃CH₂Br
  4. CH₃Br
Show solution

SN2 needs backside attack; the least hindered carbon reacts fastest. CH₃Br has no alkyl groups blocking the path.

Answer: (D)

Q2Medium

(R)-2-Bromobutane reacts with aqueous NaOH by the SN2 path. The product is:

  1. (R)-Butan-2-ol
  2. (S)-Butan-2-ol
  3. Racemic butan-2-ol
  4. But-2-ene
Show solution

SN2 inverts the configuration. OH replaces Br and takes the same priority rank (highest), so the label changes from R to S.

Answer: (B)

Q3Medium

2-Bromobutane heated with alcoholic KOH gives mainly:

  1. But-1-ene
  2. But-2-ene
  3. Butan-2-ol
  4. Butane
Show solution

Alcoholic KOH causes elimination. By Saytzeff's rule, the more substituted (more stable) alkene, but-2-ene, is the major product.

Answer: (B)

Q4Easy

Ethyl bromide reacts with AgCN to give mainly:

  1. Ethyl cyanide
  2. Ethyl isocyanide
  3. Ethanol
  4. Ethene
Show solution

AgCN is largely covalent (Ag–C bond), so the nitrogen lone pair attacks, forming C₂H₅–NC (ethyl isocyanide). KCN would give the nitrile.

Answer: (B)

Q5Hard

Chlorobenzene is less reactive than chloroethane towards nucleophilic substitution. Which reason is NOT correct?

  1. Partial double-bond character of C–Cl due to resonance
  2. sp² hybridised carbon holds the bond more tightly
  3. Chlorine is more electronegative in chlorobenzene
  4. Instability of the phenyl cation
Show solution

The electronegativity of chlorine is the same in both compounds. The real reasons are resonance, sp² carbon and the unstable phenyl cation (and repulsion by the π cloud).

Answer: (C)

Q6Hard

The rate of SN1 hydrolysis is highest for:

  1. CH₃CH₂Cl
  2. (CH₃)₂CHCl
  3. C₆H₅CH₂Cl
  4. C₆H₅Cl
Show solution

SN1 depends on carbocation stability. The benzyl cation (C₆H₅CH₂⁺) is stabilised by resonance with the ring, more than a 2° cation. Chlorobenzene does not ionise.

Answer: (C)

Q7Numerical

How many structural isomers are possible for C₄H₉Br?

Show solution

1-Bromobutane, 2-bromobutane, 1-bromo-2-methylpropane and 2-bromo-2-methylpropane. Total 4 (only 2-bromobutane is chiral).

Answer: 4

Q8Medium

Which reaction converts CH₃CH₂Br into CH₃CH₂I in good yield?

  1. Swarts reaction
  2. Finkelstein reaction
  3. Wurtz reaction
  4. Sandmeyer reaction
Show solution

Finkelstein: NaI in dry acetone. NaBr is insoluble in acetone and precipitates, pushing the equilibrium forward. Swarts makes fluorides; Wurtz couples alkyl groups; Sandmeyer works on diazonium salts.

Answer: (B)

12. Quick Revision Summary

Remember before the exam
  • Reactivity of R–X: RI > RBr > RCl > RF.
  • SN2: one step, inversion, CH₃ > 1° > 2° > 3°. SN1: carbocation, racemisation, 3° > 2° > 1°.
  • Aq. KOH → alcohol; alc. KOH → alkene (Saytzeff).
  • KCN → nitrile; AgCN → isocyanide; KNO₂ → nitrite; AgNO₂ → nitroalkane.
  • Haloarenes: resonance + sp² carbon make them unreactive; o/p-NO₂ activates them.
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