NEET UG Chemistry · Chapter 16

Haloalkanes and Haloarenes

Preparation, SN1 and SN2 reactions, elimination, and reactions of haloarenes.

NCERT Class 12NEET: 1 Q per paper 9 min read

1. Chapter at a Glance

Haloalkanes (alkyl halides, R–X) and haloarenes (aryl halides, Ar–X) contain a halogen atom bonded to an sp³ or an sp² carbon. This chapter covers their classification, preparation, physical properties, nucleophilic substitution by the SN1 and SN2 mechanisms, elimination, stereochemistry, the low reactivity of aryl halides, and important polyhalogen compounds. NEET usually asks one question from here, very often on SN1/SN2 reactivity order or a named reaction.

What NEET asks most
  • Preparation from alcohols (SOCl₂, PCl₅, HX) and named reactions (Finkelstein, Swarts, Sandmeyer).
  • SN1 vs SN2: rate law, reactivity order and stereochemistry.
  • Elimination and Saytzeff's rule.
  • Why haloarenes are less reactive; nucleophilic substitution activated by –NO₂.
  • Wurtz–Fittig, Fittig and Grignard reactions; uses and effects of CHCl₃, CCl₄, DDT, freons.

2. Classification and Nature of the C–X Bond

  • By the carbon bearing X: 1°, 2°, 3° alkyl halides; allylic (X on carbon next to C=C); benzylic (X on carbon next to a benzene ring); vinylic (X on a C=C carbon); aryl (X on the ring).
  • The C–X bond is polar (Cδ+–Xδ−). Bond length increases and bond strength decreases from C–F to C–I, so reactivity is R–I > R–Br > R–Cl > R–F.
  • Boiling points rise with molar mass (RI > RBr > RCl > RF) and fall with branching. Haloalkanes are only slightly soluble in water.
  • Among isomeric dihalobenzenes, the para isomer has the highest melting point (most symmetrical, packs best).

3. Preparation

MethodReactionNEET Note
From alcohol + HXROH + HCl (ZnCl₂) → RCl + H₂O3° alcohols react fastest (Lucas test)
From alcohol + PCl₅ / PCl₃ROH + PCl₅ → RCl + POCl₃ + HCl—
From alcohol + SOCl₂ (Darzens)ROH + SOCl₂ → RCl + SO₂↑ + HCl↑Best: by-products are gases
Addition of HX to alkenesMarkovnikov (or peroxide with HBr)—
Finkelstein reactionRCl/RBr + NaI (dry acetone) → RI + NaCl↓NaCl precipitates, driving the reaction
Swarts reactionRBr/RCl + AgF (or Hg₂F₂, CoF₂) → RFFor alkyl fluorides
Sandmeyer reactionArN₂⁺Cl⁻ + CuCl/HCl → ArCl + N₂Aryl halides from diazonium salts
Electrophilic halogenationC₆H₆ + Cl₂ (FeCl₃) → C₆H₅ClAryl halides

4. Nucleophilic Substitution: SN1 and SN2

FeatureSN2SN1
StepsOne step (concerted)Two steps via a carbocation
Rate lawrate = k[RX][Nu⁻] (second order)rate = k[RX] (first order)
Reactivity orderCH₃X > 1° > 2° > 3° (steric hindrance)3° > 2° > 1° > CH₃X (carbocation stability)
StereochemistryComplete inversion (Walden inversion)Racemisation (partial)
Favoured byStrong nucleophile, polar aprotic solvent (acetone, DMSO)Weak nucleophile, polar protic solvent (water, alcohol)
NEET NoteBackside attack at 180° to the leaving groupAllylic and benzylic halides react fast (resonance-stabilised cation)
HO⁻ C Br HO C Br transition state HO–C+ Br⁻ inverted
SN2: the nucleophile attacks from the side opposite the leaving group. In the transition state, both are partly bonded; the other three groups flip over like an umbrella in the wind (inversion of configuration).

5. Elimination and Other Reactions

  • β-Elimination (dehydrohalogenation): RX + alcoholic KOH → alkene + HX. Substitution competes with it; aqueous KOH favours substitution (alcohol), alcoholic KOH favours elimination (alkene).
  • Saytzeff's (Zaitsev's) rule: the major product is the more substituted (more stable) alkene. 2-Bromobutane gives mainly but-2-ene.
  • With KCN → alkyl cyanide (C attacks); with AgCN → isocyanide (N attacks, covalent AgCN). Similarly, KNO₂ gives alkyl nitrite and AgNO₂ gives nitroalkane.
  • Grignard reagent: RX + Mg (dry ether) → RMgX. It reacts with water or any compound with acidic H to give alkane (RH), so it must be kept dry.
  • Wurtz reaction: 2RX + 2Na → R–R. Wurtz–Fittig: ArX + RX + 2Na → Ar–R. Fittig: 2ArX + 2Na → Ar–Ar.

6. Haloarenes: Low Reactivity and Reactions

  • Aryl halides are much less reactive towards nucleophilic substitution because (i) resonance gives the C–X bond partial double-bond character, (ii) the carbon is sp² (bond is shorter and stronger), (iii) the phenyl cation is unstable, and (iv) the electron-rich ring repels the nucleophile.
  • Chlorobenzene gives phenol only under harsh conditions (NaOH, 623 K, 300 atm — the Dow process).
  • Electron-withdrawing groups (–NO₂) at ortho or para positions make substitution much easier: 2,4,6-trinitrochlorobenzene hydrolyses in warm water.
  • Haloarenes undergo electrophilic substitution at ortho and para positions, but more slowly than benzene (halogens are deactivating).

7. Polyhalogen Compounds

CompoundUseHazard / NEET Note
DichloromethaneSolvent, paint removerHarms the nervous system
Chloroform (CHCl₃)Solvent; formerly an anaestheticOxidised by air and light to poisonous phosgene (COCl₂); stored in dark bottles with 1% ethanol
Iodoform (CHI₃)Antiseptic (releases I₂)Yellow solid; forms in the iodoform test
Carbon tetrachlorideSolvent, refrigerant manufactureDamages liver; depletes ozone
Freons (CFCs)Refrigerants, aerosol propellantsDestroy stratospheric ozone
DDTInsecticide (malaria control)Non-biodegradable; accumulates in food chains (biomagnification)

8. Formula and Reaction Master Table

Rule / reactionDetails
Reactivity of RXRI > RBr > RCl > RF
SN2 orderCH₃X > 1° > 2° > 3°
SN1 order3° > 2° > 1° > CH₃X (benzylic, allylic fast)
FinkelsteinRX + NaI → RI + NaX (acetone)
SwartsRX + AgF → RF + AgX
GrignardRX + Mg → RMgX (dry ether)
Wurtz–FittigArX + RX + 2Na → ArR + 2NaX
Cyanide vs isocyanideKCN → RCN ; AgCN → RNC
SaytzeffMore substituted alkene is major

9. Master Summary Table

ConceptKey ideaNEET Note
C–X bondPolar; weakest for C–IRI most reactive
SOCl₂ methodGaseous by-productsPure alkyl chloride
SN2InversionSteric hindrance matters
SN1RacemisationCarbocation stability matters
Alc. KOHEliminationAq. KOH gives alcohol
Aryl halidesUnreactive to Nu⁻–NO₂ at o/p activates
PolyhalogensUseful but harmfulCHCl₃ → phosgene

10. Mind Map

Haloalkanes and Haloarenes
Preparation
  • From alcohols
  • Finkelstein, Swarts
  • Sandmeyer
Substitution
  • SN1
  • SN2
  • Stereochemistry
Other reactions
  • Elimination
  • Grignard
  • Wurtz, Fittig
Aryl and poly
  • Low reactivity
  • CHCl₃, CHI₃
  • DDT, freons

11. Tips and Tricks

Shortcuts
  • "Primary loves SN2, tertiary loves SN1."
  • Silver salts (AgCN, AgNO₂) are covalent, so the attack comes through nitrogen: AgCN gives isocyanides and AgNO₂ gives nitroalkanes. Ionic KCN and KNO₂ give cyanides and alkyl nitrites.
  • Aqueous KOH → alcohol; alcoholic KOH → alkene.
  • If a question asks for a reaction "without rearrangement", SN2 is implied.
Common NEET traps
  • Giving the SN2 reactivity order the same as SN1.
  • Expecting chlorobenzene to hydrolyse like chloroethane.
  • Calling halogens meta-directing in haloarenes.
  • Forgetting that Grignard reagents are destroyed by water and alcohols.

12. Important One-liners

  • Thionyl chloride is the best reagent for converting alcohols into alkyl chlorides.
  • SN2 reactions proceed with inversion of configuration.
  • Tertiary butyl bromide reacts by SN1.
  • Benzyl chloride is much more reactive than chlorobenzene.
  • Chloroform is stored in dark coloured bottles filled to the brim.
  • Iodoform is used as an antiseptic.
  • DDT was the first chlorinated organic insecticide.

13. NEET-pattern Questions with Detailed Answers

These questions are original and based on concepts that NEET UG asks again and again.

Q1Easy

The best reagent to convert ethanol into chloroethane is:

  1. Cl₂
  2. SOCl₂
  3. NaCl
  4. HCl alone
Show solution

SOCl₂ gives SO₂ and HCl as gases, which escape and leave pure chloroethane behind. HCl alone reacts slowly with a primary alcohol (it needs ZnCl₂). Cl₂ and NaCl do not convert an alcohol into a chloride.

Answer: (B)

Q2Easy

An SN2 reaction at a chiral carbon gives:

  1. Racemisation
  2. Retention of configuration
  3. Inversion of configuration
  4. No change
Show solution

The nucleophile attacks from the back, so the configuration turns inside out, like an umbrella in the wind (Walden inversion). Racemisation happens in SN1 reactions.

Answer: (C)

Q3Medium

Which reacts fastest by the SN1 mechanism?

  1. CH₃Cl
  2. CH₃CH₂Cl
  3. (CH₃)₂CHCl
  4. (CH₃)₃CCl
Show solution

SN1 depends on how stable the carbocation is. The tertiary one is the most stable, so (CH₃)₃CCl reacts fastest. CH₃Cl would be fastest in SN2.

Answer: (D)

Q4Medium

2-Bromobutane is heated with alcoholic KOH. The major product is:

  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 alkene, but-2-ene, is the major product. Butan-2-ol would form with aqueous KOH.

Answer: (B)

Q5Medium

Ethyl bromide reacts with AgCN to give mainly:

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

AgCN is largely covalent (Ag–C bond), so the nitrogen lone pair attacks the carbon. This gives the isocyanide, C₂H₅NC. Ionic KCN would give the cyanide C₂H₅CN instead.

Answer: (B)

Q6Medium

The Finkelstein reaction is used to prepare:

  1. Alkyl fluorides
  2. Alkyl iodides
  3. Aryl chlorides
  4. Alkanes
Show solution

RCl + NaI in dry acetone gives RI. NaCl is insoluble in acetone and precipitates, which drives the reaction forward. Alkyl fluorides come from the Swarts reaction, and aryl chlorides from the Sandmeyer reaction.

Answer: (B)

Q7Hard

Which undergoes nucleophilic substitution most easily?

  1. Chlorobenzene
  2. 4-Nitrochlorobenzene
  3. 2,4-Dinitrochlorobenzene
  4. 2,4,6-Trinitrochlorobenzene
Show solution

–NO₂ groups at the ortho and para positions stabilise the negative intermediate by resonance. The more such groups there are, the easier the substitution. With three of them, the compound hydrolyses in warm water.

Answer: (D)

Q8Hard

Chloroform on exposure to air and light forms a poisonous gas. The gas and the method of preventing it are:

  1. Cl₂; storing in plastic
  2. COCl₂; storing in dark bottles with a little ethanol
  3. CO; adding water
  4. HCl; adding NaOH
Show solution

2CHCl₃ + O₂ → 2COCl₂ + 2HCl. The product, phosgene, is highly toxic. Dark bottles filled to the brim keep out light and air, and about 1% ethanol converts any phosgene formed into a harmless ester.

Answer: (B)

14. Quick Revision Summary

Remember before the exam
  • RI > RBr > RCl > RF; SOCl₂ best; Finkelstein (RI), Swarts (RF), Sandmeyer (ArX).
  • SN2: one step, inversion, CH₃ > 1° > 2° > 3°.
  • SN1: carbocation, racemisation, 3° > 2° > 1°.
  • Alc. KOH elimination (Saytzeff); KCN → RCN, AgCN → RNC; Grignard needs dry ether.
  • Aryl halides unreactive unless o/p –NO₂; CHCl₃ → phosgene; DDT persistent.
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