Quick Summary
Haloalkanes (alkyl halides) and haloarenes are compounds where one or more hydrogen atoms of a hydrocarbon are replaced by halogen atoms. The C−X bond is polar (C⁺−X⁻) — this polarity makes these compounds reactive toward nucleophiles and bases. The SN1 and SN2 mechanisms are among the most tested organic chemistry concepts in NEET.
Classification
| Class | Example | Type |
|---|---|---|
| Haloalkane (alkyl halide) | CH₃Cl, CH₃CH₂Br | Aliphatic, C−X single bond |
| Vinyl halide | CH₂=CHCl | C−X on sp² carbon, C=C |
| Allyl halide | CH₂=CHCH₂Cl | C−X adjacent to C=C |
| Aryl halide (haloarene) | C₆H₅Cl | C−X on aromatic ring |
| Benzyl halide | C₆H₅CH₂Cl | C−X on CH₂ adjacent to ring |
Types by number of halogens: Mono- (1X), di- (2X), tri- (3X), tetra- (4X).
Preparation of Haloalkanes
| Method | Reaction |
|---|---|
| From alkane (free radical) | R−H + Cl₂ → R−Cl + HCl (light) |
| From alkene (addition) | R−CH=CH₂ + HX → R−CHX−CH₃ (Markovnikov) |
| From alcohol (with HX) | R−OH + HX → R−X + H₂O |
| From alcohol (with PCl₃/PCl₅) | 3R−OH + PCl₃ → 3R−Cl + H₃PO₃ |
| Halogen exchange — Finkelstein | R−Cl + NaI → R−I + NaCl (acetone) |
| Halogen exchange — Swarts | R−Cl + AgF → R−F + AgCl |
Nucleophilic Substitution Reactions
SN2 (Bimolecular Nucleophilic Substitution)
- One step, concerted
- Backside attack by nucleophile
- Inversion of configuration (Walden inversion)
- Rate = k[substrate][nucleophile]
- Favoured by: primary substrates, strong nucleophile, polar aprotic solvent (acetone, DMSO)
- Order: methyl > 1° > 2° > 3°
SN1 (Unimolecular Nucleophilic Substitution)
- Two steps: ionisation → nucleophilic attack
- Intermediate: planar carbocation
- Racemisation (attack from both sides)
- Rate = k[substrate] only
- Favoured by: tertiary substrates, weak nucleophile, polar protic solvent (water, ethanol)
- Order: 3° > 2° > 1°
Elimination Reactions
Dehydrohalogenation: R−CHX−CH₂ + KOH (alc.) → R−CH=CH₂ + KX + H₂O
Saytzeff's rule: The major alkene product has the more substituted double bond (more stable).
Competition: SN2 favoured by concentrated nucleophile at low temperature; elimination favoured at high temperature with strong base.
Haloarenes — Reactions
Haloarenes are less reactive toward nucleophilic substitution (resonance reduces C−X bond polarity).
| Reaction | Conditions | Product |
|---|---|---|
| Electrophilic substitution | Normal EAS conditions | Ring-substituted products |
| Nucleophilic substitution | Requires drastic conditions (high T, pressure) or activating groups (−NO₂) | — |
| Reaction with Mg (Grignard) | Dry ether | Aryl Grignard reagent |
| Wurtz−Fittig reaction | Na, dry ether, alkyl halide + aryl halide | Alkyl benzene |
Directive influence of halogens: Halogens are ortho/para directors (due to +M effect) but are ring-deactivating (−I effect stronger than +M).
Worked Examples
Example 1: What is the product of (CH₃)₃CBr + NaOH (aq)? SN1 (tertiary, weak nucleophile, protic solvent): (CH₃)₃COH (tert-butanol). Racemisation if chiral centre present.
Example 2: Write the Finkelstein reaction of CH₃CH₂Cl. CH₃CH₂Cl + NaI (acetone) → CH₃CH₂I + NaCl↓
Example 3: Why does CH₃Br react faster with CN⁻ in DMSO than in water? SN2 reaction — DMSO is a polar aprotic solvent that does not solvate nucleophiles strongly, keeping CN⁻ "naked" and more reactive.
Important Questions
One Mark
- Define a primary haloalkane. Give one example.
- State Saytzeff's rule.
- What is Walden inversion?
- Which solvent favours SN2 reactions?
Short Answer (3 marks)
- Explain why haloarenes are less reactive than haloalkanes towards nucleophilic substitution.
- Write the mechanism of SN2 reaction of CH₃Br with OH⁻. What is the stereochemical outcome?
- How do you prepare iodoethane from bromoethane? Write the reaction and name it.
Long Answer (5 marks)
- Compare SN1 and SN2 mechanisms with reference to (a) mechanism (b) stereochemistry (c) substrate preference (d) solvent preference (e) rate equation.
- Explain the electrophilic aromatic substitution of chlorobenzene. Why does −Cl direct ortho/para? Write the product of nitration of chlorobenzene.
Common Mistakes
- SN2 stereochemistry: forgetting that SN2 gives inversion (NOT retention) of configuration.
- SN1 stereochemistry: expecting pure inversion — SN1 gives racemisation (carbocation is planar, attack from both sides).
- Saytzeff's rule: applying it to SN instead of elimination — it applies to elimination products only.
- Haloarenes: assuming they are as reactive as haloalkanes toward nucleophiles — they are much less reactive due to resonance.
Board Exam Tips
- SN1 vs SN2 comparison table is a very common 5-mark question — prepare all five points.
- Mechanism of SN2 (with stereochemistry diagram) is a 3-mark standard question.
- Directive influence of halogens in electrophilic substitution is a 2–3 mark question.
- NEET: Reactivity order of alkyl halides, nucleophilicity vs basicity, and stereochemistry are tested regularly.