Class 12 — Haloalkanes and Haloarenes

Chapter Overview & Weightage

Haloalkanes and Haloarenes is a high-weightage organic chapter in CBSE Class 12, contributing $5$ – $8$ marks across the board paper. Reactions, mechanisms (SN1, SN2, E1, E2), and reagent recognition dominate. Students who can sketch the mechanism and predict the product reliably score full marks.

Year	Marks	Pattern
2024	7	SN1 vs SN2 mechanism + named reaction
2023	5	Reaction sequence with reagents
2022	6	Stereochemistry of SN2
2021	5	Aryl halide reactivity comparison
2020	5	Wurtz reaction + Williamson

CBSE loves three reactions: Wurtz, Sandmeyer, and Finkelstein. Drill these three plus their mechanisms.

Key Concepts You Must Know

Nomenclature — IUPAC vs common names for primary/secondary/tertiary haloalkanes.
SN1 vs SN2 — mechanism, kinetics, stereochemistry, factors affecting rate.
E1 vs E2 — competition with substitution.
Reactivity order — primary > secondary > tertiary for SN2; reverse for SN1.
Aryl halides — low reactivity, mesomeric effect, partial double-bond character.
Named reactions — Wurtz, Wurtz-Fittig, Fittig, Sandmeyer, Finkelstein, Swarts.
Polyhalogen compounds — $\text{CHCl}_3$ , $\text{CCl}_4$ , $\text{DDT}$ , freons.

Important Formulas / Reactions

$\text{Nu}^- + \text{R-X} \to [\text{Nu}\cdots\text{R}\cdots\text{X}]^{\ddagger} \to \text{Nu-R} + \text{X}^-$

Single step, bimolecular, inversion of configuration (Walden inversion).

When to use: primary haloalkanes with strong nucleophiles in polar aprotic solvents.

Step 1: $\text{R-X} \to \text{R}^+ + \text{X}^-$ (slow, rate-determining). Step 2: $\text{R}^+ + \text{Nu}^- \to \text{Nu-R}$ (fast).

Two-step, unimolecular, racemisation, carbocation intermediate.

When to use: tertiary haloalkanes, polar protic solvents.

$\text{ArN}_2^+\text{X}^- + \text{CuX} \to \text{Ar-X} + \text{N}_2 + \text{Cu}^+$

Converts aromatic diazonium salts to aryl halides ( $X$ = Cl, Br, CN).

Solved Previous Year Questions

PYQ 1 (CBSE 2024, 7 marks)

Compare SN1 and SN2 mechanisms with respect to (a) molecularity, (b) stereochemistry, (c) substrate effect, (d) solvent. Give one example of each.

Solution.

Feature	SN1	SN2
Molecularity	Unimolecular	Bimolecular
Stereochemistry	Racemisation	Inversion
Substrate	Tertiary > Secondary > Primary	Primary > Secondary > Tertiary
Solvent	Polar protic (water, alcohol)	Polar aprotic (DMSO, DMF, acetone)

Example SN1: $(\text{CH}_3)_3\text{C-Br} + \text{H}_2\text{O} \to (\text{CH}_3)_3\text{C-OH} + \text{HBr}$ .

Example SN2: $\text{CH}_3\text{Br} + \text{OH}^- \to \text{CH}_3\text{OH} + \text{Br}^-$ .

PYQ 2 (CBSE 2022, 6 marks)

Why is haloarene less reactive than haloalkane towards nucleophilic substitution? Give two reasons.

Solution. (a) Resonance effect — lone pair on halogen overlaps with the aromatic ring, giving partial double-bond character to C-X bond and shortening it. Bond is harder to break.

(b) Hybridisation effect — carbon attached to halogen is $sp^2$ in haloarenes (more electronegative, holds bond tighter) vs $sp^3$ in haloalkanes.

PYQ 3 (CBSE 2021, 5 marks)

Arrange in order of increasing reactivity towards SN2: $\text{CH}_3\text{Cl}, (\text{CH}_3)_2\text{CHCl}, (\text{CH}_3)_3\text{CCl}, \text{CH}_3\text{CH}_2\text{Cl}$ .

Solution. $(\text{CH}_3)_3\text{CCl} < (\text{CH}_3)_2\text{CHCl} < \text{CH}_3\text{CH}_2\text{Cl} < \text{CH}_3\text{Cl}$ .

Reasoning: SN2 needs back-side attack. Bulky tert-butyl group blocks the nucleophile completely. Methyl chloride has minimal steric hindrance.

Difficulty Distribution

Easy ( $\sim 25\%$ ): nomenclature, identifying primary/secondary/tertiary.
Medium ( $\sim 50\%$ ): reaction prediction, comparing SN1 vs SN2, named reactions.
Hard ( $\sim 25\%$ ): stereochemistry of mechanisms, multi-step synthesis sequences.

Expert Strategy

For mechanism questions, always draw the curved arrows and label the rate-determining step. Examiners give partial credit for arrows even if the product is wrong.

The five-step routine:

Memorise the SN1/SN2/E1/E2 comparison table cold.
Drill all six named reactions (Wurtz, Wurtz-Fittig, Fittig, Sandmeyer, Finkelstein, Swarts).
Practice five problems on substrate-mechanism prediction.
Learn the resonance reasoning for haloarene unreactivity.
Revise polyhalogen uses (CHCl3 anaesthetic, CCl4 fire extinguisher, etc.).

Common Traps

Trap 1: Confusing SN1 with E1. Both have carbocation intermediate; SN1 gives substitution, E1 gives elimination. Strong base favours E1 over SN1.

Trap 2: Forgetting Walden inversion in SN2. Configuration always inverts.

Trap 3: Predicting SN2 on tertiary substrate. Steric hindrance kills SN2; tertiary goes SN1 or E1.

Trap 4: Confusing Wurtz with Wurtz-Fittig. Wurtz: 2 alkyl halides $\to$ alkane. Wurtz-Fittig: alkyl + aryl halide $\to$ arene with alkyl group.

Trap 5: Writing diazonium without temperature. Sandmeyer needs the diazonium below $5°\text{C}$ to prevent decomposition.

Quick Revision Notes

SN1 favours: tertiary substrate, polar protic solvent, weak nucleophile, stable carbocation.
SN2 favours: primary substrate, polar aprotic solvent, strong nucleophile, no steric crowding.
Haloarenes are unreactive in SN due to resonance and $sp^2$ carbon.
Vinyl halides are also unreactive (similar resonance argument).
Allyl and benzyl halides are more reactive than expected because the resulting carbocation is resonance-stabilised.

A high-scoring chapter — drill the mechanisms and named reactions, and you walk away with $7$ + marks in the boards.