CBSE Class 12 Haloalkanes and Haloarenes PYQs Analysis (2021-2025)

Based on detailed analysis of last 5 years' papers. Perfect for 2026 Boards prep!

Key Trends

Weightage: 6 marks – Focus on mechanisms (SN1/SN2) and elimination (E1/E2)
Repeated Topics: SN1 vs SN2 (rate, stereochemistry), factors affecting substitution, E1/E2 mechanisms, Saytzeff vs Hofmann rule, stereochemistry (inversion/racemization)
Difficulty: Medium – Mechanism diagrams and reasoning are common

Most Important and Repeated Questions

Question Example	Type/Marks	Years Repeated	Notes
Compare SN1 and SN2 mechanisms. Give rate law, stereochemistry, and example for each.	Short Answer (3 marks)	2021 Term 2, 2022, 2023, 2024, 2025	Repeated 5x; SN2: inversion, bimolecular; SN1: racemization, unimolecular.
Explain factors affecting SN1 vs SN2: nature of substrate, nucleophile strength, solvent, leaving group.	Reasoning (3 marks)	2022, 2023, 2024, 2025	Repeated 4x; Polar protic favors SN1, aprotic favors SN2.
Draw mechanism of E2 elimination. Why is anti-periplanar geometry preferred?	Mechanism (3 marks)	2021 Term 1, 2022, 2023, 2025	Repeated 4x; Concerted, one-step, anti-periplanar for stereospecificity.
Assertion: SN2 reaction shows inversion of configuration. Reason: Backside attack.	Assertion-Reason (1 mark)	2023, 2024, 2025	Repeated 3x; Both true, reason explains.
Explain Saytzeff vs Hofmann rule in elimination reactions with examples.	Reasoning (2 marks)	2022, 2023, 2024	Repeated 3x; Saytzeff: more substituted alkene (E2); Hofmann: less substituted (bulky base).
Convert 1-bromopropane to propene (elimination) and to propan-1-ol (substitution).	Conversion (2 marks)	2021 Term 2, 2023, 2025	Repeated 3x; Alcoholic KOH → propene; aq KOH → propan-1-ol.
Why does SN1 reaction lead to racemization?	Reasoning (2 marks)	2023, 2024	Repeated 2x; Planar carbocation intermediate allows attack from both sides.
MCQ: Which undergoes SN1 fastest? (a) 1° alkyl halide (b) 3° alkyl halide (c) aryl halide	MCQ (1 mark)	2024, 2025	Repeated 2x; (b) 3° alkyl halide.
Case-based: Stereochemistry in SN2 and E2 reactions. Draw transition state.	Case-Based (4 marks)	2022, 2025	Repeated 2x; SN2: backside attack; E2: anti-periplanar.
Explain Finkelstein reaction and its use.	Short Answer (2 marks)	2021 Term 1, 2023	Repeated 2x; RCl + NaI (acetone) → RI + NaCl (for iodoalkanes).

Best Format for Preparation and Answering

Study Format: Draw mechanisms (curved arrows), tables for SN1 vs SN2 comparison.
Answer Format: Mechanism: Step-by-step with intermediate/transition state. Stereochemistry: Mention inversion/racemization.
Tips: Practice drawing SN1/SN2 and E2 diagrams. Time: 8-10 mins.

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