Haloalkanes and Haloarenes

Class 12

9 previous-year questions from this chapter every option and the correct answer, free · where the marks are

1. Nucleophilic substitution: SN1 and SN2

Exam focus: SN2 is a single backside attack that gives complete inversion of configuration (Walden inversion) at the stereocentre; SN1 goes through a planar carbocation attacked from either face, giving near-racemisation instead — "inversion versus racemisation" is the fastest way to tell the two mechanisms apart in a question stem. SN1 favours 3° substrates and polar protic solvents that stabilise the carbocation; SN2 favours 1° substrates and is blocked by bulky groups at the reacting carbon.

Haloalkanes and Haloarenes — Full Chapter (incl. SN1/SN2)

Lakshya NEET

Full-chapter lecture that covers SN1 and SN2 mechanisms as part of the whole chapter, not an isolated SN1/SN2-only video.

ChemistryNEET 2025show ▾

The major product of the reaction shown in the figure is: Question 78 as printed in the NEET 2025 booklet

A.Structure (1)
B.Structure (2)✓
C.Structure (3)
D.Structure (4)

Solution

Excess $C H_{3} MgBr$ attacks both electrophilic sites: the ketone becomes a tertiary alcohol (–C(OH)(CH $_{3})$ Ph), and the nitrile adds one equivalent to form an imine salt that hydrolyses to a methyl ketone on work-up. Product (2): the hydroxy-ketone.

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ChemistryNEET 2025show ▾

Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A): The first compound shown undergoes the

S_{N} 2

reaction faster than the second compound shown. Reason (R): Iodine is a better leaving group because of its large size. In the light of the above statements, choose the correct answer from the options given below: Question 84 as printed in the NEET 2025 booklet

Question 84 as printed in the NEET 2025 booklet

A.Both A and R are true and R is the correct explanation of A✓
B.Both A and R are true but R is not the correct explanation of A
C.A is true but R is false
D.A is false but R is true

Solution

Iodide is the best leaving group among the halides — the large, polarisable ion spreads its charge and is a weak base — so 1-iodobutane reacts faster than 1-chlorobutane in $S_{N} 2$ . Both statements are true and R explains A.

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ChemistryNEET 2024show ▾

The compound (from the four structures shown) that will undergo the

S_{N} 1

reaction with the fastest rate is: Question 75 as printed in the NEET 2024 booklet

A.Structure (1)
B.Structure (2)
C.Structure (3)✓
D.Structure (4)

Solution

$S_{N} 1$ rate depends on carbocation stability. 1-Bromo-1-phenylethane (3) ionises to a secondary benzylic cation stabilised by resonance with the ring — far more stable than the secondary cyclohexyl (1) or the primary (4) cations; bromobenzene (2) does not undergo $S_{N} 1$ at all.

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ChemistryNEET 2023show ▾

Consider the reaction shown in the figure and identify the product (P). Question 73 as printed in the NEET 2023 booklet

A.Structure (1)✓
B. $C H_{3} CH = CH - C H_{3}$
C.Structure (3)
D.Structure (4)

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ChemistryNEET 2021show ▾

The major product formed in the dehydrohalogenation reaction of 2-bromopentane is pent-2-ene. This product formation is based on:

A.Saytzeff's rule✓
B.Hund's rule
C.Hofmann rule
D.Hückel's rule

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ChemistryNEET 2021show ▾

The correct sequence of bond enthalpy of the C–X bond is:

A. $C H_{3} - F < C H_{3} - Cl < C H_{3} - Br < C H_{3} - I$
B. $C H_{3} - F > C H_{3} - Cl > C H_{3} - Br > C H_{3} - I$ ✓
C. $C H_{3} - F < C H_{3} - Cl > C H_{3} - Br > C H_{3} - I$
D. $C H_{3} - Cl > C H_{3} - F > C H_{3} - Br > C H_{3} - I$

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ChemistryNEET 2020show ▾

The elimination reaction of 2-bromopentane to form pent-2-ene is:

(a) A β -

elimination reaction (b) In accordance with Zaitsev's rule (c) A dehydrohalogenation reaction (d) A dehydration reaction

A.(a), (b), (c)✓
B.(a), (c), (d)
C.(b), (c), (d)
D.(a), (b), (d)

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2. Haloarenes: reactions and directing effects

Exam focus: Aryl C–X bonds are shorter and stronger than alkyl C–X bonds, because the halogen's lone pair conjugates into the ring, which is exactly why chlorobenzene resists the SN1/SN2 chemistry that works readily on an alkyl halide. Nucleophilic substitution on a haloarene needs either forcing conditions (high temperature and pressure with NaOH) or a strong electron-withdrawing group like NO₂ at the ortho/para position to make the ring susceptible.

Chemical Reactions of Aryl Halides and Directing Effects

Competition Wallah

Dedicated lecture on aryl halide reactions and directing effects, part of a numbered haloalkanes/haloarenes series.