CBSE Class 12 Chemistry 2026: question 18

Hints

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Step-by-step solution

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1. Resonance effect: a lone pair on the halogen is delocalised into the benzene ring. The C–X bond therefore gets partial double-bond character, becomes stronger and shorter, and is difficult to break.

   1 mark

2. Hybridisation of carbon: the C attached to X is $s{p}^2$ hybridised. With more s-character it is more electronegative, holds the bonding pair more tightly, and the C–X bond is shorter and stronger than in haloalkanes ($s{p}^3$ C). Other acceptable reasons: the phenyl cation is unstable (so $S_{N}1$ is not possible), and the electron-rich ring repels the incoming nucleophile (so backside $S_{N}2$ attack is hindered).

   1 mark

Answer: (1) Resonance gives the C–X bond partial double-bond character; (2) the $s{p}^2$ carbon holds the C–X bond more tightly (also: phenyl cation unstable, nucleophile repelled by the π-electron cloud).

Revise the chapter

• Chapter: Haloalkanes and Haloarenes (notes, formulas, where marks are lost and every past question)