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JEE Main levelSingle correct optionDifficulty 2 of 3o3016

In a polar protic solvent, which of the following halides ionises (SN1) most rapidly?

A $(CH X_{3}) X_{2} CH - Cl$
B $CH X_{3} CH X_{2} CH X_{2} - Cl$
C $CH X_{3} - Cl$
D $CH X_{3} - O - CH X_{2} - Cl$

Answer: D

An oxygen lone pair adjacent to the cationic carbon is donated into the empty p orbital, giving the resonance-stabilised oxocarbenium ion

CH X_{3} - O X^{+} = CH X_{2}

. This stabilisation exceeds that available to a secondary alkyl cation, so the chloromethyl ether ionises fastest.

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