Chlorobenzene labelled with $^{14} C$ at the carbon bearing chlorine (C-1) is treated with $NaNH X_{2}$ in liquid $NH X_{3}$ . Assume the labelled and unlabelled benzyne carbons are equally susceptible to attack and ignore isotope effects. The percentage of the aniline formed in which $NH X_{2}$ is attached to the labelled carbon is

A25%
B75%
C50%
D100%

Answer: C

Elimination of

HCl

gives benzyne with the triple bond between C-1 (labelled) and C-2. The amide ion can add to either carbon of the triple bond with equal probability. Half of the aniline therefore has

NH X_{2}

on the labelled carbon and half on the neighbouring unlabelled carbon.

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Answer: C

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