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Chlorobenzene labelled with C-14 at carbon-1 reacts with NaNH2 in liquid ammonia to give aniline-type products where the amino group is found at both C-1 and C-2 positions (scrambling). Which mechanistic statement best explains this observation?

1. The reaction proceeds via an elimination-addition pathway involving a benzyne intermediate
2. The reaction proceeds via an SN1 mechanism
3. The reaction proceeds via an SN2 mechanism
4. The reaction proceeds via electrophilic aromatic substitution

Correct answer: The reaction proceeds via an elimination-addition pathway involving a benzyne intermediate

Solution

The C-14 label appears equally at C-1 and C-2 in the product, which is only possible if the reaction goes through a symmetrical benzyne intermediate formed by elimination of HCl. Addition of NH2- to either end of the triple bond in benzyne gives the observed scrambling.

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