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Chlorobenzene labelled with C-14 at position 1 (the carbon bearing Cl) is treated with NaNH2 in liquid ammonia. Which of the following correctly describes the product(s) formed?

1. Aniline with C-14 only at position 1 (50%) and aniline with C-14 at position 2 (50%)
2. Only aniline with C-14 exclusively at position 1
3. Aniline with C-14 equally distributed at all six carbons
4. No reaction occurs under these conditions

Correct answer: Aniline with C-14 only at position 1 (50%) and aniline with C-14 at position 2 (50%)

Solution

The reaction proceeds via benzyne mechanism: NaNH2 eliminates HCl from chlorobenzene-1-C14 to give 1,2-didehydrobenzene (benzyne) with C-14 at position 1. The benzyne triple bond is between C1 (C-14) and C2 (C-12). Nucleophilic addition of NH2⁻ can occur at either C1 or C2 with equal probability, giving two products: aniline with NH2 at C-14 position (50%) and aniline with NH2 at the adjacent unlabelled carbon with C-14 still in ring (50%). This is the classic isotopic scrambling evidence for the benzyne mechanism.

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