= Solution
The first stage is a <Stork enamine reaction>. <Pyrrolidine> attacks the <ketone> <carbonyl>, and <proton> transfers give a <carbinolamine>. Loss of <water> gives an <iminium ion>; removal of the benzylic alpha <proton> produces the <enamine> conjugated with the <aromatic ring>. Removal of <water> and prolonged heating favour this condensation. That conjugated <enamine> is preferred over the alternative with its double bond on the other side of the original <carbonyl>.
The nucleophilic <enamine> <carbon> attacks the methylene <carbon> of bromoacetonitrile by an <SN2 reaction>, displacing bromide and making the <carbon>–<carbon> bond. It is the <carbon> beside the aromatic fusion that acquires $\mathrm{CH_2CN}$, as in D. Finally, <water> adds to the alkylated iminium <carbon>; <proton> transfers and cleavage of the C–N bond regenerate the <ketone> and release <pyrrolidine>. This explains <carbon> alkylation, its position, and restoration of the <carbonyl> rather than treating the amine as a permanent <substituent>.
\Image[/past-exam-of-the-chemistry-b-course-of-the-university-of-cambridge/2025/paper-1-enamine-alkylation.png]
{title=Enamine formation, carbon alkylation by bromoacetonitrile, and hydrolysis}
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