The product of reaction of acetone with HCN is:

Solution

Chemistry ma’am · 3 steps

Cyanide adds to the carbonyl carbon of acetone, so the cyanohydrin keeps both methyl groups.

1. Nucleophile forms. A trace of base pulls H⁺ off HCN and gives CN⁻, the real nucleophile. The carbonyl carbon of acetone carries a partial positive charge, so CN⁻ attacks it.
2. Addition happens. The C=O π bond breaks and oxygen takes a negative charge. This alkoxide then picks up H⁺ from HCN, so -OH and -CN end up on the same carbon.

   (CH₃)₂C=O + HCN → (CH₃)₂C(OH)CN

3. So. The product is 2-hydroxy-2-methylpropanenitrile, called acetone cyanohydrin, which is option (b).

The idea to keep. Aldehydes and most ketones add HCN across C=O: the CN goes to carbon and the H goes to oxygen. The starting carbon skeleton stays, plus one extra carbon from CN.

Answer from the NTA final answer key. Solution by PrepWiser.