Q.What is the action of HCN on

Step 1. HCN is a nucleophile that adds to any carbonyl compound's electrophilic carbonyl carbon (Section 12.5.A.1): the cyanide carbon bonds to the former carbonyl carbon, and the carbonyl oxygen is protonated to -OH, giving a cyanohydrin in every case.

Step 2. (i) Propanone (acetone), CH3-CO-CH3, + HCN gives acetone cyanohydrin, (CH3)2C(OH)(CN) -- 2-hydroxy-2-methylpropanenitrile.

Step 3. (ii) 2,4-Dichlorobenzaldehyde, an aromatic aldehyde bearing two ring chlorines, + HCN adds across its -CHO exactly as any other aldehyde would, giving the cyanohydrin 2-(2,4-dichlorophenyl)-2-hydroxyacetonitrile, (2,4-Cl2C6H3)-CH(OH)-CN -- the ring chlorines are simply carried through unchanged, since HCN reacts only at the carbonyl, not at the aromatic ring. …

Apply the single, general rule for HCN addition -- CN bonds to the carbonyl carbon, OH appears on the same carbon -- to each of the three named substrate …

• Assuming the aromatic ring's chlorines in (ii) somehow react with HCN or block the addition -- HCN addition occurs exclusively at the carbonyl carbon; substituents elsewhere on t …