The compound C6H5CHO is benzaldehyde. The functional group present in benzaldehyde is an aldehyde group -CHO .When benzaldehyde is treated with HCN hydrogen cyanide , it undergoes a nucleophilic addition reaction called the cyanohydrin formation. In this reaction, the cyanide ion CN- acts as a nucleophile and attacks the carbonyl carbon of the aldehyde group, resulting in the formation of a cyanohydrin. The product formed is called phenylglycolonitrile C6H5CH OH CN .