Reactions of Aldehydes & Ketones with HCN (Cambridge International A-Level Chemistry)

Reaction of Hydrogen Cyanide with Aldehydes & Ketones

• The carbonyl group -C=O, in aldehydes and ketones is polarised
• The oxygen atom is more electronegative than carbon drawing electron density towards itself
• This leaves the carbon atom slightly positively charged and the oxygen atom slightly negatively charged
• The carbonyl carbon is therefore susceptible to attack by a nucleophile, such as the cyanide ion

Nucleophilic addition

• The nucleophilic addition of hydrogen cyanide to carbonyl compounds is a two-step process
• In step 1 the cyanide ion attacks the carbonyl carbon to form a negatively charged intermediate
• In step 2 the negatively charged oxygen atom in the reactive intermediate quickly reacts with aqueous H⁺ (either from HCN, water or dilute acid) to form 2-hydroxynitrile

The nucleophilic addition of HCN mechanism

The cyanide ion attacks the carbonyl carbon to form a negatively charged intermediate which quickly reacts with a proton to form a 2-hydroxynitrile compound

Exam Hint The actual negative charge on the cyanide ion is on the carbon atom and not on the nitrogen atom