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