Quick Notes Carbonyls - Nucleophilic Addition

  • Hydrogen cyanide reacts with carbonyl groups in nucleophilic addition reactions to form hydroxynitriles. nucleophilic addition of HCN to carbonyl mechanism to form hydroxynitrile
  • The length of the carbon chain in a carbonyl molecule can be extended using this reaction.
  • Hydrogen cyanide is highly reactive (and toxic); it can be prepared ‘in situ’ using a metal cyanide (KCN or NaCN) and dilute sulfuric acid (H2SO4).

Full Notes Carbonyls - Nucleophilic Addition

When hydrogen cyanide is added to a carbonyl group, nucleophilic addition occurs.

carbonyl with KCN and HCN to form Hydroxynitrile

Hydrogen cyanide is a highly toxic, dangerous substance. So, it’s often produced in solution with the carbonyl compound that it’s going to react with. A metal cyanide ion (usually NaCN or KCN) is added with dilute sulfuric acid. The metal cyanide dissociates in solution releasing CN- ions; the dilute sulfuric acid releases H+ ions.

Potassium Cyanide to cyanide ions dissociation

Cyanide ions (CN-) are strong nucleophiles that are attracted to the carbon with a partial positive charge in a carbonyl.


nucleophilic addition mechanism of potassium cyanide to carbonyl hydroxynitrile first step

The lone pair of electrons on the carbon in the nitrile move to the partially positive carbon in the carbonyl. The C=O bond breaks, with the oxygen atom gaining a pair of electrons and a negative charge.

nucleophilic addition of potassium cyanide to carbobnyl hydroxynitrile second step mechanism

A H+ ion in solution (from the sulfuric acid) then bonds with the negatively charged oxygen atom, forming an –OH group.

nucleophilic addition of potassium cyanide to carbonyl hydroxynitrile mechanism final step