Explain the mechanism of nucleophilic addition to the carbonyl group. Why are aldehydes more reactive than ketones? Write the reactions of ethanal with HCN and with hydroxylamine.

1. 1 · State the idea. The δ+ carbonyl carbon is attacked by a nucleophile; the C=O π bond breaks. This is nucleophilic addition.
2. 2 · Write the mechanism. Nu⁻ attacks at right angles to the plane → tetrahedral alkoxide (sp² → sp³) → takes H⁺ from the medium → >C(OH)Nu.
3. 3 · Draw it. Draw the planar C=O with a curved arrow from Nu⁻; then the tetrahedral carbon with -O⁻, Nu and two groups. Label sp² and sp³.
4. 4 · Give both reasons. Steric: two bulky groups in ketones hinder attack. Electronic: two alkyl groups (+I) lower the δ+ charge of a ketone's C=O carbon.
5. 5 · Ethanal with HCN. CH₃CHO + HCN → (OH⁻) CH₃CH(OH)CN, ethanal cyanohydrin.
6. 6 · Ethanal with hydroxylamine. CH₃CHO + H₂NOH → (H⁺) CH₃CH=NOH + H₂O, ethanal oxime: addition, then loss of water.
7. 7 · Close in one line. So aldehydes add nucleophiles faster than ketones, and ammonia derivatives add and then eliminate water.

Scoring tip: Name both reasons, steric and electronic. Show the sp² to sp³ change in the mechanism; examiners look for it.