Potential energy profile
ReactantsInt.Product
Step 0
Ready
Ethanal (CH₃CHO) — the carbonyl C is δ+ due to the electronegative O. CN⁻ nucleophile is ready to attack.
Step 1 — slow
CN⁻ attacks C=O
CN⁻ donates a lone pair to the δ+ carbonyl carbon. The C=O π bond breaks — both electrons shift to oxygen.
Intermediate
Anionic intermediate
Tetrahedral anionic intermediate — carbon is now sp³. Oxygen carries a negative charge (O⁻). Simulation paused.
Step 2 — fast
Protonation by HCN
O⁻ abstracts a proton from HCN (Brønsted acid). HCN regenerates CN⁻ — a catalytic cycle.
Product
Cyanohydrin formed
Hydroxynitrile (cyanohydrin) product: CH₃CH(OH)CN. Carbon centre is tetrahedral.
View:
Speed:
1.0×
Ready
Why CN⁻ attacks C
The carbonyl carbon is δ+ due to the electronegative oxygen pulling electron density away. CN⁻ acts as a nucleophile, donating its lone pair to this electron-deficient carbon.
C=O π bond breaks
As CN⁻ forms a bond with C, the C=O π bond breaks heterolytically. Both electrons shift to oxygen, giving it a negative charge. Carbon rehybridises from sp² to sp³.
Protonation step
The anionic O⁻ intermediate is protonated by HCN (acting as a Brønsted acid). This regenerates CN⁻, making KCN a catalyst — only a trace amount is needed.
Product: cyanohydrin
The product is a hydroxynitrile (cyanohydrin): CH₃CH(OH)CN. The reaction adds both OH and CN across the C=O bond. The product carbon is tetrahedral (sp³).