A cyanohydrin forms when cyanide (as the nucleophile CN⁻, typically supplied from NaCN or KCN together with HCN) adds to an aldehyde or ketone via the same universal nucleophilic addition mechanism from earlier in this sub-subject: cyanide's carbon lone pair attacks the electrophilic carbonyl carbon, the carbonyl pi electrons collapse onto oxygen, and protonation during workup gives the final product — a molecule bearing both a hydroxyl and a nitrile group on what was the original carbonyl carbon, written R₂C(OH)CN.

What makes this particular nucleophilic addition especially valuable in synthesis is that cyanide's carbon becomes a genuinely new carbon atom permanently incorporated into the product skeleton — this reaction is one of the small number of nucleophilic additions in this sub-subject that actually extends the carbon count of the molecule, rather than simply installing a new heteroatom-containing group onto an existing carbon skeleton.

The hub notes directly that aldehydes react more readily than ketones in cyanohydrin formation, and the reaction is thermodynamically favorable with formaldehyde and acetaldehyde specifically, while the equilibrium disfavors cyanohydrin formation with more hindered ketones. This tracks with a combination of two familiar effects working in the same direction: aldehydes are both less sterically hindered (only one substituent plus a hydrogen, versus two substituents on a ketone) and more electronically electrophilic at the carbonyl carbon (a ketone's two alkyl groups donate some electron density inductively, making its carbonyl carbon comparatively less electrophilic than an aldehyde's).

For a bulky, hindered ketone specifically, the equilibrium constant for cyanohydrin formation can become genuinely unfavorable — the steric strain introduced by forcing a fourth substituent (the newly added CN and OH groups) onto an already crowded carbon can outweigh whatever electronic driving force exists, shifting the reaction back toward the free ketone and HCN rather than the cyanohydrin product. This is a useful, concrete illustration of a principle worth remembering broadly: not every nucleophilic addition proceeds equally well on every carbonyl substrate, and steric hindrance specifically can make an otherwise favorable addition reaction reversible in the wrong direction.