cyanohydrin
/ sy-AN-oh-hy-drin /
A cyanohydrin is the product of adding hydrogen cyanide across a carbonyl: a carbon that now carries both an -OH group and a -CN (nitrile) group. The name says it all: 'cyano' for the CN and 'hydrin' for the OH. It is one of the rare carbonyl additions that builds a new carbon-carbon bond, which makes it valuable in synthesis.
The mechanism is textbook nucleophilic addition with cyanide ion (CN-) as the nucleophile. Cyanide attacks the electron-poor carbonyl carbon, the carbonyl electrons go up onto oxygen to form an alkoxide, and that oxygen then grabs a proton (from HCN or water) to become the hydroxyl. Because cyanide is a weak base and a good nucleophile, the addition is reversible, and a little base is usually needed to generate enough CN- from HCN to get a workable rate. Aldehydes and unhindered ketones react well; bulky ketones less so.
Cyanohydrins are useful handles because the new groups can be transformed: the nitrile can be hydrolysed to a carboxylic acid (giving an alpha-hydroxy acid, one carbon longer than you started), or reduced to an amine (giving a beta-amino alcohol). Nature uses cyanohydrins too, sometimes defensively: some plants store cyanogenic glycosides that release a cyanohydrin and then toxic HCN when the tissue is crushed, which is why bitter almonds and apple seeds carry a warning.
Acetone plus HCN gives acetone cyanohydrin, (CH3)2C(OH)CN; industrially this is a route to methyl methacrylate, the monomer of Plexiglas/acrylic glass.
A cyanohydrin: one carbon bearing both OH and CN, from cyanide adding to a carbonyl.
HCN and cyanide are acutely toxic; the chemistry is straightforward but the reagent is lethal. The reaction's reversibility is also why crushed cyanogenic plant tissue can liberate HCN gas.