Crotonic acid, or trans-2-butenoic acid, is a short-chain unsaturated carboxylic acid, described by the formula CH3CH=CHCO2H. Crotonic acid is so named because it was erroneously thought to be a saponification product of croton oil.[citation needed] It crystallizes as needles from hot water.
Racemic threonine can be prepared from crotonic acid by alpha-functionalization using mercury(II) acetate.[2]