Isocrotonic acid (also known as quartenylic acid; formally named (Z)-2-butenoic acid) is the cis isomer of crotonic acid. It is an oil, possessing an odor similar to that of brown sugar. At its boiling point of 171.9 ðC, it converts into crotonic acid. The compound can be prepared from 1,3dibromo-2butanone via the Favorskii rearrangement.
Ethyl isocrotonate can be prepared by semihydrogenation of ethyl tetrolate.
Rudolph Fittig and Hugo Erdmann showed that the ó-phenyl structural analog of isocrotonic acid forms ñ-naphthol when dehydrated, an observation that provided useful evidence in understanding the nature of naphthalene.