Fumaric acid or trans-butenedioic acid has the formula HOOC-CH=CH-COOH (C4H4O4). This white crystalline compound is one of two isomeric unsaturated dicarboxylic acids, the other being maleic acid. In fumaric acid the carboxylic acid groups are trans and in maleic acid they are cis. Fumaric acid has a fruity taste. The salts and esters are known as fumarates. The primary hazard is the threat to the environment. Immediate steps should be taken to limit spread to the environment. It is combustible but may be difficult to ignite.
Fumaric acid is currently produced by isomerization of maleic acid, which is produced from maleic anhydride. Maleic anhydride, in turn, is industrially produced by catalytic oxidation of suitable hydrocarbons in the gas phase.
Maleic acid is converted by thermal or catalytic cis–trans isomerization into fumaric acid. The most common catalysts used are mineral acids, peroxy compounds, and thiourea. The crude fumaric acid obtained in this way is purified by crystallization from water.