Maleic acid is an organic compound

Organic compound

An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

 that is a dicarboxylic acid

Dicarboxylic acid

Dicarboxylic acids are organic compounds that contain two carboxylic acid functional groups. In molecular formulae for dicarboxylic acids, these groups are often written as HOOC-R-COOH, where R may be an alkyl, alkenyl, alkynyl, or aryl group...

, a molecule with two carboxyl groups. Maleic acid is the cis-isomer of butenedioic acid, whereas fumaric acid

Fumaric acid

Fumaric acid or trans-butenedioic acid is the chemical compound with the formula HO2CCH=CHCO2H. 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...

 is the trans-isomer. It is mainly used as a precursor to fumaric acid, and relative to its parent maleic anhydride

Maleic anhydride

Maleic anhydride is an organic compound with the formula C2H22O. It is the acid anhydride of maleic acid and in its pure state it is a colourless or white solid with an acrid odour....

, maleic acid has few applications.
Maleic acid is a less stable molecule than fumaric acid. The difference in heat of combustion

Heat of combustion

The heat of combustion is the energy released as heat when a compound undergoes complete combustion with oxygen under standard conditions. The chemical reaction is typically a hydrocarbon reacting with oxygen to form carbon dioxide, water and heat...

is 22.7 kJ·mol⁻¹.