Maleic and Fumaric Acids

geometrical isomers of ethylene dicarboxylic acid.

The geometrical isomerism of ethylene compounds has been studied in great detail using the example of maleic and fumaric acids. Both acids are colorless crystals. Maleic acid, the cis isomer (I), has a melting point of 130°C and a boiling point of 160°C; fumaric acid, the trans isomer (II), has a melting point of 286°C and a boiling point of 290°C.

Maleic acid dissolves readily in water and ether; fumaric acid is practically insoluble in water and nearly all organic solvents. Maleic acid is stronger than fumaric acid but less stable. It is readily converted to fumaric acid under exposure to light or upon heating to temperatures above 200°C. Fumaric acid is converted to maleic acid upon exposure to ultraviolet irradiation.

Fumaric acid is found in certain species of fungus, lichen, and other plants, including Fumaria officinalis. Maleic acid is not found in nature. Both acids can be obtained by heating malic acid. Maleic anhydride (colorless crystals; melting point, 52.8°C; boiling point, 199.9°C) is prepared by oxidizing benzene or furfurol over V₂O₅ and is used primarily for industrial purposes (for example, in the manufacture of unsaturated polyesters and alkyds).