The two simplest and important unsaturated dicarboxylic acids are
Maleic acid
cis-But-2-ene-1, 4-dioic acid
Fumaric acid
trans-But-2-ene-1, 4-dioic acid
The two acids represent the best known example of geometrical isomerism (cis-trans isomerism).
Maleic acid is manufactured by the oxidation of benzene with air in presence of vanadium pentoxide as catalyst at about 450°C.
Maleic acid is stronger than fumaric acid. However, both give similar products on addition of hydrogen, Br2, H2O, H2O + O forming succinic acid, monobromo succinic acid, malic acid and tartaric acid respectively. On heating, maleic acid easily undergoes cyclisation forming makes anhydride.
