China maleic anhydride manufacturers

How is maleic acid converted to fumaric acid?

Maleic acid is quite different from Fumaric acid in a considerable number of ways. Here are some ways in which Maleic acid differs from Fumaric acid:
Maleic acid is a less stable molecule than fumaric acid. The difference in heat of combustion is 22.7 kJ·mol−1.
Maleic acid is more soluble in water, than fumaric acid.
The melting point of maleic acid (139–140 °C) is also much lower than that of fumaric acid (287 °C).
Intramolecular hydrogen bonding that takes place in maleic acid at the expense of intermolecular interactions is not possible in fumaric acid for geometric reasons.
The major industrial use of maleic acid BP is its conversion to fumaric acid by isomerization. This isomerization is catalyzed by a variety of reagents, such as mineral acids and thiourea.
Maleic acid and fumaric acid do not spontaneously interconvert because rotation around a carbon carbon double bond is not energetically favourable. However, conversion of the cis isomer into the trans isomer is possible by photolysis in the presence of a small amount of bromine. Light converts elemental bromine into a bromine radical, which attacks the alkene in a radical addition reaction to a bromo-alkane radical; and now single bond rotation is possible. The bromine radicals recombine and fumaric acid is formed.

There is another way in which maleic acid can be coverted into fumaric acid food grade. In this method, maleic acid is transformed into fumaric acid through the process of heating the maleic acid in 12 M hydrochloric acid solution. Reversible addition (of H+) leads to free rotation about the central C-C bond and formation of the more stable and less soluble fumaric acid.