What is the Difference Between Maleic Acid and Fumaric Acid?

Maleic acid and fumaric acid are both dicarboxylic acids with the same molecular formula, C₄H₄O₄, but they have different structural formulas, making them isomers of each other. The key differences between maleic acid and fumaric acid are:

1. Structural Isomerism: Maleic acid is the cis-isomer, while fumaric acid is the trans-isomer of butenedioic acid.
2. Acidity: Maleic acid is more acidic than fumaric acid due to its ability to form intramolecular hydrogen bonds after losing one H+ ion, making the conjugate base more stable than the trans form (fumaric acid).
3. Solubility in Water: Maleic acid is more soluble in water than fumaric acid.
4. Melting Point: The melting point of maleic acid (135 °C) is much lower than that of fumaric acid (287 °C).
5. Production: Maleic acid is produced by hydrolysis of maleic anhydride, which is derived from the oxidation of benzene or butane. Fumaric acid is produced through a catalytic isomerization of maleic acid at low pH in an aqueous solution.

Both maleic acid and fumaric acid have industrial applications. Maleic acid is mainly used as a precursor to fumaric acid and as an industrial raw material for the production of glyoxylic acid by ozonolysis. Fumaric acid is used as a food additive due to its fruity taste and as an adhesion promoter for different substrates, such as nylon and zinc-coated metals.

Comparative Table: Maleic Acid vs Fumaric Acid

The key difference between maleic acid and fumaric acid lies in their isomerism. Maleic acid is the cis-isomer of butenedioic acid, while fumaric acid is the trans-isomer. Here is a table comparing the differences between maleic acid and fumaric acid:

Despite having the same chemical formula and molar mass, maleic acid and fumaric acid have different structural formulas, which leads to their distinct physical and chemical properties.