What Is Maleic Acid?
Maleic acid is a type of unsaturated dicarboxylic acid, characterized by two carboxyl groups. When heated above its melting point, maleic acid dehydrates and condenses to form maleic anhydride. It is a highly reactive compound commonly used as a raw material in organic synthesis.
Physical and Chemical Properties of Maleic Acid
1. Name
English name: Maleic acid
IUPAC name: (2Z)-but-2-enedioic acid
2. Molecular Formula
C4H4O4
3. Molecular Weight
116.1
4. Melting Point
133-134°C
5. Solvent Solubility
Soluble in water, ether, and alcohols; insoluble in benzene
Differences Between Maleic Acid and Fumaric Acid as Geometric Isomers
Maleic acid and fumaric acid are geometric isomers. The cis form of ethylenedicarboxylic acid is maleic acid, while the trans form is fumaric acid.
The physical and chemical properties of these compounds differ significantly:
1. Difference in Ease of Forming Acid Anhydride
In cis maleic acid, the proximity of the carboxyl groups facilitates dehydration and condensation upon heating to form anhydride. In contrast, the trans configuration of fumaric acid keeps the carboxyl groups apart, making intramolecular dehydration-condensation unlikely.
2. Differences in Aqueous Solubility and Melting Point
Maleic acid is highly soluble in water, while fumaric acid is not. This is attributed to the formation of intramolecular hydrogen bonds in maleic acid, hindering intermolecular hydrogen bonding. Maleic acid’s lower melting point compared to fumaric acid is also explained by its cis configuration, which facilitates intramolecular hydrogen bonding.
Characteristics and Uses of Maleic Acid
Maleic acid is used in various fields, from food additives to industrial applications. It is employed in the pharmaceutical industry as an acid component for pH adjustment and as a counterion in basic compounds. Industrially, it is a key material in producing glyoxylic acid via ozone decomposition and is used in synthetic resins (unsaturated polyesters), paints, resin modifiers, PVC stabilizers, surfactants, etc.
Maleic Acid Synthesis Method
Maleic anhydride, synthesized by the vapor-phase oxidation of benzene using a vanadium oxide (V) catalyst, is hydrolyzed to produce maleic acid.