Maleic Acid

C₄H₄O₄ acid

Molar Mass

116.072 g/mol

Properties

State	Solid (white crystalline)
Color	White
Solubility	Soluble in water (478 g/L at 20 °C); soluble in ethanol and acetone
Melting Point	135 °C
Boiling Point	Decomposes to maleic anhydride above 135 °C

About Maleic Acid

Maleic acid is the cis-dicarboxylic acid HOOC-CH=CH-COOH, and almost every interesting thing about it traces back to the geometric constraint that its two COOH groups sit on the same side of the central double bond. That single structural fact makes it a textbook contrast to its trans isomer, fumaric acid: maleic melts at 135 °C, fumaric at 287 °C; maleic dissolves freely in water at 478 g/L, fumaric only at 6 g/L; maleic dehydrates cleanly to maleic anhydride above 135 °C, fumaric refuses to form a cyclic anhydride at all because its COOH groups are pointing in opposite directions. The proximity of the carboxyls also produces a textbook anomaly in the Ka values — maleic has pKa1 = 1.92 and pKa2 = 6.23, a separation of more than 4 log units, because the mono-anion is stabilized by an internal hydrogen bond between the deprotonated and protonated carboxyl groups, and the second ionization has to fight against that ring. Industrial maleic acid is mostly a way station to maleic anhydride, the third-largest commodity anhydride after acetic and phthalic, made directly by the V2O5/MoO3-catalyzed vapor-phase oxidation of n-butane. Maleic anhydride and acid feed unsaturated polyester resins (gelcoats and fiberglass laminates), copolymers like the alternating poly(styrene-co-maleic anhydride) used in immunoassay plates, and the Diels-Alder textbook examples chemistry students cut their teeth on. Biologically, maleic acid is the imposter twin: fumarate is a Krebs cycle intermediate, while maleate has no metabolic role and is acutely nephrotoxic in animal models, used experimentally to induce Fanconi syndrome.

Where you'll encounter it

If you have ever cured a fiberglass boat-repair patch with peroxide-initiated polyester resin, run a Diels-Alder undergraduate prep with cyclopentadiene plus maleic anhydride, or compared maleate buffer to phosphate for a pH-7 enzyme assay, you have been using maleic acid chemistry. Boatyards that mix MEKP catalyst into unsaturated polyester gelcoat are crosslinking the maleate double bonds embedded along the polyester backbone, which is why the resin gels in 20 minutes and cures fully within the day. Sophomore organic students running the cyclopentadiene plus maleic anhydride reflux in toluene watch the dienophile disappear and a clean white norbornene-dicarboxylic anhydride precipitate drop out — one of the few teaching preps where the product crystallizes in real time. Pharmacists weighing chlorpheniramine maleate or enalapril maleate are working with the same diacid as the salt counterion.

Common Uses

Feedstock for maleic anhydride and unsaturated polyester resins
Comonomer in styrene-maleic anhydride (SMA) and ethylene-maleic anhydride copolymers
Dienophile in Diels-Alder synthesis (norbornene-dicarboxylic acid prep)
Acidulant E296 in beverages and dairy desserts
Buffer system in the pH 1.5-3 and 5-7 ranges for biochemical assays
Pharmaceutical salt former (chlorpheniramine maleate, enalapril maleate)
Teaching example for cis/trans isomerism and intramolecular H-bonding in pKa anomalies

Safety Information

GHS H302 (harmful if swallowed), H315 (skin irritation), H317 (skin sensitizer), H319 (eye irritation). Oral LD50 (rat) about 700 mg/kg. Use nitrile gloves and goggles when weighing the solid; the dust irritates mucous membranes. Maleate salts are nephrotoxic at high parenteral doses (the mechanism behind experimental maleate-induced Fanconi syndrome) but the small counter-ion mass in pharmaceutical formulations is well below this threshold. Maleic anhydride (the dehydration product) is more aggressive — STEL 0.25 ppm, OSHA PEL 0.25 ppm — because it hydrolyzes back to the diacid on contact with mucosa.

This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.

Constituent Elements

C — Carbon H — Hydrogen O — Oxygen

Frequently Asked Questions

What is the molar mass of maleic acid?

Maleic acid (C4H4O4) is 116.072 g/mol: 4 C (48.044) + 4 H (4.032) + 4 O (63.996). Identical to fumaric acid in elemental composition because they are geometric isomers — the molar mass cannot tell them apart, but mp and aqueous solubility instantly can.

What is the difference between maleic acid and fumaric acid?

Pure cis vs trans geometry around the central C=C. In maleic acid both COOH groups are on the same face of the double bond, close enough to hydrogen-bond to each other and close enough to dehydrate into a 5-membered cyclic anhydride. In fumaric acid the COOH groups are 180 degrees apart, packing into a tight planar lattice that gives it the much higher melting point (287 vs 135 °C) and very low water solubility (6 vs 478 g/L). The metabolic difference is just as sharp — fumarate is a Krebs cycle intermediate handled smoothly by fumarase, while maleate has no enzyme tuned to it and accumulates harmlessly only in small amounts.

Why does maleic acid form an anhydride so easily?

The cis geometry holds the two COOH oxygens within hydrogen-bonding distance, so the activation barrier for closing a 5-membered cyclic anhydride is small. Heat the diacid above its 135 °C melting point and water leaves spontaneously: 2 COOH -> O=C-O-C=O + H2O. Fumaric acid cannot perform this trick because its COOH groups are too far apart; forcing the geometry would require breaking the C=C bond. This is the classic 'geometric isomers do different chemistry' demo in undergraduate organic textbooks.