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Fumaric Acid

C4H4O4 organic
Molar Mass
116.072 g/mol
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Properties

StateSolid (white crystalline powder or granules)
ColorWhite
SolubilitySlightly soluble in water (7 g/L at 25°C); soluble in ethanol
Melting Point287°C (sublimes)
Boiling PointSublimes at 200°C

About Fumaric Acid

Fumaric acid is the trans (E) isomer of butenedioic acid (HOOC-CH=CH-COOH, 116.072 g/mol), and it is one half of the textbook cis-trans pair every undergraduate organic course teaches: fumaric (trans, mp 287 °C, water solubility 7 g/L) on one side, maleic (cis, mp 131 °C, water solubility 790 g/L) on the other. The trans geometry locks the two carboxyls 180° apart across the C=C, allowing them to engage in extended hydrogen-bonded chains in the crystal lattice — the source of both the high melting point and the dismal water solubility. The cis isomer is forced into an intramolecular H-bond that disrupts that lattice and behaves more like a normal small-molecule diacid. Biochemically, fumarate is a citric-acid-cycle intermediate produced when succinate dehydrogenase (Complex II of the electron transport chain) oxidizes succinate, transferring two electrons to FAD; fumarase (a perfect-enzyme example with kcat/KM near the diffusion limit) then hydrates it to L-malate. Fumarate also exits the urea cycle when argininosuccinate lyase cleaves argininosuccinate. The medicinal angle is dimethyl fumarate (Tecfidera, Bafiertam): a Michael acceptor that covalently modifies KEAP1 cysteines, releasing NRF2 to drive the antioxidant response — approved for relapsing-remitting multiple sclerosis and psoriasis.

Where you'll encounter it

If you have ever fermented a tarragon vinegar or a sourdough culture, you have made fumaric acid as a minor metabolite of Rhizopus or Lactobacillus. In food formulation it is the acidulant of choice for tortillas, gelatin desserts, and powdered drink mixes because, unlike citric acid, it is non-hygroscopic — it does not absorb humidity and turn the powder into a brick. In a teaching lab the maleic-to-fumaric isomerization (UV light or trace HBr, heat overnight) is the classic demonstration that the trans isomer is the thermodynamic sink.

Common Uses

  • Acidulant E297 in tortillas, gelatin desserts, and beverage powders for non-hygroscopic tartness
  • Citric-acid-cycle intermediate produced by succinate dehydrogenase (Complex II)
  • Dimethyl fumarate (Tecfidera) prodrug for relapsing-remitting multiple sclerosis
  • Crosslinking diacid in unsaturated polyester resin manufacturing for fiberglass
  • Feed additive at 1-2% in pig and broiler diets to reduce gastric pH
  • Demonstration substrate for cis-trans isomerism in undergraduate organic teaching labs
  • Aspergillus terreus and Rhizopus oryzae fermentation product for bio-based manufacture
  • Iron deficiency oral therapy as ferrous fumarate (most stable Fe(II) salt for shelf life)

Safety Information

GRAS-listed by FDA for food use; not classified as hazardous under GHS at food-grade concentrations. Concentrated dust may cause mild skin and eye irritation (transient). No OSHA PEL specified. Dimethyl fumarate (the medicinal form) is a different beast — Michael acceptor reactivity makes it a strong skin sensitizer (it caused a wave of contact dermatitis in 2008-2009 from anti-mold sachets in Chinese-made furniture, leading to an EU ban below 0.1 mg/kg in consumer products). Tecfidera patients commonly experience flushing and GI upset; rare cases of progressive multifocal leukoencephalopathy (PML) require lymphocyte monitoring. Fumaric acid esters in psoriasis treatment occasionally produce eosinophilia and lymphopenia.

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 fumaric acid?
Fumaric acid is C4H4O4 with a molar mass of 116.072 g/mol. The breakdown: 4 × C (4 × 12.011 = 48.044) + 4 × H (4 × 1.008 = 4.032) + 4 × O (4 × 15.999 = 63.996). Same molecular formula as maleic acid — the cis isomer — and therefore the same molar mass, which means you cannot distinguish them by elemental analysis or mass spectrometry on the parent ion alone. Melting point or NMR (the vinyl protons are inequivalent in maleic but equivalent in fumaric by symmetry) will tell them apart immediately.
What is the difference between fumaric acid and maleic acid?
They are the trans (fumaric) and cis (maleic) geometric isomers of butenedioic acid. The trans arrangement of fumaric acid lets the two carboxyls form intermolecular H-bonds in the solid, building a tight crystal lattice with a high melting point (287 °C) and very low water solubility (7 g/L). The cis arrangement of maleic acid forces an intramolecular H-bond that prevents that packing — melting point drops to 131 °C and water solubility shoots up to 790 g/L. Maleic also has a much lower pKa1 (1.9 vs 3.0) because the intramolecular H-bond stabilizes the monoanion.
Where does fumarate appear in metabolism?
Two main places. In the TCA cycle, succinate dehydrogenase (Complex II of the ETC) oxidizes succinate to fumarate, reducing FAD to FADH2 and feeding electrons directly into the ubiquinone pool. Fumarase then hydrates fumarate stereospecifically (it adds water across the trans double bond, giving exclusively L-malate, never D-malate). In the urea cycle, argininosuccinate lyase cleaves argininosuccinate into arginine and fumarate — the cytosolic fumarate is then routed back into the mitochondrion to rejoin the TCA pool. Loss-of-function fumarase mutations cause hereditary leiomyomatosis and renal-cell carcinoma, an interesting case of a TCA enzyme acting as a tumor suppressor.