Maleic anhydride is the cyclic anhydride of maleic acid. The two carboxylate oxygens are joined through a C–O–C bridge, forming a strained 5-membered ring. This ring:

• Stores ring strain energy that drives rapid ring-opening with nucleophiles
• Makes MAH far more reactive than either diacid - the anhydride ring opens much faster than a carboxylic acid reacts with an alcohol or amine
• Can only form because the parent diacid is cis (maleic acid) - geometry places the two –COOH groups close enough to cyclise
• Produces MW 98 (= maleic acid 116 – H₂O 18) - lowest MW per mole of C4 diacid functionality

Maleic acid is the open-chain form - the hydration product of MAH. Both carboxylic acid groups are on the same side of the C=C double bond (Z/cis configuration):

• High water solubility (~780 g/L at 25°C) - cis geometry allows intramolecular hydrogen bonding with water
• Strong acid - first pKa = 1.94 (very acidic; –COOH groups mutually destabilise by induction)
• Can re-form MAH on strong heating (>150°C dehydration) - back-reaction possible
• Less stable thermodynamically than fumaric acid - tends to isomerise to fumaric acid under acidic conditions or with heat
• Melting point 138°C - higher than MAH (52.8°C); white crystalline solid

Fumaric acid is the geometric isomer of maleic acid - the two carboxylate groups are on opposite sides of the C=C bond (E/trans configuration):

• Low water solubility (~6.3 g/L at 25°C) - trans geometry prevents effective hydrogen bonding with water; this is why fumaric acid precipitates during its industrial production from maleic acid
• Weaker acid than maleic acid - first pKa = 3.03 (–COOH groups farther apart; less mutual destabilisation)
• Cannot form a cyclic anhydride under normal conditions - trans geometry places the two –COOH groups too far apart for 5-membered ring closure
• More thermodynamically stable than maleic acid - the trans configuration has lower steric strain
• Sublimes at 287°C - does not have a normal melting point at atmospheric pressure

Why MAH melts at only 52.8°C but maleic acid melts at 138°C: The anhydride ring prevents intermolecular hydrogen bonding between carboxylate groups - the ring seals off the H-bond donors. Maleic acid, with two free –COOH groups, forms extensive intermolecular hydrogen bonds that require more energy to disrupt.

Why maleic acid is much more water-soluble than fumaric acid: The cis geometry of maleic acid allows both –COOH groups to hydrogen-bond with water simultaneously. Fumaric acid's trans geometry means the groups point in opposite directions, reducing effective hydration. This solubility difference is used industrially to separate fumaric acid from maleic acid solution by precipitation.

Why fumaric acid sublimes rather than melts: Fumaric acid's trans geometry and extended crystal packing gives a very high lattice energy - at atmospheric pressure, the vapour pressure reaches 1 atm before the crystal has enough thermal energy to melt. It sublimes at 287°C. Under pressure (>0.5 atm), fumaric acid can melt at ~302°C.

Global consumption: ~3–4 million MT/year

• Unsaturated polyester resins (UPR): ~45% - fibreglass composites, marine, wind energy, construction
• SMA copolymers: ~15% - engineering plastics, paper sizing, detergent builders
• MAH-grafted polyolefins: ~8% - GF-PP composites, PA6/PP alloys, WPC tie-layers
• Alkyd resins & maleate esters: ~8% - coating intermediates, DBM/DOM plasticisers
• Maleic/fumaric acid: ~15% - MAH is the industrial source for both
• Water treatment polymers (HPMA): ~6% - scale inhibitors, dispersants

Produced primarily from MAH + H₂O

• Food acidulant (E296): Beverages, jams, fruit preparations - approved food additive in EU/US
• Malic acid precursor: Enzymatic hydration of maleic acid → malic acid (apple acid, E296) used in fruit-flavoured foods
• Pharmaceutical intermediate: Maleic acid salt form (maleate salt) is used to increase solubility of basic drug substances - e.g., chlorphenamine maleate (antihistamine), timolol maleate (glaucoma)
• Textile anti-crease: Maleic acid crosslinks cellulose in durable press fabrics
• Water-soluble polymer monomer: Co-monomer in scale inhibitors and superabsorbents

Produced industrially from maleic acid by isomerisation

• Food acidulant (E297): Baking powder, tortilla making, fruit drinks - non-hygroscopic; longer shelf life than citric acid in dry mixes
• Pharmaceutical - psoriasis: Dimethyl fumarate (Tecfidera®, Skilarence®) - oral treatment for moderate-to-severe plaque psoriasis and multiple sclerosis
• Polyester resin co-monomer: Used in some UPR grades as partial MAH replacement for improved water resistance (pure fumarate UPR); also used in polyester for PET-type applications
• Animal feed supplement: Reduces enteric methane production in ruminants (cattle, sheep); growing market driven by agricultural climate commitments
• Rosin ester modifier: Fumaric acid reacts with rosin similarly to maleic acid; used in some varnish and adhesive resin applications

Key interconversion facts: (1) MAH + water → maleic acid: fast, irreversible at ambient T; exothermic; (2) Maleic acid → MAH: requires >150°C dehydration; partial even at 200°C; (3) Maleic acid → fumaric acid: acid- or heat-catalysed cis-to-trans isomerisation; thermodynamically driven (fumaric acid is ~4 kJ/mol more stable); fumaric acid precipitates from solution driving reaction to completion; industrial process uses HCl or thiourea catalyst at 100–150°C; (4) Fumaric acid → maleic acid: reverse trans-to-cis isomerisation requires higher energy; partial only; not used industrially; (5) In UPR synthesis: maleate units in the growing polyester chain partially isomerise to fumarate above 180°C - this is intentional and beneficial (fumarate cures faster with styrene).

Q1: What is the difference between maleic anhydride and maleic acid?

Maleic anhydride (CAS 108-31-6, MW 98 g/mol, formula C₄H₂O₃) and maleic acid (CAS 110-16-7, MW 116 g/mol, formula C₄H₄O₄) are structurally related but chemically distinct compounds that are not interchangeable in industrial processes. Maleic anhydride is the cyclic anhydride form - the two carboxylic acid groups of maleic acid have been condensed with loss of one water molecule to form a strained five-membered ring containing one C–O–C anhydride bridge. The key practical differences: (1) Reactivity: MAH is far more reactive - its anhydride ring opens instantly with water, alcohols, and amines at ambient temperature; maleic acid requires elevated temperature and often a catalyst for the same esterification or amidation reactions; (2) Physical form: MAH melts at 52.8°C (solid at room temperature); maleic acid melts at 138°C; (3) Water contact: MAH hydrolyses irreversibly to maleic acid when exposed to moisture - MAH does not "dissolve" in water, it converts to the acid; maleic acid dissolves readily in water (~780 g/L); (4) Industrial role: MAH is the reactive intermediate used in polymer synthesis (UPR, SMA, HPMA); maleic acid is typically a downstream product of MAH hydrolysis or an end-product used as a food acidulant and pharmaceutical salt precursor; (5) Hazard: MAH is a skin corrosive and respiratory sensitiser (much more hazardous than maleic acid); maleic acid is an irritant but not corrosive or a sensitiser. They are never interchangeable in UPR synthesis - substituting maleic acid for MAH changes the stoichiometry (MW 116 vs 98) and reaction mechanism, giving wrong acid number and molecular weight in the polyester product.

Q2: What is the difference between maleic acid and fumaric acid?

Maleic acid (CAS 110-16-7) and fumaric acid (CAS 110-17-8) are geometric isomers - they have the same molecular formula (C₄H₄O₄, MW 116 g/mol) and the same connectivity, differing only in the configuration of the two –COOH groups relative to the central C=C double bond. Maleic acid is the cis (Z) isomer - both –COOH groups on the same side; fumaric acid is the trans (E) isomer - –COOH groups on opposite sides. This single structural difference produces dramatically different properties: (1) Water solubility: maleic acid ~780 g/L (freely soluble); fumaric acid ~6.3 g/L (sparingly soluble) - this difference is used industrially to separate fumaric acid by precipitation from aqueous maleic acid; (2) Acidity: maleic acid pKa₁ = 1.94 (stronger acid); fumaric acid pKa₁ = 3.03 (weaker acid) - the cis arrangement of two electron-withdrawing groups in maleic acid creates stronger mutual destabilisation; (3) Anhydride formation: maleic acid can dehydrate to maleic anhydride above 150°C; fumaric acid cannot form a stable cyclic anhydride - the trans geometry prevents ring closure; (4) Thermal stability: fumaric acid is thermodynamically more stable (~4 kJ/mol lower energy); maleic acid isomerises to fumaric acid under acidic or thermal conditions; (5) Industrial uses: maleic acid - food acidulant (E296), pharmaceutical maleate salts, malic acid precursor; fumaric acid - food acidulant (E297, more stable in dry mixes), dimethyl fumarate pharmaceutical, animal feed. Despite having the same molecular weight, they cannot substitute for each other in food applications (different taste profiles, solubility, hygroscopicity) or pharmaceutical applications (different specific activities).

Q3: Can maleic acid substitute for maleic anhydride in UPR synthesis?

Technically yes, but with important corrections - and in practice, it is rarely done because MAH is more economical and more reactive. If maleic acid (MW 116) is used instead of MAH (MW 98) in a UPR recipe, the following adjustments are needed: (1) Charge correction: Because maleic acid has MW 116 vs MAH's MW 98, you need 18.4% more kg of maleic acid to provide the same molar amount of diacid (one mole of maleic acid = 116g vs one mole of MAH = 98g); (2) Water of reaction: MAH already has the water removed (it is the anhydride); maleic acid will generate one additional mole of water per mole of diacid during polycondensation compared to MAH, which must be accounted for in the reactor water removal design; (3) Reaction rate: The initial ring-opening step that makes MAH react so quickly with glycols at low temperature is absent with maleic acid - the reaction must be conducted at elevated temperature (160–200°C) with catalyst, and reaction times are longer; (4) Stoichiometric impact: The glycol charge must be adjusted based on actual maleic acid purity and, importantly, on the actual OH:COOH ratio - any deviation from 1:1.05 correction ratio will give incorrect AN in the final polyester. In summary: maleic acid can be used in UPR but requires a recipe reformulation, longer reaction time, and careful stoichiometry correction. For any new UPR formulation, always specify which form of the diacid (MAH or maleic acid) the recipe was developed with, and never substitute one for the other without full reformulation.

Q4: How is fumaric acid produced industrially and what is its connection to maleic anhydride?

Fumaric acid is produced industrially from maleic anhydride via a two-step process: (1) MAH is hydrolysed with water to form maleic acid (the cis-diacid); (2) maleic acid is isomerised to fumaric acid (the trans-diacid) using an acid catalyst (HCl, HBr, or thiourea) at 100–150°C. The isomerisation is thermodynamically driven - fumaric acid is more stable than maleic acid by ~4 kJ/mol - and the reaction is pushed to completion by the low water solubility of fumaric acid (~6.3 g/L vs ~780 g/L for maleic acid), which causes fumaric acid to precipitate from solution as it forms, preventing the back-reaction. The precipitated fumaric acid is filtered, washed, and dried to give the commercial product. This means that essentially all fumaric acid in commercial use is derived from maleic anhydride - making MAH the indirect raw material for the food, pharmaceutical, and animal feed industries that consume fumaric acid. Alternative bio-based routes to fumaric acid (from glucose via fermentation using Rhizopus fungi) have been studied but are not commercially dominant. The global fumaric acid market is approximately 150,000–200,000 MT/year, consuming roughly 130,000–170,000 MT/year of MAH as raw material.

Q5: Does Sinolook Chemical supply maleic anhydride for UPR, SMA, and water treatment applications?

Yes - Sinolook Chemical supplies maleic anhydride (CAS 108-31-6) for all major MAH applications including UPR synthesis, SMA copolymer production, HPMA water treatment polymers, MAH-grafted polyolefins, alkyd resin modification, and maleated rosin production. We supply two grades: Standard grade (purity ≥99.0%, APHA ≤30 molten, Fe ≤5 ppm, maleic acid ≤0.3%, crystallisation point ≥52.5°C) for UPR, grafting, HPMA, and general synthesis applications; Premium grade (purity ≥99.5%, APHA ≤10, Fe ≤1 ppm, maleic acid ≤0.1%) for SMA personal care, gel-coat UPR, and ink varnish applications. Both grades are available in 25 kg PE-lined bags, 500 kg/1,000 kg big bags, and by arrangement as molten material in heated ISO tanks for large UPR producers. Sinolook Chemical does not supply maleic acid or fumaric acid - we focus on maleic anhydride as the primary industrial C4 intermediate. For MAH pricing, samples, COA, and REACH/TSCA documentation, contact us at sales@sinolookchem.com or WhatsApp 0086 18150362095. Product details at sinolookchem.com/personal-care-chemicals/maleic-anhydride.html