Why only the cis (maleic acid) isomer can form the anhydride: Ring closure to a 5-membered anhydride requires both carbonyl groups to be within ~2.5 Å of each other. In cis-maleic acid, the two –COOH groups on the same side of the double bond are geometrically proximate and can readily undergo intramolecular dehydration. In trans-fumaric acid, the carboxylate groups point in opposite directions - the geometry is wrong for 5-membered ring formation, and fumaric acid does not form a corresponding stable cyclic anhydride under normal conditions.

Ring atom sequence	O–C(=O)–C=C–C(=O)–[O]
C=C bond length	1.326 Å (shorter than alkene; conjugated)
C–C(=O) bond length	1.478 Å (single bond conjugated)
C=O bond length	1.192 Å (normal carbonyl)
C–O–C (ring oxygen)	1.389 Å (C–O single bond)
Ring planarity	Essentially planar (all sp² carbons)
H–C=C–H dihedral	0° (cis, both H on same face)

The electronic character of MAH's two functional groups is critical to understanding its reactivity:

The anhydride ring is a strained, electrophilic functional group that opens rapidly with nucleophiles. The driving force is relief of ring strain combined with formation of two stable C–O or C–N bonds:

The C=C of MAH is flanked by two electron-withdrawing carbonyl groups, making it the most electrophilic of the common reactive alkenes. This electron deficiency drives two distinct reaction modes:

Process: n-Butane/air mixture passes over a vanadium-phosphorus-oxide (VPO) catalyst bed in a multi-tube fixed-bed or fluidised-bed reactor. The highly exothermic oxidation generates MAH plus water; the product gas is cooled and the MAH absorbed in a circulating solvent (dibutyl phthalate) or condensed directly. Crude MAH is purified by stripping and distillation.

Process: Benzene vapour mixed with excess air is passed over the vanadium-molybdenum oxide catalyst. The six-carbon ring is oxidatively fragmented; two carbons are lost as CO₂ (with some CO and maleic acid as by-products). The MAH yield per kg of benzene is lower than the butane route on a weight basis due to the two carbons lost as CO₂.

Ester	From	Application
Dibutyl maleate (DBM)	MAH + n-BuOH	Plasticiser, reactive monomer
Dioctyl maleate (DOM)	MAH + 2-EH alcohol	PVC plasticiser co-monomer
Monomethyl maleate	MAH + MeOH	Pharmaceutical intermediate
Propylene glycol maleate	MAH + PG	UPR monomer building block

Property	MAH	PA
Ring-opening rate	Fast ⭐	Moderate
C=C for crosslinking	Yes ⭐	No
MW per anhydride	98 g/mol (lower cost/mol)	148 g/mol
Role in UPR	Crosslinkable segment	Flexible / rigid spacer

The maleimide product retains the reactive C=C double bond, which is now activated by two flanking carbonyl groups (even more electrophilic than MAH's C=C). Bismaleimide (BMI) resins exploit this structure for high-temperature crosslinking.

• MAH-grafted polymer compatibilisation: The pendant anhydride groups on MAH-g-PP react with the terminal –NH₂ groups of polyamide (PA6, PA66) chains in a reactive extrusion process; forms a PP-b-PA copolymer at the interface that compatibilises the blend - this is the commercial basis of the PA6/PP engineering plastic alloy industry
• Bismaleimide (BMI) resins: MAH + diamine → bismaleamic acid → bismaleimide; the bismaleimide crosslinks via the reactive C=C bonds at 180–230°C; used in aerospace structural composites, printed circuit board laminates (FR-5), and high-temperature adhesives
• N-Substituted maleimides as comonomers: N-phenylmaleimide and N-cyclohexylmaleimide are comonomers that raise the Tg of acrylate copolymers for heat-resistant coatings

MAH is an exceptionally poor monomer for homopolymerisation because its electron-poor C=C has low reactivity toward adding another MAH radical (like repels like in radical polymerisation). However, it undergoes near-perfect alternating copolymerisation with electron-rich monomers such as styrene, vinyl acetate, and vinyl ethers because:

• The electron-poor MAH radical accepts electrons readily from an electron-rich monomer
• The electron-rich monomer radical in turn accepts the electrophilic MAH monomer
• This donor-acceptor alternation gives near-perfect 1:1 alternating sequences even at non-equimolar feed ratios
• Reactivity ratios: r(MAH) ≈ 0.01 with styrene; r(styrene) ≈ 0.01 - essentially zero homopolymerisation tendency for both

Reactive extrusion of polypropylene (PP) with MAH (0.5–2 wt%) and a peroxide initiator at 180–220°C installs pendant anhydride groups on the PP chain. The grafting mechanism:

Key process controls: MAH concentration (higher = more grafting but also more crosslinking/degradation); peroxide type and concentration; extruder temperature profile; residence time. Typical grafting degree: 0.3–1.5 wt% MAH on PP.

Q1: What is the molecular formula of maleic anhydride?

The molecular formula of maleic anhydride is C₄H₂O₃ with a molecular weight of 98.06 g/mol. The formula reflects the unusual hydrogen-to-carbon ratio of 2:4 - there are only two hydrogen atoms in the entire molecule, both located on the carbon-carbon double bond (vinyl hydrogens at δ 7.52 ppm in ¹H NMR). The three oxygen atoms are distributed as: two carbonyl oxygens (one on each C=O group) and one ring-bridging oxygen in the anhydride linkage (C–O–C). Compared to its parent diacid, maleic acid (C₄H₄O₄, MW 116), maleic anhydride has lost one water molecule (H₂O = 18 mass units), giving 116 – 18 = 98 g/mol. This relationship between MAH and maleic acid - differing by exactly one water molecule - is fundamental to MAH chemistry: adding water to MAH regenerates maleic acid, and dehydrating maleic acid regenerates MAH. The empirical formula C₄H₂O₃ can also be written as (CHO)₂·CO, highlighting the two equivalent carbonyl carbon environments and the bridging oxygen.

Q2: How is maleic anhydride made from n-butane?

The industrial production of maleic anhydride from n-butane proceeds by catalytic vapour-phase selective oxidation: C₄H₁₀ + 3½ O₂ → C₄H₂O₃ + 4 H₂O. The process uses a vanadium-phosphorus-oxide (VPO) catalyst - specifically vanadyl pyrophosphate (VO)₂P₂O₇ - at 350–450°C. The mechanism involves: (1) adsorption of n-butane on the V⁴⁺/V⁵⁺ active site; (2) sequential hydrogen abstraction to form a butene intermediate; (3) further oxidation and ring-closure to 1,3-butadiene; (4) further oxidation and ring-closure to maleic anhydride by oxygen insertion. The net reaction is highly exothermic (–1,257 kJ/mol), requiring careful reactor temperature management to avoid over-oxidation to CO₂ and CO. Product MAH is recovered by absorption in a circulating organic solvent (dibutyl phthalate) or by partial condensation, then purified by stripping and fractional distillation to commercial grade. The butane route achieves approximately 53–58 mol% yield from butane and has largely displaced the older benzene oxidation route due to lower feedstock cost, better atom efficiency (no carbon wasted as CO₂), and avoidance of carcinogenic benzene handling.

Q3: Why can maleic acid form an anhydride but fumaric acid cannot?

The key difference is the geometric relationship between the two carboxylic acid groups: maleic acid is the cis-isomer (both –COOH groups on the same side of the C=C double bond), and fumaric acid is the trans-isomer (–COOH groups on opposite sides). For a cyclic anhydride to form, both carboxylate oxygens must be close enough in space to come together and eliminate a water molecule, forming a ring. In cis-maleic acid, the geometry places the two –COOH groups at approximately 2.5 Å proximity - well within the distance needed for 5-membered ring formation. The intramolecular cyclisation to give the 5-membered anhydride ring is geometrically and thermodynamically favourable. In trans-fumaric acid, the two –COOH groups point in opposite directions with a distance of approximately 5–6 Å - too far apart for 5-membered ring formation. Fumaric acid would need to form a much larger ring (which would be unstable) or undergo cis-trans isomerisation before cyclisation. In practice, if you heat fumaric acid strongly enough (>200°C), it first isomerises to maleic acid (thermally), which then dehydrates to maleic anhydride. This is why heating fumaric acid above its sublimation point (287°C) produces small amounts of maleic anhydride - through the maleic acid intermediate.

Q4: What is a Diels-Alder reaction of maleic anhydride and why is it important?

The Diels-Alder reaction is a [4+2] cycloaddition between a conjugated 1,3-diene (the 4π component) and an alkene (the 2π dienophile). Maleic anhydride is one of the most reactive Diels-Alder dienophiles because its C=C double bond is strongly electron-deficient - flanked by two electron-withdrawing carbonyl groups - which makes it highly reactive toward the electron-rich diene. The reaction proceeds as a concerted, thermally allowed pericyclic mechanism that forms two new C–C bonds simultaneously, generating a six-membered ring fused to the anhydride. Industrial importance: (1) Maleated rosin: Abietic acid in natural rosin contains a conjugated diene system; reaction with MAH gives maleated rosin with higher softening point and acid value, used in hot-melt adhesives and printing ink varnishes; (2) Tetrahydrophthalic anhydride (THPA): Butadiene + MAH gives THPA, an important epoxy curing agent for electrical and automotive applications; (3) Nadic anhydride: Cyclopentadiene + MAH gives the endo/exo mixture used to modify high-temperature epoxy resins for aerospace; (4) α-Phellandrene/terpene adducts: Terpene-MAH resins for traffic paint and adhesives. The textbook cyclopentadiene/MAH reaction (endo product preferred by secondary orbital interactions; fast at 25°C) is the single most commonly cited Diels-Alder example in organic chemistry teaching.

Q5: What is MAH-grafted polypropylene and what is it used for?

MAH-grafted polypropylene (MAH-g-PP, also written PP-g-MAH or MAPP) is a functional polyolefin produced by reacting polypropylene with maleic anhydride (typically 0.5–2 wt%) and a peroxide free-radical initiator in a twin-screw extruder at 180–220°C. The peroxide generates carbon radicals on the PP backbone by hydrogen abstraction; these backbone radicals then add across the MAH C=C double bond, installing pendant anhydride groups on the polypropylene chain. The grafting degree (typically 0.3–1.5 wt% MAH) determines the density of anhydride functionality. MAH-g-PP is used as a compatibiliser in three main application families: (1) Glass fibre/mineral-filled PP composites: The anhydride groups on MAH-g-PP react with –OH groups on glass fibre surfaces (via hydrogen bonding and covalent bonds with the silane sizing), dramatically improving interfacial adhesion, tensile strength, and impact resistance of the composite - a standard technology in automotive plastic parts; (2) PA6/PP polymer alloys: When PA6 (nylon) and PP are blended without compatibiliser, the immiscible phases give poor mechanical properties; MAH-g-PP acts as a reactive compatibiliser by reacting its anhydride groups with the terminal –NH₂ groups of PA6 chains in the melt, forming a PA6-co-PP block copolymer at the interface that stabilises the blend morphology - the basis of the PP/PA6 engineering plastic alloy used in automotive and consumer goods; (3) Multilayer film adhesion: MAH-g-PE (polyethylene equivalent) is used as the tie-layer adhesive between incompatible polymer layers (e.g., PE/nylon/PE barrier packaging), where it bonds to polar substrates via anhydride-amine reactions.

Q6: What is the difference between maleic anhydride and phthalic anhydride in UPR synthesis?

Maleic anhydride (MAH, C₄H₂O₃, MW 98) and phthalic anhydride (PA, C₈H₄O₃, MW 148) are both cyclic anhydrides used in unsaturated polyester resin (UPR) synthesis, but they serve fundamentally different structural roles. MAH contributes the crosslinkable C=C double bond - without MAH, the polyester would have no unsaturation to crosslink with styrene, and the resin would not cure. MAH is essential for the thermoset character of UPR. PA contributes rigidity and controls flexibility - phthalic anhydride contains an aromatic ring that imparts stiffness to the polymer backbone but no reactive double bond; it acts as a structural spacer between MAH units. By adjusting the MAH:PA molar ratio, formulators control the crosslink density and therefore the final resin properties: higher MAH ratio = more crosslinks = harder, more brittle, more chemically resistant resin; higher PA ratio = fewer crosslinks = softer, tougher, more flexible resin. Practical formulation: a typical general-purpose UPR uses a MAH:PA molar ratio of approximately 1:1 with propylene glycol as the diol. Marine-grade resins use isophthalic acid (iso-PA) instead of ortho-PA for improved water resistance. For cost reasons, some low-end UPR producers substitute part of the MAH with less expensive adipic acid (no double bond), accepting lower crosslink density and performance.