C4H4O42

C4H4O42 is not a standard neutral formula; the well-established neutral formula is C4H4O4, which corresponds to two closely related dicarboxylic acids: maleic acid (cis-butenedioic acid) and fumaric acid (trans-butenedioic acid). These two isomers share the same molecular formula but differ in the geometry around their carbon–carbon double bond.

Structure and isomerism: Both acids consist of a four-carbon skeleton with two adjacent carboxyl groups and a central C=C bond. In maleic acid the carboxyl groups are on the same side of the double bond (cis), while in fumaric acid they are on opposite sides (trans). This geometric difference leads to distinct physical properties and reactivities, though both are categorized as unsaturated dicarboxylic acids.

Properties and occurrence: Fumaric acid is a white, crystalline solid with a high melting point around 286–287 °C and limited water solubility; maleic acid melts around 135–140 °C and is more soluble in water. Both occur in small natural amounts in certain plants and fungi and are produced commercially, typically starting from butadiene-derived precursors via maleic anhydride, followed by hydrolysis and, if desired, isomerization to the other form. In many contexts, the active species are the fumarate or maleate ions in salts rather than the free acids themselves.

Uses and reactions: They are used as acidity regulators in foods (fumaric acid is labelled as E297 in some regions) and as chemical intermediates. They serve as monomers or co-monomers in the production of unsaturated polyesters and various ester derivatives; their salts (e.g., sodium fumarate, potassium fumarate) are common in chemical and pharmaceutical applications.

Dianion and related forms: If both acidic hydrogens are removed, the dianion C4H2O4^{2-} forms, known in salt form as fumarates or maleates; fumarate also appears as an essential metabolic intermediate in the citric acid cycle.

Safety: Both acids are irritants and can cause skin, eye, or respiratory irritation; handling should follow standard chemical safety practices.