<unk>  (ionised maleate in biology) or (Z)-butenedioic acid or cis-butenedioic acid or<unk>  or<unk> ic acid or toxilic acid is an organic compound which is a dicarboxylic acid (molecule with two carboxyl groups). The molecule consists of an ethylene group flanked by two carboxylic acid groups.<unk>  is the cis isomer of butenedioic acid, whereas fumaric acid is the trans isomer.  The cis isomer is the less stable one of the two; the difference in heat of combustion is 22.7 kJ/mol. The physical properties of<unk>  are very different from that of fumaric acid.<unk>  is soluble in water whereas fumaric acid is not and the melting point of<unk>  (130 - 139 °C) is also much lower than that of fumaric acid (287 °C). Both properties of<unk>  can be explained on account of the intramolecular hydrogen bonding that takes place at the expense of intermolecular interactions.<unk>  should not be confused with<unk>  or malonic acid, both of which are different types of dicarboxylic acids.
Maleic acid
Tartaric acid
Meglutol
Citicoline
Meconic acid
Mandelic acid
Stearic acid
Cyanic acid