Phthalaldehyde

o-Phthalaldehyde is the chemical compound with the formula C6H4(CHO)2. Often abbreviated OPA, the molecule is a dialdehyde, consisting of two formyl (CHO) groups attached to adjacent carbon centres on a benzene ring. This pale yellow solid is a building block in the synthesis of heterocyclic compounds and a reagent in the analysis of amino acids.

Synthesis
The molecule was first described in 1887 when it was prepared from &alpha;,&alpha;,&alpha;',&alpha;’-tetrachloro-ortho-xylene. A more modern synthesis is similar: the hydrolysis of the related tetrabromoxylene using potassium oxalate, followed by purification by steam distillation. The reactivity of OPA is complicated by the fact that with water it forms both a mono- and dihydrate, C6H4(CHO)(CH2OH) and C6H4(CH2OH)2, respectively.

Isomeric phthalaldehydes
Related to o-phthalaldehyde are the meta and para isomers, which are respectively named isophthalaldehyde (m.p. 87-88 °C, CAS# 626-19-7) terephthalaldehyde (m.p. 114-116 °C, CAS# 623-27-8).