Process for the production of unsubstituted or polysubstituted o-phthalaldehydes

A process for the production of an unsubstituted or substituted o-phthalaldehyde of the formula I ##STR3## wherein R.sub.1 and R.sub.2, each independently of the other, represent hydrogen, lower alkyl, hydroxyl, halogen, nitro, cyano or carboxyl, which comprises hydrolysing at a pH-range between 4-11 an unsubstituted or substituted .alpha.,.alpha.,.alpha.',.alpha.'-tetrahalogeno-o-xylene of the formula II ##STR4## wherein R.sub.1 and R.sub.2 are as defined for formula (I) and X represents a halogen atom, with an alkali metal salt, alkaline earth metal salt or ammonium salt of a lower alkanecarboxylic acid or of an aromatic carboxylic acid in the presence of an optionally substituted tetra lower alkyl ammonium as phase transfer catalyst and of an alkali metal carbonate or alkaline earth metal carbonate at a temperature above 60.degree. C.