Process for the preparation of 2,5-di(phenylamino)-terephthalic acid and its dialkyl esters in high purity

A process for the preparation of 2,5-di(phenylamino) terephthalic acid, or its dialkyl esters, of formula (I):

• ##STR4## in which R is a hydrogen atom or a methyl group and R' is a hydrogen atom or a methyl or ethyl group, said process comprising (1) reacting a dialkyl(C.sub.1 -C.sub.2) succinate with a sodium alcoholate in xylene, in the manner of a Dieckmann condensation, to give the disodium salt of the dialkyl (C.sub.1 -C.sub.2)2,5-dihydroxycyclohexadiene-1,4-dicarboxylate, (2) reacting the resulting condensation product, after decomposition of the disodium salt with acid, with a phenylamine of formula (II):
• ##STR5## in which R is as defined above, in the presence of an organic acid, in xylene, to give the dialkyl(C.sub.1 -C.sub.2) 2,5-di(phenylamino)-3,6-dihydroterephthalate, (3) dehydrogenating (oxidizing) the resulting 1,4-cyclohexadiene derivative with oxygen to give the corresponding dialkyl(C.sub.1 -C.sub.2) 2,5-di(phenylamino)-terephthalate, said steps (1) through (3) being optionally carried out without isolation of any of the intermediate products, and then (4) optionally saponifying the resulting dialkyl ester in methanolic sodium hydroxide solution to give the corresponding disodium 2,5-di(phenylamino) terephthalate, and (5) optionally liberating the 2,5-di(phenylamino) terephthalic acid from said disodium salt with acid, and wherein said process further comprises carrying out the oxidation in stage (3) in such a way that the reaction mixture of stage (2), in the form of a solution or suspension containing solid particles, is blanketed with nitrogen in a stirred vessel, this reaction mixture is circulated by pumping and the oxygen is metered into a partial volume of the circulated reaction mixture in such a way as to produce effective thorough mixing.