Colour in relation to chemical constitution of the phthalein dyes. Phthaleins of the mixed type

Abstract

1. Mixed or unsymmetrical phthaleins obtained by the condensation of pureo-benzoyl-benzoic acid with aromatic phenols and amino-phenols, many of which have been prepared for the first time, have been shown to have much less intense colour development on solution in alkali than pure or symmetrical phthaleins obtained by the condensation of the same phenols and aminophenols with phthalic anhydride. 2. The cause of the above interesting phenomenon has been traced to the existence of less intense tautomerism between the lactonoid and the quinonoid forms of the unsymmetrical phthaleins as compared with those of the symmetrical phthaleins. In some cases the tautomerism only begins to make itself manifested under drastic circumstances, that is in hot solutions of concentrated alkali. 3. The colour phenomena exhibited by the unsymmetrical and symmetrical phthaleins on treatment with alkali are quite comparable with similar phenomena exhibited by isonitroso-1:3-diphenyl-thiohydantoin and isonitroso-1:3-diphenylthiobarbituric acid under identical conditions as studied by Dutt and his pupils, and due to the same cause.