Benzoquinone imines. Part VI. Mechanism and kinetics of the reaction of p-benzoquinone di-imines with m-phenylenediamines

Oxidation of mixtures of a p-phenylenediamine and a m-phenylenediamine is shown to give aminoindamine dyes by a reaction between the p-benzoquinonedi-imine and the m-diamine. At pH > 7 the latter reaction involves electrophilic attack by the conjugate acid of the di-imine on the electron-rich carbon [C(4)] of the m-diamine as the rate-controlling step. The resulting 2,4,4′-triaminodiphenylamine then undergoes rapid oxidation by a second molecule of the di-imine to give the dye. At pH < 7 attack on the m-diamine by the diprotonated di-imine makes a significant contribution to the reaction. C-Methylation of the di-imine reduces the rate of coupling, while C-methylation of the m-diamine increases the rate. In the indamine dyes, methylation of the benzenoid ring derived from the p-diamine results in a bathochromic shift of the visible absorption band, while methylation of the quinonoid ring derived from the m-diamine produces a small hypsochromic shift.