Acid–base properties of highly substituted diaminonaphthalenes

Abstract

The kinetic and equilibrium acid-base behaviour of four new highly hindered diaminonaphthalenes has been studied in aqueous solution or in Me₂SO–H₂O mixtures containing hydroxide ion. In concentrated aqueous sodium hydroxide solutions, 1,8-bis(dimethylamino)-2,7-diethoxynaphthalene and 1,8-bis(diethylamino)-2,7-diethoxynaphthalene are almost fully protonated and aqueous pK_a values of 16.1 and 15.9, respectively, have been measured from comparative studies with other diaminonaphthalenes in 60%(v/v) Me₂SO–H₂O. Studies of the protonation of 1,8-dimorpholino-2,7-dimethoxynaphthalene (pK_a 13.0) and 1,8-dimorpholino-2,7-diethoxynaphthalene (pK_a 12.5) were made in aqueous sodium hydroxide. The reversible acid-base reactions of the diaminonaphthalenes in the presence of hydroxide ion occur slowly. For example the approach to an equilibrium mixture of the protonated and unprotonated forms of 1,8-bis(diethylamino)-2,7-diethoxynaphthalene in 60%(v/v) Me₂SO–H₂O with 0.01 mol dm^–3 NaOH occurs with a half-life of 8s. It is found that 2- and 7-substituents have large effects in increasing the basicity and decreasing the rates of proton transfer and this is due in part to an increase in strain in the amines and in the transition state for proton transfer.