Exceptional basic strength of 1,8-bis(dimethylamino)- and 1,8-bis(diethylamino)-2,7-dimethoxynaphthalenes: kinetic and equilibrium studies of the ionisation of the protonated amines in Me2SO–H2O mixtures with hydroxide ion

Abstract

Exceptionally high pK_a values of 16.1 and 16.3 have been obtained for 1,8-bis(dimethylamino)- and 1,8-bis(diethylamino)-2,7-dimethoxynaphthalene respectively from measurements of the equilibrium between the protonated amines and hydroxide ion in Me₂SO–H₂O mixtures. These values make the amines more basic by four pK_a units than 1,8-bis(dimethylamino)naphthalene. This may be a result of an increase in strain in the free amines and of a strengthening of the intramolecular hydrogen bond in the protonated amines. Proton transfer from protonated 1,8-bis(dimethylamino)-2,7-dimethoxynaphthalene to hydroxide ion in 35%(v/v) Me₂SO–H₂O occurs in the millisecond range (k_OH– 110 dm³ mol^–1 s^–1) but for 1,8-bis(diethylamino)-2,7-dimethoxynaphthalene the reaction is extremely slow with half-lives in the range of minutes in 50%(v/v) Me₂SO–H₂O (k_OH– 0.18 dm³ mol^–1 s^–1). The steady increase in the rate coefficient for proton transfer from protonated 1,8-bis(diethylamino)-2,7-dimethoxynaphthalene to hydroxide ion over the solvent range 50–90%(v/v) Me₂SO–H₂O is due to a weakening of the intramolecular hydrogen bond in the protonated amine.