Polar and Steric Contributions of Saturated Substituents to the Acid-Base Equilibria of Aqueous Amines

Abstract

A simple algebraic technique permits an unambiguous separation of polar and steric contributions of saturated substituents to the modification of the fundamental acidity of substituent ammonium ions. Two assumptions are made: (1) an empirical mathematical relationship between the acidic dissociation constant and the polar and steric factors for the substituents, and (2) the steric effect of a hydrogen atom is constant in all types of amine. Extensive data then show that (a) each substituent group possesses a characteristic polar and steric, factor, (b) the polar effects obey a purely additive combination law, and (c) the steric effects of a given substituent are accumulative and consist of the product of a characteristic steric factor and the total number of substituents borne by the nitrogen atom. The substituent factors may be used to compute the acidic ionization constant of any secondary or tertiary ammonium ion for whose substituents the corresponding factors are known. It is to be noted that these factors represent the first system in which polar and steric effects of substituents attached directly to a site of reaction have been calculated.