Medium effects on tetraalkylammonium ions in aqueous acid solutions

Abstract

. Medium effects upon 1H and '3C chemical shifts have been determined for some tetraalkylammonium ions in 0-10 M aqueous solutions of H2S04, HCIO,, and H3P04. The observed effects are very weak in magnitude, thus justifying the choice of tetraalkylammonium ions as reference standards for studies of ionic species in aqueous solutions of strong acids. The trends in the observed low-field shifts are interpreted in terms of changes in cation hydration and in hydrogen bonding between alkyl hydrogens and acid anions.TOMASZ A. MODRO, WILLIAM F. REYNOLDS et KEITH YATES. Can. J. Chem. 54,1439Chem. 54, (1976. On a dCterminC les effets du milieu sur les dCplacements chimiques du ' H et du 13C pour quelques ions tCtraalkylammonium dans des solutions aqueuses de &I0 M e n H2S04, HCIO, et H3P04. Les effets observCs sont de trks faible amplitude ce qui justifie le choix des sels des ions tCtraalkylammonium comme produits de rCfCrence pour des Ctudes d'espkces ioniques dans des solutions aqueuses d'acides forts. On interprkte les tendances observCes dans les dCplacements B bas champs en termes de changements dans l'hydratation du cation et dans les liens hydrogknes entre les hydrogknes des groupes alkyles et les anions des acides.[Traduit par le journal]A knowledge of the behavior of activity coefficients of cationic species is of primary importance in studies of acid-base equilibria and acid-catalyzed reactions in nondilute solutions.' In the acidity function approach, the medium dependence of the activity coefficients of the indicator base (f,) and its conjugate acid CfBH+) provide a key to the understanding of the very individual behavior of different classes of organic bases in their protonation equilibria. For acid-catalyzed reactions it has been demonstrated (2) that it is possible to determine the acidity-activity coefficient dependence for the transition state of the rate-determining reorganization of the protonated substrate for some reactions in strongly acidic media. In this case, the activity coefficient under consideration, .fS, also involves an ionic system and its behavior can be directly coinpared with those of various stable cationic species of related structure. Since the activity coefficients of single ions are experimentally inaccessible, the generally accepted approach introduced by Boyd (3) involves the use of standard reference ions, and presents the ion activity coefficient relative to the reference ion, i.e.where X+ is the standard reference cation. The most commonly used reference cation is the tetraethylammonium ion (TEA+) which is expected to be relatively free of medium-dependent specific solute-solvent interactions? This rather intuitive choice can be, in principle, tested experimentally by examining medium effects on the TEA+ ion in aqueous solutions of strong acids and comparing them with similar effects on some structurally related ions. It is known (1) that the activity coefficients of some tetraalkylammonium ions, relative to that of TEAf, vary significantly with a change of acidity for aqueo...