Specific salt effects on the exchange reaction between 4,4′-dimethyldiphenylmethyl thiocyanate and ionic thiocyanate in acetone

Abstract

Rates of exchange of 4,4′-dimethyldiphenylmethyl thiocyanate and MeSCN (Me = Li, Na, Buⁿ₄N) have been measured in acetone at 25°. With NaSCN the kinetics have also been investigated in the presence of a large amount of NaClO₄, in all runs the total concentration of electrolytes, (NaSCN)+(NaClO₄) being kept constant at 0·1 or 0·03M. With no salt other than MeSCN is the first-order rate coefficient, k_obs, linearly dependent on the stoicheiometric concentration of the salt over a large interval (650 fold). With NaClO₄, k_obs is correlated with the stoicheiometric concentration of the nucleophile by a more complex kinetic expression. From the order of reactivity of different salts, LiSCN > NaSCN > Buⁿ₄NSCN, and the form of the kinetic equation a mechanism for the reaction is suggested which includes salt-assisted as well as solvent-assisted ionization paths. It is suggested that electrophilic assistance by salts leads to ion quadruplets which may return to covalent material without undergoing exchange or react with the nucleophilic salt to undergo exchange.