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, kobs, linearly dependent on the stoicheiometric concentration of the salt over a large interval (650 fold). With NaClO4, kobs 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.
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