The Role of Weak Bases in Bimolecular Elimination Reactions

Abstract

It has previously been found that mercaptide ions have a greater kinetic affinity for a proton in bimolecular elimination (E2) reactions then more strongly basic alkoxide or hydroxide ions. This apparent anomaly is discussed and. is shown to be in accord with the nucleophilic properties of mercaptide ions towards other elements such as carbon. The number of substrates undergoing bimolecular elimination reactions with mercaptide ions (thiophenoxide in particular) is extended, and it is found that thiophenoxide is more or less reactive than alkoxide ions in alcoholic media depending on the nature of the substrate. There is a correlation between the relative reactivities of these two bases towards a given substrate and the nature of the transition state for E2 reaction. The Brönsted relationship is employed as a diagnostic tool of transition state character, and the results are in accord with presently accepted views on the effect of substrate structure on E2 transition state character.