A Qualitative Valence-Bond Approach to Organic Reactivity. Application to Elimination Reactions

A general method for building up the reaction profile for the entire range of elimination reactions, using simple valence-bond configurations, is described. Rules which enable the method to be applied to any organic reaction are presented. The approach enables predictions to be made regarding the nature of the transition state, the possible intervention of intermediates along the reaction pathway, and the effect of substituent changes on rates and reaction mechanism. Application of the model to elimination reactions leads to conclusions in general agreement with current views. A number of new features, however, are observed. For example, the possibility of an elimination mechanism not yet observed, involving a radical-anion intermediate, is deduced, and a theoretical justification for the E2C-E2H mechanism is presented. Also, the concertedness of C-H and C-X bond cleavage in proper E2 substrates is questioned.