Application of linear free energy relationships to elucidate E2 transition state structure

2 Application of linear free energy relationships to elucidate E2 transition state structure 

Kinetic isotope effects provide useful information concerning the transition state structure of E2 reactions but they suffer from some serious shortcomings. Not all elements exist in more than one stable isotopic form and isotope effects for heavier atoms are extremely small and cannot be measured using presently available experimental techniques. Moreover, even carbon and hydrogen isotope effects can give ambiguous results. An alternative approach makes use of the linear free energy relationship concept, namely the relation of rate or equilibrium constant of an unknown reaction to those of known standard reactions. 

Rates of elimination of substrates possessing suitably substituted alpha or beta phenyl groups can be correlated by the Hammett equation , viz. 

In all solvents, the p value for the ammonium salts is greater than for the sulphonium salts, a result in agreement with the isotope studies (Section 2.2.1) which show that elimination from the former proceeds through a more carbanion-like E2 transition state. 

HAMMETT REACTION CONSTANTS FOR ELIMINATION FROM 2-PHENYLETHYL AND RELATED 

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