ISOTOPE EFFECTS IN NUCLEOPHILIC-SUBSTITUTION REACTIONS .9. THE EFFECT OF BOND STRENGTH ON SUBSTITUENT EFFECTS ON THE STRUCTURE OF S(N)2 TRANSITION-STATES

The effects of substituents on the structure of S(N)2 transition states suggested by kinetic isotope effects and Hammett p values are often different and, moreover, often do not agree with substituent effects predicted by current theories whether the change in substituent is made in the nucleophile, in the leaving group, or at the oc-carbon. The importance of the strength of the reacting bonds in determining the effects of substituents on transition-state structure is investigated. A bond strength hypothesis that suggests there will be a significant change in the weaker reacting bond but little or no change in the stronger reacting bond in an S(N)2 transition state when a substituent in the nucleophile, the substrate, or the leaving group is altered in an S(N)2 reaction, predicts a high percentage of the experimental results.