GEM EFFECT .4. ACTIVATION PARAMETERS ACCOMPANYING INCREASED STERIC REQUIREMENTS OF 3,5' SUBSTITUENTS IN SOLVOLYSIS OF MONO-P-BROMOPHENYL GLUTARATES

The kinetics for the solvolyses of 11 p-bromophenyl 3-R- and 3,3′-R,R′-glutarate monoesters with R and R’ groups of differing steric requirements have been studied at several temperatures. The values of ΔH± have been found, within experimental error, to be independent of the nature of R and R’ so that steric acceleration is reflected in TΔS±. The values of TΔS± are in accord with intramolecular nucleophilic displacement of p-bromophenoxide by the carboxyl anion. An explanation is offered for the observed change of mechanism from intramolecular nucleophilic attack when a cyclic anhydride can be formed to intramolecular general base assisted attack of water when the cyclic anhydride cannot be formed. © 1968, American Chemical Society. All rights reserved.