The limiting factor to account for the increased rate of intramolecular reactions between functional groups as opposed to their intermolecular counterparts can and has been explained both by proximity effects and by activation energy. Neither explanation has emerged as the single most important reason in all or even the majority of cases studied. We have therefore reexamined the spatiotemporal hypothesis of Menger and the transition-state energy approach of Houk on a consistent set of compounds subjected to the Barton oxidation or related reactions in an effort to more clearly define the reasons for the proximity effect. For the 26 structures studied, neither hypothesis provides a consistent, quantitative explanation although the transitionstate energy hypothesis offered the most promise. 0 1993 by John Wiley & Sons, Inc. *Several theories have been put forward to account for rate acceleration in intramolecular and enzymatic reactions. Jencks"" suggested that rate enhancements of a maximum of 55-fold could occur when two reactants were held in the correct proximity. Other theories include the "orbital steering" effect of Koshland'Ih and entropic effects by Carothers'l' and Jencks.'ld