Correlation of Activation Energy with Charge Transfer

1. Epoxidation of Cyclopentene, Cyclohexene and Cycloheptene by Acetylperoxyl Radicals J. R. Lindsay Smith, D. M. S. Smith, M. S. Stark and D. J. Waddington Department of Chemistry University of York, York, YO10 5DD, UK Introduction To further the current understanding of how radicals add to C=C double bonds,eg. 1-3 a series of cyclic alkenes (cyclopentene, cyclohexene and cycloheptene) were reacted with acetylperoxyl radicals. The example given here is for CH3C(O)O2 addition to cyclopentene, which ultimately forms the product cyclopentene oxide. Transition State Adduct Cyclopentene Oxide AM1 Geometries for the Addition of Acetylperoxyl Radicals to Cyclopentene Experimental and Results These reactions were studied via the co-oxidation of acetaldehyde (to produce acetylperoxyl radicals), the cyclic alkene and a previously studied reference alkene, cis-2-butene. Arrhenius parameters determined over the temperature range 373 - 433 K are given in Table 1. Rate parameters for cis-2-butene are also given for comparison,4 as it would be expected that a relatively unstrained cycloalkene would behave in a similar manner to this non-cyclic alkene. This is indeed found to be so, with no statistically significant difference between the parameters for cyclohexene and cis-2-butene. Correlation of Activation Energy with Charge Transfer Activation energies for the addition of peroxyl radicals to The measured activation energy for the non-cyclic, unsubstituted mono-alkenes all correlate well relatively unstrained cyclohexene fits in with estimates of the energy released by charge transfer well with this correlation between EC and on formation of the transition state (EC).2-5 activation energy.1 Only cyclohexene could be included in this correlation, as electron affinities for cyclopentene and cycloheptene have not yet been determined. Effect of Ring Strain on A-factors Cyclopentene, which is planar and consequently highly strained, has a significantly larger pre-exponential factor and activation energy for its reaction with acetylperoxyl than either cyclohexene or cis-2-butene. Geometries for the transition states and isolated reactants calculated at the AM1 level6 indicate that radical addition to cyclopentene can release a significant amount of entropy and consequently increase the pre-exponential factor for the reaction. The calculated ratio of the A-factors for cyclopentene with respect to cyclohexene is comparable with the experimental determination. An alternative interpretation is that the transition state for addition to cyclopentene is comparatively early with the C-O bond length being slightly longer than for cyclohexene. Consequently, the transition state is looser, consistent with a relatively large pre-exponential factor in comparison with addition to cyclohexene. Effect of Ring Strain on Activation Energy The degree of charge transfer at the transition state is determined by the degree of overlap between the free electron orbital of the radical, and the  orbital of the C=C double bond. Everything else being equal, the longer the C-O bond at the transition state, the less the overlap, hence there is less charge transfer and less energy released by this charge transfer. Consequently the barrier for radical addition to cyclopentene is higher than for addition to cyclohexene by 148 kJ mol-1. References Acknowledgements (1) Stark, M. S. J. Phys. Chem.1997, 101, 8296; Stark, M. S., J. Am. Chem. Soc.2000, 122, 4162. DMSS would like to thank the EPSRC for financial support. (2) Ruiz Diaz, R.; Selby, K.; Waddington, D. J. J. Chem. Soc. Perkin Trans. 21977, 360. (3) Baldwin, R. R.; Stout, D. R.; Walker, R. W. J. Chem. Soc. Faraday Trans. 11984, 80, 3481. (4) Ruiz Diaz, R.; Selby, K.; Waddington, D. J. J. Chem. Soc. Perkin Trans. 21975, 758. (5) Parr, R. G.; Pearson, R. G. J. Am. Chem. Soc.1983, 105, 7512. (6) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. J. Am. Chem. Soc.1985, 107, 3902.