Epoxidation of 2,3-Dimethyl-2-Butene, Conjugated Dienes and 1,5-Hexadiene 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
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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
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.
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,
Arrhenius parameters determined over the temperature
range 373 - 433 K are given in Table 1.
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 cyclohexene
Correlation of Activation Energy with Charge Transfer
Activation energies for the addition of peroxyl radicals to non-cyclic, unsubstituted
mono-alkenes all correlate well with estimates of the energy released by charge
transfer on formation of the transition state (EC).5
The measured activation energy for the relatively unstrained cyclohexene fits in well
with this correlation between EC and activation energy (Figure 1).1
Only cyclohexene could be included in this correlation, as electron affinities for
cyclopentene and cycloheptene have not yet been determined.
Figure 1: Relationship between EC and the activation energy for the epoxidation of
alkenes by: acetylperoxyl radicals, cyclohexene, 9 other monoalkenes.
Other radicals: methylperoxyl: × hydroperoxyl: + iso-propylperoxyl:
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 than either cyclohexene or cis-2-butene.
Geometries for the transition states and isolated reactants calculated using MOPAC 6 with the AM1 hamiltonian6 indicate that radical addition to cyclopentene can release a significant amount of entropy and consequently increase the pre-exponential factor for the reaction.
The transition state for addition to cyclopentene is comparatively early and therefore loose, implying a relatively large pre-exponential factor in comparison with addition to cyclohexene.
The calculated ratio of the A-factors for cyclopentene with respect to cyclohexene (1.8) is comparable with the experimental determination (2.00.9).
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.
(1) Stark, M. S. J. Phys. Chem.1997, 101, 8296. Stark, M. S., J. Am. Chem. Soc.2000, 122, 4162. DMSS would like to that 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.