Addition of Acetylperoxyl to 2,3-Dimethyl-2-Butene The first example of addition of oxygen

1. 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 Addition of Acetylperoxyl to 2,3-Dimethyl-2-Butene The first example of addition of oxygen centred radicals to alkenes to be investigated was for acetylperoxyl addition. eg.1 The variation of rate of reaction with the ionisation energy of the alkene identified the reaction as an electrophilic addition.1 However, the most polar of this class of reaction, the addition of acetylperoxyl to 2,3-dimethyl-2-butene has not previously been examined. This reaction was studied here over the temperature range 393 to 433 K, and Arrhenius parameters found (Table 1). Addition of Acetylperoxyl to Dienes To examine how radical addition to dienes differs from addition to unsubstituted mono-alkenes, Arrhenius parameters for the reaction of acetylperoxyl radicals with three conjugated and one unconjugated diene were determined (Table 1). Transition State for Acetylperoxyl Addition to 1,3-Butadiene Transition State This is perhaps surprising, considering that the resultant adduct radical is resonance stabilised. The activation energy for addition to 1,3-butadiene is in fact comparable to values for terminal mono-alkenes, in spite of having a lower ionisation energy. Activation Energy vs. Alkene Ionisation Energy The activation energy for addition of acetylperoxyl radicals to 1,3-butadiene is higher than would be expected from the relationship between alkene ionisation energy and activation energy for addition to unsubstituted mono-alkenes. Appropriate Structure Activity Relationships for Radical Addition to Alkenes Consideration of just the ionisation energy of the alkene can be misleading. The value for 1,3-butadiene is lower than, for example, that for propene. However, the electron affinity of 1,3-butadiene is also lower than that of propene, so the electronegativities for both are comparable. The difference in electronegativities between the alkene and the attacking radical controls the rate of addition, so peroxyl radical addition to 1,3-butadiene has a similar activation energy to that for propene. This is shown graphically here (the gradient for a zero charge transfer represents the absolute electronegativity).6 The addition shows no sign of steric hindrance, in fact the pre-exponential factor is slightly larger than for other peroxyl radical addition reactions. Activation Energy vs. Alkene Ionisation Energy This work on 2,3-dimethyl-2-butene now extends the reactions investigated to cover alkenes with ionisation energies ranging from 8.3 to 9.7 eV. The measured barrier for this reaction conforms with the correlation between alkene ionisation energy and the activation energy for addition of acetylperoxyl to alkenes previously found.1 Activation Energy vs. Charge Transfer Energy The activation energy for addition to 1,3-butadiene is quite consistent with the correlation between activation energy for the addition of peroxyl radicals to mono-alkenes and the energy released by charge transfer to the radical (EC).7 This demonstrates the need to also consider the electron affinity of the alkene, and not just its ionisation energy, when examining its reactivity. Activation Energy vs. Radical Electonegativity With this measurement, Arrhenius parameters are now available for a wide range of peroxyl radicals attacking the one alkene.1-3 The difference in electronegativity between the radical and the alkene can be considered to control the rate of the addition. The relationship between radical electronegativity and activation energy for addition to 2,3-dimethyl-2-butene is given here. As a comparison, values for two other oxygen centred species (ozone4 and the nitrate radical5) are also given. They also fall on the same correlation as the peroxyl radicals. References (1) Ruiz Diaz, R.; Selby, K.; Waddington, D. J. J. Chem. Soc. Perkin Trans. 21977, 360. (2) Baldwin, R. R.; Stout, D. R.; Walker, R. W. J. Chem. Soc Faraday Trans. 11984, 80, 3481. (3) Stark, M. S. J. Phys. Chem.1997, 101, 8296. (4) Wayne, R. P. et al. Atmos. Environ. 1991, 25A, 1. Acknowledgements DMSS would like to thank the EPSRC for funding this work. (5) Atkinson, R. J. Phys. Chem. Ref. Data1997, 26, 215. (6) Parr, R. G.; Pearson, R. G. J. Am. Chem. Soc.1983, 105, 7512. (7) Stark, M. S., J. Am. Chem. Soc.2000, 122, 4162.