Activation Energy vs. Charge Transfer Energy

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