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
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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
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
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
(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.