Addition of Acetylperoxyl to 2,3-Dimethyl-2-Butene

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
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).
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).
Transition State for Acetylperoxyl Addition to
1,3-Butadiene
Transition State
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.
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.

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
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 acetlyperoxyl to alkenes
previously found.1
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. 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 Wi
th 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. 2 1977, 360. (2) Baldwin, R.
R. Stout, D. R. Walker, R. W. J. Chem. Soc
Faraday Trans. 1 1984, 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. Data 1997, 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.