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 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
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).
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 abs
olute 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. peroxy
l 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 Wi
th 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. 2 1977, 360. (5) Atkinson, R. J.
Phys. Chem. Ref. Data 1997, 26, 215. (2)
Baldwin, R. R. Stout, D. R. Walker, R. W. J.
Chem. Soc Faraday Trans. 1 1984, 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.