Transcript Addition of Peroxyl Radicals to Alkenes

Addition of Peroxyl Radicals to Alkenes and the Reaction of Oxygen with Alkyl Radicals
Moray Stark
Department of Chemistry
University of York, York, YO10 5DD, UK
Frontier Orbital Description of Radical Addition to Alkenes
Addition of Peroxyl Radicals to Alkenes
One of the most studied examples of radical reactions with alkenes is peroxyl radical addition, investigated in detail by Waddington et al.
(acetyl and alkylperoxyl radicals) and Baldwin and Walker et al.
eg.2
eg.1
(hydroperoxyl radicals).
k1
k2
Hydroperoxyl + Ethene
k-1
Transition State
Hydroxyl + Ethene Oxide
Epoxides are the dominant product of these reactions, with the initial addition being the rate determining step (k2 >> k-1).1,2
Activation Energy vs. Alkene Ionisation Energy
Adduct
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This dependence of activation energy for the
However, the radical in the 2A’ first
addition reaction on the energy of the first
excited state correlates with the
excited state of the radical is what would be
2A’
expected if the radical and alkene approached
point (an ROHF representation of this
in the same plane (Cs symmetry).
is shown here).
ground state of the Cs saddle
The peroxyl radical 2A” ground state correlates
with an energetically unfavourable 2A” excited
state of the Cs saddle point for the addition.
Conical Intersections and Activation Energies
It is well known that one of the factors controlling
This identifies the addition as an
If the radical and alkene approach in the
The conical intersection guarantees
the rate of addition of peroxyl radicals to alkenes
electrophilic reaction; the greater
same plane (Cs symmetry), then the 2A”
that for a particular relative orientation
is the ionisation energy of the alkene; the reaction
the charge transfer to the radical
and 2A’ surfaces cross at a conical
and separation of the radical and
being faster the lower the alkene ionisation energy.1,2
during the reaction, the faster the
intersection.
alkene, the energy of the system must
be higher than the reactants.
rate.
The transition state for the addition
reaction has C1 symmetry, and is
The proximity of the transition state to
reached from the conical intersection
the conical intersection ensures that it
by moving along the symmetry breaking
too has an energy higher than the
co-ordinate of the branching space.
reactants, with a barrier height related to
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Charge Transfer During Reaction
the energy of the first excited state of
the radical.
The parabola model of Pearson and Parr allows
Shown here is how the system’s
the degree of charge transfer at the transition
energy varies with charge transfer
state for the addition to be estimated by using
for the addition of acetylperoxyl
the ionisation energies (I) and electron affinities
radicals to 2-methyl-2-butene.
Reaction of Oxygen with Alkyl Radicals
Alkyl radicals react with oxygen to give alkylperoxyl radicals
Ab-initio studies have shown that
The energy released (EC) by the
which at low pressure or high temperature can decompose to
alkylperoxyl radicals decompose
charge transfer (NC) can be viewed
an alkene and hydroperoxyl radical.
directly to the alkene + HO2 via a
(A) of the isolated reactants.3
2A”
as the driving force for the reaction.
The energy released by charge transfer to
the radical as it approaches the alkene (EC)
However, there has been controversy over the mechanism of
hydroperoxylalkyl radical as once
this decomposition, as the reverse reaction, HO2 + alkene, gives
thought.8
the epoxide + OH and not the alkylperoxyl radical as might be
Activation Energy vs. Charge Transfer Energy
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transition state and not via the
expected on the grounds of reversibility.eg.2,7
with the above description for
The relationship also suggests that
peroxyl radical addition to alkenes.
the activation energy for a reaction
correlates very well with the activation energy
involving no charge transfer would
for the reaction4 (as indeed do data for nitrate
be 80 - 90 kJ mol-1.
This mechanism is combined here
radical addition).5
Coincidentally, 80 - 90 kJ mol-1 is
Potential Energy Diagram for Ethyl + Oxygen
This suggests that the energy released by
also typical of the energy required to
charge transfer lowers the barrier for the
promote these radicals to their first
Discussion of this class of reaction has centred on
A potential energy diagram for this
addition approximately in proportion,
electronically excited states.6
the most studied example, ethyl + oxygen , with
system is suggested here, based on
disagreement over not just the mechanism, but
the above mechanism, and with
also of barrier heights for key steps in the reaction.
barrier heights chosen to be as
becoming barrierless for EC  60 kJ mol-1.
References
(1)
(2)
(3)
(4)
Ruiz Diaz, R.; Selby, K.; Waddington, D. J. J. Chem. Soc. Perkin Trans. 2 1977, 360.
(5) Wayne, R. P. et al. Atmos. Environ. 1991, 25A, 1.
Stothard, N. D.; Walker, R. W. J. Chem. Soc. Faraday Trans. 1990, 86, 2115.
(6) Stark, M. S. J. Am. Chem. Soc. 2000, 122, 4162.
Parr, R. G.; Pearson, R. G. J. Am. Chem. Soc. 1983, 105, 7512.
(7) Wagner, A. F.; Slagle, I. R.; Sarzynski, D.; Gutman, D. J. Phys. Chem. 1990, 94, 1853.
Stark, M. S. J. Phys. Chem. 1997, 101, 8296.
(8) Quelch, G. E.; Gallo, M. M.; Schaefer, H. F. J. Am. Chem. Soc. 1992, 114, 8239.
compatible as possible with
experimental observations.