Publication
579
J. Electroanal.
Chem., 564, 99-113, 2004
DOI: 10.1016/j.jelechem.2003.10.011 |
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Dimerization of electrochemically generated ion radicals: mechanisms
and reactivity factors
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Cyrille
Costentin and Jean-Michel Savéant
Laboratoire d'Electrochimie Moléculaire,
(CNRS UMR 7591), Université Paris 7, 2 place
Jussieu, Case 7107, 75251, Paris cedex 05, France
Electrohydrodimerization of activated olefins, unlike
other families of compounds, has given rise to vigorous
controversies as to the radical–radical character
of the coupling of the anion radicals as originally
proposed. Careful examination of the facts shows
that the original proposals were correct.
In ion radical dimerizations, the reactivity is
governed by the interplay between three factors,
bond formation, coulombic repulsion, and solvation.
The latter factor is essential to counteract coulombic
repulsion, making exergonic or slightly endergonic,
a reaction that would be strongly endergonic in
the gas phase. Solvation plays also a central role
in the kinetics of the reaction. Thanks to the
ensuing energy gain, the activation energy may
be very small. The payback is however a strong
negative contribution of solvation to the entropy
of activation making the reaction much slower than
the diffusion limit, unlike what happens with simple
neutral radicals. These various features are illustrated
with the example of the electrodimerization of
acrylonitrile in a polar solvent such as N,N-dimethylformamide
or acetonitrile, with the help of quantum chemical
calculations. |