Everything about Free Radical Reaction totally explained
A
free radical reaction is any
chemical reaction involving
free radicals. This reaction type is abundant in
organic reactions.
Two pioneering studies into free radical reactions have been the discovery of the
triphenylmethyl radical by
Moses Gomberg (1900) and the
lead-mirror experiment described by
Friedrich Paneth in 1927. In this last experiment
tetramethyllead is
decomposed at elevated temperatures to methyl radicals and elemental lead in a
quartz tube. The gaseous methyl radicals are moved to another part of the chamber in a carrier gas where they react with lead in a mirror film which slowly disappears.
When radical reactions are part of
organic synthesis the radicals are often generated from
radical initiators such as peroxides or azobis compounds. Many radical reactions are chain reactions with a
chain initiation step, a
chain propagation step and a
chain termination step.
Reaction inhibitors slow down a radical reaction. Radical reactions occur frequently in the gas phase, are often initiated by light, are rarely acid or base catalyzed an are not dependent on polarity of the reaction medium . Reactions are also similar whether in the gas phase or solution phase .
Kinetics
The
chemical kinetics of a radical reaction depend on all these individual reactions. In
steady state the concentrations of initiating (I
.) and terminating species T
. are negligent and rate of initiation and rate of termination are equal. The overall
reaction rate can be written as :
»
with a
broken-order dependence of 1.5 with respect to the initiating species.
The reactivity of different compounds toward a certain radical is measured in so-called competition experiments. Compounds bearing
carbon-hydrogen bonds react with radicals in the order
primary <
secondary >
tertiary <
benzyl <
allyl reflecting the order in C-H
bond dissociation energy
Further Information
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