A Radical reaction is species that has a single and unpaired electron. Since these radicals electrically neutral, they are called as Free radicals. These pairs of electrically neutral free radicals are actually formed by the breakage of hemolytic bond. This bond breakage is formed thro' heating of non-polar solvents. At an elevated temperature these molecular species dissociates into the free radicals. And any chemical reaction that involves these free radicals is called as free radical reaction. This radical reaction is largely found in case of organic reactions.
Following are the important types of radical reaction that involve free radicals.
- Free radical substitution like free radical halogenation and free radical autoxidation.
- Free radical addition reaction.
- Intra molecular free radical reactions which may involve substitution or addition. For instance Hofmann – Loffler reaction which is also called Barton reaction.
- Free radical rearrangement reactions which are rare when compared to rearrangements that involves carbonations. But these radical reactions are restricted to aryl migrations.
Other radical reactions are:
- Fragmentation reactions which is also called homolysis. Some of the fragmentation reactions are Norrish reaction, Hunsdiecker reaction and decarboxylations.
- Another radical reaction is Electron transfer one of which involves the decomposition of peresters by means of Cu (I). It is called as One-electron reduction reaction. One more example for this type of radical reaction is called as Kolbe electrolysis.
- Radical – nucleophilic aromatic substitution is again one of the radical reactions, which is one of the special cases of nucleophilic aromatic substitution.
This type radical reaction that involves aromatic substitution is actually produced by the photochemical reaction and a thermal fission reaction. Or else it can also be formed by means of an Oxidation reaction. Some other special reactions that involves these free radicals are called as combustion reaction, pyrolysis, and cracking. Actually a radical called Triphenylmethyl radical was discovered by Moses Gomberg in 1900 and also an experiment named as lead – mirror experiment was described by a person called Friedrich Paneth in 1927, when pioneering studies had been made on this free radical reaction.
In the last mirror experiment of the radical reaction, at an elevated temperature decomposition of tetramethyllead in a quartz tube lead to the formation of methyl radicals and elemental lead. These gaseous methyl radicals later move to other part of the chamber in the carrier gas. There they get reacted in a mirror film along with the lead and then slowly disappears. When the radical reactions are considered in case of organic synthesis, radical initiators like peroxides or azobis compounds are used to generate the radical reaction.
Mostly the radical reactions are the chain reactions that involve the following three steps:
- A Chain initiation step,
- A Chain Propagation step, and
- A Chain termination step.
A Reaction inhibitor in a radical reaction would make the reaction to slow down its process. Generally the type of radical reactions that occur in the gas phase are more frequent and they are initiated by means of using light and they acid or base catalyzed rarely. They also do not depend on the polarity of the reaction medium. At the same time, whether they may occur in gas phase or solution phase the radical reactions would be normally similar.
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