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Additions
The most common addition reaction which radicals undergo is
addition to C=C bonds.
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Both carbon and halogen centred radicals will attack double
bonds and add to them, and the most important consequence of
this is the radical polymerisation
mechanisms. The general mechanism is given here - it follows
the general scheme for chain reactions
given on the first page. The initiator
radical is abbreviated 'In' - this is not supposed to be indium! |
Substitutions
These can not be classified under specific mechanisms e.g. SN1,
SN2 etc., because the substitution that is referred
to is the overall result of the reaction.
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A classic radical substitution reaction is the Goldfinger
reaction, where an allylic hydrogen is replaced by a
halogen (usually bromine). The important reagent to note is
the N-Bromo-Succinimide ('NBS'),
which in conjunction with HBr produces a constant supply of
Br2 for the reaction. It is also worth noting that
only a small amount of HBr is needed to initiate the reaction
and after that it is produced as a by-product - therefore HBr
is often not mentioned amongst the reagents as it is already
present in a catalytic quantity with the NBS itself. |
The radical initiator (here abbreviated In) is usually some
sort of alkyl initiator - the most commonly used is called AIBN
(azo-iso-butyl-nitrile), which decomposes as shown below
to release N2(g) (a highly stable molecule, which
provides the thermodynamic driving force for the decomposition)
and two radical species.
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