Arylation(XXXVI)that it is impossible to work with them in sunlight. It should, how
ever, be emphasised that this autoxidation is an addition rather than
a displacement reaction.
(Hi) Arylation. Phenyl or other aryl radicals can take part in a
number of rMBffons, one of the most common being the Sandmeyer
reaction of a diazonium salt with cuprous chloride or bromide as a
complex ion, e.g. CuCl 2 e. Here the aryl radical is believed to be
generated by an electron transfer, coupled with loss of nitrogen
ArlV + CuCl,^9 -+ Ar'+N.+ CuQ.which is then followed by a displacement reaction on a chlorine atom
of the cupric chloride:
Ar-+CI—Cu—CI -t- Ar—Cl + Cu—CIAgain no reaction chain is set up as the reaction In which the radical
is consumed does not lead to the production of a second one in its
place. It is found that
Rate a [ArNa®] [CuClje]indicating that the first reaction is the slower, i.e. rate-determining,
one. The actual intervention of radicals can be confirmed, under
suitable conditions, by their initiation of the chain polymerisation of
acrylonitrile. Reactions of diazonium solutions can also proceed
through an ionic mechanism as has already been mentioned (p. 112).
Phenyl or other aryl radicals, generated in a number of ways, can
also react with aromatic species, e.g. the Gomberg-Bachmann reac
tion in which aryl radicals ar^pierated by the decomposition of
diazo hydroxides (XXXVII), in contact with the aromatic compound
that is to be;Jayl>flS:
Ar—N=N—OH -+ Ar« + N,+ OH
(XXXVII)