Radicals and Their Reactions *The chief characteristics of radical reactions are their rapidity, their
initiation by radicals themselves or substances known to produce
them (initiators), and their inhibition or termination by substances
which are themselves known to react readily with radicals (inhibitors),
e.g. hydroquinone, diphenylamine, iodine etc. Apart from the short
lived radicals that occur largely as reaction intermediates, some others
are known which are more stable and which consequently have a
longer life; these will be considered first.LONG-LIVED RADICALS
The colourless solid hexaphenylethane, Ph 8 C—CPh 3 , is found to
yield a yellow solution in non-polar solvents such as benzene. This
solution reacts very readily with the oxygen of the air to form tri-
phenylmethyl peroxide, Ph 3 C—O—O—CPh 3 , or with iodine to yield
triphenylmethyl iodide, Ph 3 C—I. In addition, the solution is found
to be paramagnetic, i.e. to be attracted by a magnetic field, indicating
the presence of unpaired electrons (compounds having only paired
electrons are diamagnetic, i.e. are repelled by a magnetic field). These
observations have been interpreted as indicating that hexaphenyl
ethane undergoes reversible dissociation into triphenylmethyl
radicals: #
Ph 3 C:CPh 3 Ph 3 C-+ CPh 3
In support of this hypothesis, it is significant that the C—C bond
energy in hexaphenylethane is only 11 kcals/mole compared with
83 kcals/mole for this bond in ethane itself.
The degree of dissociation of a 3 per cent solution in benzene has
been estimated as about 0-02 at 20° and about 0 -1 at 80°. The reason
for this behaviour, in contrast to hexamethylethane which does not
exhibit it, has been ascribed to the stabilisation of the triphenylmethyl
radical, with respect to undissociated hexaphenylethane, that can
arise from the delocalisation of the unpaired electron via then-orbitals
of the benzene nuclei: