Radicals and Their Reactions •
A mixture of several of the "eight possible geometrical isomers of
hexachlorocyclohexane is obtained. In the absence of sunlight and
radicals, no addition of chlorine can take place, while if catalysts are
present (p. 106) electrophilic substitution occurs. With toluene, under
radical conditions, attack on the methyl group offers an easier reac
tion path leading to predominant side-chain cblorination (substitu
tion), rather than addition to the nucleus as with benzene, because of
the stability and consequent ease of formation of the initial product,
the benzyl radical, PhCHa\
(ii) Hydrogen halide. The addition of hydrogen bromide to
propylene via ionic intermediates to yield isopropyl bromide, has
already been referred to (p. 141). In the presence of peroxides or other
radical sources, however, th^ addition proceeds via a rapid chain
reaction to yield n-propyl broBlJrle (XVII) (i.e. the so-called 'anti-
Markownikov* addition or the peroxide effeqty. The difference in
product under differing conditions is due to the fact that in the former
case the addition is initiated by H®, while in the latter it is initiated by
Br-:Br Br Br Br H Br
\ / Br- \ / -Br- \ /
C=C === H^C—C- ===== C=C
/ \ / \ / \
H H Br H Br H
(XV) (XIV) (XVI)The radical (XIV) formed initially can then eliminate Br- very rapidly
and so be reconverted to the cis starting material, (XV), or rotation
about the C—C bond can take place first followed by subsequent
elimination of Br* to yield the trans isomeride, (XVI). As the latter is
the more stable, it will come to preponderate, leachng to an overall
conversion of (XV)->-(XVI). The addition of iodine is even more
readily reversible at room temperature.
The addition of chlorine to benzene—one of the few addition
reactions of an unactivated benzene nucleus—has also been shown to
proceed via a radical mechanism, i.e. it is catalysed by light and the
presence of peroxides, and is slowed or prevented by the usual inhi
bitors. This presumably proceeds:
C1 ^ ,C1
Further addition