0Radicals and Their Reactions *
It will be seen that the initiating step, the photochemical fission of a
molecule of chlorine, will flMkto the formation of two reactive
entities, i.e. free chlorine atoms, which are, of course, radicals. In
support of this it is found that *a—a
caa==cci,+ -ci -* 'ecu—cci 3 (xii)
(XI) t la,•a + ccis—cci 3 (xiii)
Rate oc ^Intensity of absorbed lightA further possibility is the attack of benzoate or phenyl radicals on
as yet undecomposed benzoyl peroxide leading to the formation in the
O
II
system of new radicals, X—C 6 H 4 —C—O* and X—C 8 H 4 % which can
give rise to a further range of possible products. As this is only a
simple case, the possible complexity of the mixture of products that
may result from radical reactions in general will readily be realised.
The most important group of radical reactions are probably those
involving addition.
(b) Addition reactions: (i) Halogens. As has already been men
tioned (p. 137) the addition of halogens to unsaturated systems can
follow either an ionic or a radical mechanism. In the vapour phase in
sunlight, it is almost entirely radicals that are involved, provided the
containing vessel has walls of a non-polar material. The same is true
in solution in non-polar solvents, again in the presence of sunlight.
In more polar solvents, in the absence of sunlight, and particularly if
catalysts, e.g. Lewis acids, are present, the reaction proceeds almost
entirely by an ionic mechanism. It thus follows that in solution in
non-polar solvents in the absence of sunlight or catalysts, little or no
reaction takes place between olefines and halogens as neither ionic
sgecies nor radicals will normally be formed under these conditions
without tome specific initiating process.
The photochemically catalysed addition of chlorine to tetrachloro-
ethylene (XI), for example, may be formulated as: