Addition of Halogens
i.e. each quantum of energy absorbed did, in fact, lead to the initia
tion of two reaction chains. The addition of a free chlorine atom to
the unsaturated compound results in the formation of a second
radical (XII) which is capable of undergoing a radical displacement
reaction with a molecule of chlorine to yield the final addition pro
duct (XIII) and a free chlorine atom. This is capable of initiating a
similar reaction cycle with a second molecule of unsaturated com
pound and so the process goes on, i.e. an extremely rapid, continuing
chain reaction is set up by each initiating chlorine atom produced
photochemicalljfcwOuch a continuing chain reaction, self-perpetuating
once initiated, is perhaps the most characteristic feature of reactions
proceeding via a radical mechanism. In support of the above reaction
scheme, it is found that each quantum of energy absorbed leads to
the conversion of several thousand molecules of (XI) ->(XIII). Until
the later stages of the reaction, i.e. when nearly all the unsaturated
compound, (XI), is used up, the concentration of CI * will always be
very low compared to that of CI 2 C=CC1 2 , so that termination of
reaction chains owing to • '
ci'+'Ci -> a,
- or mutual interaction of other active intermediates, e.g. of (XII), wtll
be a very uncommon happening, and hence chain termination
relatively infrequent. The reaction is inhibited by oxygen as the
latter's molecule contains two unpaired electrons, 0—CV, causing
it to behave as a diradical, albeit a not very reactive one. It can thus
act as an effective inhibitor by converting highly active radicals to the
much less reactive peroxy radicals, Ra—O—O. That the oxygen is
reacting largely with pentachloro-ethyl radicals (XII) is shown by the
O
II
formation of trichloro-acetyl chloride, C1 3 C—C—Cl, when the addi
tion of chlorine to tetrachloro-ethylene is inhibited by oxygen. Photo
chemical addition of bromine is usually slower as the reaction chains
are shorter.
The addition of chlorine to manyHnsaturated compounds is found
to be irreversible at room temperature and for some way above (cf
p. 250), whereas the addition of bromine is often readily reversible.
This results in the use of bromine radicals for the cis -> trans isomerisa-
tion of geometrical isomerides:
- J
243
