A Guidebook to Mechanism in Organic Chemistry

Radicals and Their Reactions

For radicals which do not have their configuration thrust upon

them in this way, or which are not notably stabilised by delocalisation,

the evidence available to date while not conclusive is certainly sugges­

tive. Thus spectroscopic evidence indicates that the methyl radical and

its deutero-derivative are planar or nearly so and with other simple

alkyl radicals any stabilisation by hyperconjugation that may be pos­

sible will tend to favour the planar configuration; though this tendency

is presumably less marked than with the corresponding carbonium

ions as the stabilisation of the radicals is less pronounced than that

of the ions.

(iv) Reactions

As with the carbonium ions and carbanions that have already been

considered, radicals, once formed, can take part in three principal

types of reaction: addition, displacement, and rearrangement, the

latter normally being followed by one or other of the former. Before

these reaction types are considered in detail, however, reference will be

made to the formation and behaviour of a typical radical to illustrate

the complexity of the secondary reactions that may result and, conse­

quently, the wide variety of products that may be formed.

{a) The thermal fission of benzoyl peroxide: Benzoyl peroxide (a

crystalling solid obtained by the reaction of benzoyl chloride with

hydrogen peroxide in alkaline solution under Schotten-Baumann

conditions) undergoes extremely ready thermal decomposition to

yield benzoate radicals:

Ph—C—O—O—C—Ph -v Ph—C—0-+-0—C—Ph

It can be looked upon as consisting of two dipoles joined negative
end to negative end as indicated above, and part, at least, of its
inherent instability may stem from this cause. It would thus be
expected that substitution of^K* benzene nucleus with electron-
donating groups would enhance this instability leading to even more
ready decomposition and this is, in fact, found to be the case. Electron-
withdrawing groups are, correspondingly, found to exert a stabilising
influence as compared with the unsubstituted compound.