A Guidebook to Mechanism in Organic Chemistry

Radicals and Their Reactions

which has already been encountered in the reaction of aromatic

ketones with sodium in the absence of air (p. 169); it also resembles

the radical ion obtained in the first stage of the acyloin reaction (p.

169). The above are but two cases of electrolytic reaction, several

examples of which have considerable synthetic importance.

(ii) Methods of detection
The classical work on the detection of short-lived radicals was done
by Paneth using thin metal, e.g. lead, mirrors deposited on the inside
wall of glass tubes. These mirrors disappeared wbm attacked by
radicals, so by varying (a) the distance of the mirrors from the point
where the radicals were generated (by thermal decomposition of
metal alkyls), and (b) the velocity of the inert carrier gas by which the
radicals were transported, it was possible to estimate their half-lives.
That of methyl, under these conditions proved to be ca. 8 x 10"^8
seconds.
Some, more stable, radicals, e.g. Ph 3 C*, may be detected by
molecular weight determinations, but it is only rarely that this can be
accomplished satisfactorily. Several radicals are coloured, though the
compounds from which they are derived are not, so that colorimetric
estimation may be possible; and even though the radicals themselves
may, not be coloured, the rate at which they discharge the colour of
the stable Radical, diphenylpicrylhydrazyl (II), may serve to deter­
mine their concentration. This is an example of the ' use of a radical
to catch a radical' already mentioned (p. 234), the evidence being
strengthened by the isolation of the mixed product formed by mutual
interaction of the two radicals, if that is possible. Another chemical
method of detection involves the ability of radicals to initiate poly­
merisation of, for example, olefines; reference is made to this below
(p. 247).
The use of magnetic fields to detect the paramagnetism arising
from the presence of unpaired electrons in radicals has already been
referred to (p. 232). Though simple in essence, it can be fraught with
much difficulty in practice, and other physical methods of detection
are commonly preferred. The most useful of these to date is electron
spin resonance spectroscopy, wmWagain depends for its utility on
the presence of unpaired electrons in radicals.
Where it is desired merely to try and discover wfiether a particular
reaction proceeds via radical intermediates or not, one of the simplest
procedures is to observe the effect on the rate of the reaction of