A Guidebook to Mechanism in Organic Chemistry

Radicals and Their Reactions *

chloride

NO, NO,

(ID

for this is very stable (due to delocalisation of the unpaired electron)

and forms stable, isolable products with many radicals. In addition,

its solutions are Bright violet in colour and its reaction with other

radicals to yield colourless products can thus be readily followed

colorimetrically.

SHORT-LIVED RADICALS

The short-lived radicals, e.g. Me-, though more difficult to handle,

are of much greater importance as participants in chemical reactions;

as their short life suggests, they are extremely reactive.

The relative stability of simple alkyl radicals is found to be in the

same order as that of the corresponding carbonium ions (p. 62)

R,C* > R,CH' > RCH,' > CH,-

the sequence reflecting decreasing stabilisation by hyperconjugation
as the series is traversed. As mlgnPbe expected, however, the differ­
ences in stability between the radicals is less majked than between
corresponding carbonium ions. Radicals involving allylic or benzylic
positions show greatly enhanced stability arising from the delocalisa­
tion via ir orbitals that is then possible:

234 , ~»

Similarly, solutions of diphenyl disulphide become yellow on

heating

PhS:SPh ^ PhS- + -SPh

and the radicals formed may be detected by the classical device of

adding a second radical and isolating a mixed product:

PhS- + -CPh 8 -+ PhS:CPh 3

The sulphide obtained is, however, rapidly decomposed in the pre­

sence of air. The best radical to use for such detection is 1,1-diphenyl-

2-picrylhydrazyl (II)

NO, NO,

picryl /' \ PbO,. /' \

Ph,N—NH, Ph,N—NH— ( ]>NO, —> Ph,N—N—(_ }NO,