A Guidebook to Mechanism in Organic Chemistry

RADICALS AND THEIR REACTIONS

MOST of the^ajctions that have been considered to date have in­
volved the participation, however transiently, of charged inter­
mediates, i.e. carbonium ions and carbanions, produced by the hetero-
lytk fission of covalent bonds:

But reactive intermediates possessing an unpaired electron, i.e.

radicals, can also be generated if a covalent bond undergoes homo-

lytic fission:

Reactions involving such radicals occur widely in the gas phase,

but they also occur in solution, particularly if the reaction is carried

out in non-polar solvents and if it is catalysed by light or the simul­

taneous decomposition of substances known to produce radicals,

e.g. peroxides. Once a radical reaction has been started, it often

proceeds with very great rapidity owing to the establishment of fast

chain reactions (see below). These arise from the ability of the first

formed radical to generate another on reaction with a neutral mole­

cule, the new radical being able to repeat the process, and so the

reaction is carried on. Thus in the bromination of a hydrocarbon,

R—H, the reaction may need starting by introduction of the radical,

Ra% but once started it is self-perpetuating:

Ra*+Br—Br -»• Ra—Br+-Br

I

R—H + • Br -> R-+H—Br

t Br,

0 •Br + R—Br

/