A Guidebook to Mechanism in Organic Chemistry

V

Radicals and Their Reactions

•CHaJ^CH^=CH, <- CH 2 =CH—CH 2 -

(ii) Autoxidation. Another displacement reaction involving

radicals is autoxidation, the reaction of organic compounds with

oxygen under mild conditions. Substances often contain impurities,

e.g. trace metals, that can act as initiators so that the reaction then

proceeds spontaneously; but, as always, added peroxides act as very

powerful initiators. The decomposition of most organic compounds

exposed to air and sunlight is due to photosensitised oxidation. Thus

a number of hydrocarbons may be converted to hydroperoxides

(XXVI), molecules of oxygen reacting extremely readily with radicals

on account of their own diradical nature (cf. p. 243):

r i -o.- i

Ra+H—C • Ra—H+—C- —> —C—O—O-

I I I

t U-H

—C- + — C—O—O—H (XXVI)

I I

In some cases the hydroperoxide formed can itself act as an

initiator so that the reaction is autocatalysed.

As peroxy radicals, R0 2 •, ar^frelatively low reactivity they do not

readily abstract hydrogen from —H and many autoxidation

reactions are highly selective. Thus tertiary hydragens are usually the

only ones attacked in simple saturated hydrocarbons but allylic,

benzylic and other positions that can yield stabilised radicals are

attacked relatively easily. Thus decalin (decahydronaphthalene) yields

L

the first-formed radical by the delocalisation that can then take place

(c/.p. 234):