A Guidebook to Mechanism in Organic Chemistry

Radicals and Their Reactions

FE<B© + H® + Ph—C'

(XXX)

o

II

Ph—O

(XXX)

? T

Ph—C—O—OH + Ph—C—H

(XXXII)

H®?

O

II

Ph—c—o—o-

(XXXI)

o

II

1 Ph—c—H

Ph—C—O—OH

(XXXII)

2Ph—C—OH

catalysed by hydrogen ions and so is accelerated as the amount of
benzoic acid formed increases. The presence of electron-donating
groups in the benzene nucleus, as might be expected, facilitates
removal of a hydrogen atom from the aldehyde to yield the initial
radical, corresponding to (XXX).
The autoxidation of aldehydes may be lessened by very careful
purification but more readily by the addition of anti-oxidants, such
as phenols fend aromatic amines, that react preferentially with any
radicals that may be present.
An interesting autoxidation is the photo-oxidation of hydro­
carbons such as anthracene (XXXV):

H. H

(XXXV) (XXXIII) (XXXIV)

The light absorbed converts thermthracene to an excited state,
such as the di-radical (XXXIII) or something Ijke it, which then
adds on a molecule of oxygen to form the photo-oxid£ (XXXIV).
This photo-oxidation proceeds so readily with higher lin aromatic
hydrocarbons such as hexacene (XXXVI)