A Guidebook to Mechanism in Organic Chemistry

Stereochemistry

adding (a) compounds that readily form radicals, e.g. organic perox­
ides, and (b) compounds known to react readily with radicals, i.e.
inhibitors such as hydroquinone.

(iii) Stereochemistry

A good deal of attention has been devoted to the question of whether

radicals in which the unpaired electron is on carbon have a planar

(VIII) or a pyramidal (IX) structure

C C

I /A\

(VIII) (IX)

i.e. whether the presence of the unpaired electron preserves the quasi-

tetrahedral state or not. There is little doubt that in radicals that may

be considerably stabilised by delocalisation of the unpaired electron,

the three bonds attached to the carbon atom will be coplanar. Thus

in triphenylmethyl, although, as has been said already (p. 233), inter­

ference between the o-hydrogen atoms of the benzene nuclei prevents

the latter from lying in a common plane, the bonds joining the radical

carbon atom to the three phenyl groups are almost certainly co­

planar, for movement of one of these bonds out of rne common plane

would lower delocalisation possibilities without any compensating

relief of steric strain. The benzene nuclei are angled to this common

plane like the blades of a propeller so as to relieve as much steric

strain as possible, while losing the minimum amount of delocalisation

stabilisation due to their non-coplanarity.

By contrast, radicals in which the radical carbon atom constitutes

the bridgehead of a rigid cyclic system will have the pyramidal con­

figuration forced upon them, e.g. the apocamphyl radical (X):

Me

Me

There is, however, evidence that such radicals are considerably less

stable than simple tertiary aliphatic radicals upon which no such

stereochemical restraint is imposed.