A Guidebook to Mechanism in Organic Chemistry

Long-Lived Radicals

A number of contributing structures of this kind can be written,
but the stabilisation thereby promoted is not so great as might, at
first sight, be expected, as interaction between the hydrogen atoms in
the o-positions prevents the nuclei attaining coplanarity. The radical
is thus not flat, but probably more like a three-bladed propeller with
angled blades, so that delocalisation of the unpaired electron, with
consequent stabilisation, is considerably inhibited.
The ready formation and stability of the radicals are, indeed, due
in no small measure to the steric crowding in hexaphenylethane that
can be relievedj^ dissociation. In support of this explanation, it is
found that the C—C distance in this compound is significantly longer
(by ca. 0-04 A) than in ethane. Also, while the introduction of a
variety of substituents into the nuclei promotes dissociation, this is
particularly marked when substituents are in the o-positions where
they would be expected to contribute most to steric crowding.
Further, it is found that the compound (I)

in which two of the benzene nuclei on each carbon atom are bonded

to each other and so held back from 'crowding* near the C—C bond,

is not dissociated at room temperature though the possibilities of

stabilising the radical, that could be obtained from (I), by delocalisa­

tion are at least as great as those for triphenylmethyl.

Somewhat less stable radicals may be obtained by warming tetra-

arylhydrazines in non-polar solvents, green solutions being obtained:

KMnOi

2PhjNH >- Ph,N:NPJjj ^ Ph,N- + -NPh,

Here, promotion of dissociation by steric crowding is clearly less

important than with hexaphenylethane; stabilisation of the radical

due to delocalisation may be more significant, but dissociation is

certainly favoured by the lower energy of the N—N bond.