A Guidebook to Mechanism in Organic Chemistry

Rearrangements

Oe O^0

but it is by ncyjjg»ns certain that this is the only factor involved.
Yields in this reaction tend to be low, partly owing to the two-
phase, aqueous/organic, system involved. They may be improved
by using substances such as N-mtroso-acetanilide, PhN(NO)—
CO—Me, or dibenzoyl peroxide (heated) as sources of phenyl radi­
cals, for the reaction may then be carried out in homogeneous solu­
tion using the substance to be phenylated as solvent, but mixed
products and tarry by-products still result.
(d) Rearrangements: The few known rearrangement reactions of
radicals nearly always involve aryl residues as migrants, and even
then only from an atom on which the attached groups are strained by
crowding. Thus the radical (XXXVIII) derived from the aldehyde,
Ph 2 C(Me)—CH 2 —CHO, may be made to undergo* loss of CO, but
the products ultimately formed are derived from radical (XXXIX),
not (XL):
-co
PhaC(Me)—CHa—C=0 > Ph 2 C(Me) CH 2 > PhC(Me)—CH 2 Ph
(XXXVIII) (XL) (XXXIX)
It has been suggested that the migration of phenyl rather than an
alkyl group is encouraged by the possibility, with the former, of pro­
ceeding via an intermediate (XLI) that is stabilised by delocalisation:

For in the above casei^is migration of methyl rather than phenyl that

would be exp&tedTo yield ^he more stable radical, Ph 2 C—CH 2 Me,

and the non-phenylated radical, EtMe 2 C—CH 2 , corresponding to
(XL) is found not to rearrange at all.