A Guidebook to Mechanism in Organic Chemistry

Radicals and Their Reactions

But migration is not confined to shifts on to carbon; thus triphenyl­
methyl peroxide (p. 232) undergoes the following changes on heating:

Ph 3 C—O—O—CPh 3

Ph 2 C—OPh
2Ph 3 C—O- -» 2Ph„C—OPh |
Ph,C—OPh
The much less common occurrence of rearrangements among radi­
cals as compared with carbonium ions (p. 86) probably reflects the
smaller difference in stability observed between primary and tertiary
radicals as compared with the corresponding carbojiium ions.

(v) Diradicals

The oxygen molecule has already been referred to as a diradical,

albeit an unreactive one, and another very simple species is the

methylene radical obtained, for example, by the photo-chemical de­

composition of ketene (XLII) or diazomethane (XLIII):

CHJNJ

(XLIII)

CHj CH 8 =C=0

(XLII)

This by contrast is extremely reactive, adding with great readiness,

and stereospecifjcally, to double bonds to form cyclopropane

derivatives:

\

H R H

H y \ H F

R

H

R.

c=c —• K-^

H H H

The photochemical excitation of anthracene and other lin aromatic

hydrocarbons has already been referred to; if the excitation is carried

out in the absence of air or oxygen, cyclisation takes place to a photo-

dimer(XLIV):

?

(XLIV)

258 J X