A Guidebook to Mechanism in Organic Chemistry

Diradicah

The isomerisation of cyclopropanes to the corresponding propylene

derivatives probably proceeds through diradicals:

ft

R I CH R H

R CH 2 R R I CH R H CH R

\ / \ / \\/ \ /

C C -> C* *C -> C *C

R' R' R/ R' R R' R'

H CH R
\ / \ /
c c
/ ^^1 ,
R R' R'
The driving force for the 1,2-hydrogen shift is provided by the
possibility of electron pairing and consequent formation of a new bond
that is thereby conferred.
Diradicals have also been encountered as intermediates in the re­
duction of ketones to pinacols (p. 168) and in the acyloin reaction on
esters (p. 169). All these diradicals, with the exception of the oxygen
molecule, are highly unstable but, surprisingly, a number ofWiradicals
are known which are quite stable. Thus the hydrocarbon (XLV) exists
in the diradical form:

Pb 2 C-f' ^—f V

CPh

«

(XLV)

This is due to the fact that the diradical is very greatly stabilised

by delocalisation and that a^quinonoid structure embracing both

nucfiei, that would result in electron pairing, cannot be formed. The

diradical (XLVI)

(XLVI)

CI CI

CPhj y* Ph 2 C= -CPhj

CI Ci

(XLVII)