A Guidebook to Mechanism in Organic Chemistry

Radicals and Their Reactions

can, in theory, be converted to a quinonoid form (XLVII) in which

its electrons are paired, but formation of the latter is inhibited as the

bulky chlorine atoms prevent the two benzene nuclei from becoming

coplanar, a necessary condition if there is to be the effective over­

lapping between their n orbitals that formation of (XLVII) requires.

Both these diradicals undergo reversible association in solution,

however.

(vi) Chemical action of X-rays

X-rays and other ionising radiations can react wMarater, in living

tissues as well as in vitro systems, in the following way:

Two radicals are formed by secondary processes, one involving a
second molecule of water. As the ejected electron may not react
immediately with a second molecule of water, the two radicals, and
any reaction chains that they set in motion, may thus occur some
distance apart.
Vinyl polymerisation in aqueous solution may be initiated in this
way but the solution must be free from oxygen which acts as a
powerful inhibitor. In living tissue, dissolved oxygen can lead to the
formation of hydrogen peroxide

while dissolved nitrogen can similarly be converted to NH 8 , etc
Many other reactions of great ultimate complexity can be set in train
by radiation in this way.^1

H,0 + fci> ->• [H,0]^9 + e'

H®+-OH HO

|H.O

HOe+'H

H-+-0,- - HO,- -* H,0,

0

\

260

O