A Guidebook to Mechanism in Organic Chemistry

Radicals and Their Reactions

but also by disproportionation:

CH,—CHj • + CHj— CH, • -»• CH,—CH,+CHr=CH,

The use of lead tetraethyl as an anti-knock agent depends in part on

the ability of the ethyl radicals that it produces to combine with radicals

resulting from the over-rapid combustion of petrol, thus terminating

chain reactions which are building up towards explosion.

In solution, of course, the relative abundance of solvent molecules

means that the initial radicals most commonly meet their end by

reaction with solvent

but a new radical is then obtained in exchange and this may possibly

be capable of establishing a new reaction chain.

The thermal fission of carbon-carbon bonds is seen in the radical-

induced cracking of long-chain hydrocarbons where the initial

radicals introduced into the system act by abstracting a hydrogen

atom from a -CH 2 - group of the chain. The radical so formed then

undergoes fission at the /J-position yielding an olefine of lower mole­

cular weight and also a new radical to maintain the reaction chain:

Ra- H Ra—H

RS-CH,—CH—CH,—CH,—R' -* R—CH 2 —CH—CH 2 —CH 2 —R'

I

R—CH,—CH=CH,+ • CHa—R'

Termination of the reaction by mutual interaction of radicals will

tend not to take place to any marked extent until the concentration of

long-chain hydrocarbons has dropped to a low level.

Bonds involving some elements other than carbon may undergo

easier thermal fission. Thus diazomethane yields methylene diradicals

on heating, the reaction chain readily building up to explosion:

low temperatures and, because or the ease with which they will form
radicals, are much used as initiators: •

CH,—CH.+H—R -» CH,—CH.+ R

Ph—C :—Ph -* Pb—C—o+- -Ph