A Guidebook to Mechanism in Organic Chemistry

Radicals and Their Reactions

At elevated temperatures (ca. 450°) propylene, Me—CH=CH 2 , is
found to undergo chlorination to allyl chloride rather than addition
of chlorine, for as the temperature rises the addition reaction be­
comes reversible (cf. p. 243) whereas the displacement reaction via a
stabilised allyl radical does not:

CI 2 +CHa—CH=CH, -> Cl—CH 2 —CH=CH 2 +HCI

At similarly elevated temperatures it is found^hat halogeno-

benzenes undergo considerable chlorination and bromination in the

m-position despite the presence in the nucleus of an o/p-directive

halogen; thus bromobenzene yields 57 per cent of m-dibromoben-

zene at 500°. This is due to increasing homolytic attack by bromine

atoms generated by thermal fission of molecules of bromine. Attack

by Br* at such elevated temperatures will tend to be less selective and

will be little influenced by relative electron availability at o-, m- and p-

positions and the usual directive effect of a substituent already present

will no longer apply: a characteristic feature of the homolytic sub­

stitution of aromatic systems at high temperatures.

Fluorination takes place with great readiness and though it appears

to proceed via a ftdical mechanism, the reaction will often take place

in the absence of fight or initiators. Fluorine atoms are then believed

to be produced, in the first instance, by the reaction:

\ \

—C—H + F—F -»• —C- + H—F+'F

/ /

The driving force of the reaction is provided by tne 100 kcals by

which the bond energy of H—F exceeds that of F—F. Bromination

is generally slower and less easy than chlorination as the stage in

which a hydrogen atom is abstracted

\ ^ \

—C—H+ BT^ _c + H—Br

/ /

is often endothermic, whereas in chlorination this stage is exothermic

due to the greater bond energy of H—Cl as compared with H—Br.

250

\