A Guidebook to Mechanism in Organic Chemistry

Radicals and Their Reactions

It might be expected that the attack on the aromatic species, C„H 8 —X,

would then proceed via a direct displacement:

Ar+C,H 5 —X -»- Ar—CjH*—X+H*

This would, however, involve the breaking of a carbon-hydrogen

bond and the formation of a carbon-carbon bond and, as the former

is usually considerably stronger than the latter, would thus lead to a

high activation energy and so to a slow reaction: this is not what is

actually observed. Also the addition of substances tbMwould readily

be reduced by free hydrogen atoms has never been found to result in

such reduction, indicating that it is unlikely that hydrogen atoms ever

do in fact become free. It seems more likely, therefore, that the reaction

proceeds as a two-stage process:

The hydrogen atom is removed by another radical or by attack on

the original source of aryl radicals:

Ar—N=N-<)H+
A?
{^T^-X -> Ar•+N,+H,Q+Ar—^^-X

Evidence for such a mechanism is provided by the fact that such aryl-

ations show no isotope effect, i.e. deuterium and tritium are dis­

placed as readily as hydrogen, indicating that arylation cannot be

initiated by fission of a carbon-hydrogen (or deuterium or tritium)

bond as the rate-determining step of the reaction.

The aryl group has here been shown entering the p-position to the

group already present, but it could as well have been the o- or

m-positions as the characteristic directing effect exerted by this group

on an attacking electrophilic or nucleophilic reagent will apply with

much less stringency to an uncharged aryl radical. Nevertheless, an

entirely random choice of positionol^ttack by the entering group is

not observed. All substituents, irrespective of Steir nature, appear

slightly to favour attack at the o- and p-positions. Tnis^may be due to

the extra stabilisation of the intermediate by delocalisation that could

result when attack is at the o- and p-, but not at the m-position