A Guidebook to Mechanism in Organic Chemistry

Addition of Hydrogen Halide

Ra * (ex peroxides)+HBr ->• Ra—H + • Br

I

Me—CH=CH,+ 'Br -* Me—CH—CH,—Br

| JHB, (XVIII>

•Br + Me—CHa—CH,—Br

(XVII)

In the addition of Br to propylene, the radical (XVIII) is formed
rather than th^Jossible alternative, Me—CHBr—CHg, since a
secondary radical is more stable than a primary one (p. 234).
The addition reaction may not need the presence of added radicals
to initiate it, however, for olefines absorb oxygen from the air
forming peroxides which can then themselves sometimes act as
initiators. Such auto-initiation can be avoided by rigorous purifica­
tion of the olefine prior to reaction, but this is not easy to achieve in
practice, and formation of isopropyl bromide, i.e. predominance of
the ionic reaction leading to so-called normal or Markownikov
addition, is more easily secured by the addition of radical acceptors
such as hydroquinone, etc., to absorb any radicals that may be pre­
sent in the system and so prevent the occurrence of the rapid chain
reaction. • •
It should not be thought that the presence of radicals in any way
inhibits the ionic mechanism; it is merely that the radical reaction
which they initiate, being a chain reaction, is so very much more
rapid that it results in the vast majority of the propylene being con­
verted to n-propyl bromide, (XVII), despite the fact that the ionic
reaction is proceeding simultaneously. The virtually complete control
of orientation of addition of HBr that can be effected by introducing
radicals or radical acceptors into the reaction is very useful pre-
paratively; it is not confined to propylene and applies to a number
of other unsymmetrical unsaturated compounds, e.g. allyl bromide,
CH2=CH—CH 2 —Br, which can be converted into 1,2- or 1,3-
dibromopropane at will. In some cases, however, the ionic mechan­
ism of addition is sufficiently fasMo^ompete effectively with that
induced by radicals and clear-cut control cannot then, of course, be
effected. 0
It should, moreover, be emphasised that the reversal of the normal
orientation of addition in the presence of peroxides is confined to
HBr. This is due to the fact that with HBr the formation of (XVIII)