Radicals and Their Reactionsand its subsequent conversion to (XVII), i.e. the steps propagating
the chain-reaction, are both exothermic, while with HF too much
energy is required to produce F', and though with HI, I is formed
readily enough, it is then not sufficiently reactive to proceed further,
i.e. the energy gained in forming a carbon-iodine bond is so much
smaller than that lost in breaking a carbon-carbon double bond as to
make the reaction energetically not worth while. With HC1 the ener
getics more closely resemble those with HBr and radical additions
have been observed in a few cases, but the radical reaction is not very
rapid, as the reaction chains are short at ordinaryfMiperatures, and
it competes somewhat ineffectively with the ionic mechanism.
Nothing has so far been said about the stereochemistry of the
addition of radicals to unsaturated compounds. It has been found,
however, that the radical addition of HBr to substituted cyclohexenes
proceeds stereospecifically trans. Thus with 1-methylcyclohexene
(XIX)
(XIX) * " (XXI) (XXII) (XX)cis-l-bromo-2-methylcyclohexane (XX) is obtained, i.e. the bromine
and hydrogen have entered trans to each other. It might be expected
that the first formed radical would be (XXI), but it has been suggested
that the observed trans addition arises from the contribution of
(XXII), corresponding to a bromonium ion (p. 138) plus an extra
electron. As with a bromonium ion, attack (by HBr) would then take
place 'from the back* leading to inversion of configuration on the
carbon atom attacked with formation of (XX)—an overall trans
addition. Though the occurrence of trans addition has been explained
in this way, (XXII) contains a bromine atom with nine electrons,
a not very likely happening, and alternative explanations of stereo-
specific trans addition have accofftlngly been put forward.
With simple acyclic olefines no such sterec^specific addition of
HBr is observed at room temperature owing to ready rotation about
the C—C single bond in the first-formed radical intermediate (cf.
p. 243). When such a radical addition is carried out at — 78°, however,