Free-Radical Mechanism

Free-Radical Mechanism

It has generally been accepted that addition of halogen to alkenes, in the absence of light, is polar. Thus, for example, stewart et al. (1935) showed that the addition of chlorine to ethylene is accelerated by light. This suggests a free-radical mechanism :

Cl2      hv     2Cl

CH2=CH2 + Cl  → CH2Cl-CH2     Cl2     CH2Cl-CH2Cl + Cl  etc.

Poutsma (1965), however, has shown that chlorine adds, e.g., to but-2-ene at 25oC in the absence of light, and proposed a spontaneous free-radical mechanism:

MeCH=CHMe + Cl2   spont.  MeCHClCHMe   Cl2   MeCHClCHClMe  +  Cl;  etc.

This mechanism is supported by the fact that the reaction was much slower in the presence of oxygen. Under these conditions, free-radical addition is inhibited and the reaction proceeds by the slower polar mechanism.

Instead of addition reactions with the halogens, the alkenes may undergo substitution provided the right conditions are used. Thus, when straight-chain alkenes are treated with chlorine at a high temperature, they form mainly monochlorides of the allyl type,i.e., in the chain –C-C-C=C-, it is the hydrogen of C that is substituted. This high temperature substitutions strongly presumptive of a free-radical mechanism, e.g.,

Cl2      400 – 600oC      2Cl

CH3CH=CH2 + Cl →  HCl + CH2CH=CH2   Cl2   ClCH2CH=CH2 + Cl ;  etc.

The reason for allyl substitution occurring is not certain, but one possibility is that, of the three types of free radicals that may be formed, CH3CH=CH, CH3C=CH2, and CH2CH=CH2, it is the allyl one that will have the greatest resonance stabilization (the contributing structures are identical):


From the M.O. point of view, in the allyl free radical, each carbon atom is associated with a pz electron. Thus an M.O. is formed covering all three carbon atoms, with consequent delocalization. Since the formation of the free radical with maximum stability means that the reaction path through this intermediate will be accompanied by the greatest decrease in free energy, the reaction will therefore proceed along this route.