Peroxide Effect

Peroxide Effect

The Peroxide effect (Kharasch, 1933). The presence of oxygen or peroxide that are formed when the alkene stands exposed to the air, or added peroxide such as benzoyl peroxide, causes the addition of hydrogen bromide to take place in the direction opposite to that predicted by Markovnikov’s rule. This departure from the rule is known as the ‘abnormal’ reaction, and was shown to be due to the ‘peroxide effect’ (Kharasch et al., 1933). Hydrogen chloride, hydrogen iodide and hydrogen fluoride do not exhibit the abnormal reaction. It has been found that the addition of hydrogen bromide is ‘abnormally’ effected photochemically as well as by peroxide catalysts.

The mechanism of the peroxide effect is a free-radical chain reaction, the peroxide generating the free radical R¹  (cf. polymerization. Below):

(R¹CO₂)₂  →  2R¹CO₂^.  →  2R^1. + 2CO₂

R₁^. + HBr  →  R¹H + Br^.

R²CH=CH2 + Br^.  →  R²CHCH₂Br  ^HBr  R²CH₂CH₂Br + Br^., etc.

In the photochemical addition, the bromine atom is produced by absorption of a quantum of light:

HBr  →  H^. + Br^.

a bell et al. (1962), using ESR spectroscopy, showed that the photochemical addition of HBr produces free radicals which appear to be best represented as bridged structures. They also showed, using DBr, that the bromine atom is the initial event in the attack.

The reason for the addition being contrary to Markovnikov’s rule is not certain. A favourted theory is that the order of stability of free radicals is the same as that for corbonium ions, i.e., t > s > primary. One explanation for this order is no bound resonance. Thus the reaction proceeds as shown in the equations above, the primary free-radical, R²CHBrCH₂^., being much less favoured energetically than the secondary, R²CHCH₂Br.

It has previously been pointed out that, among other factors, the energy of activation in a given reaction is lower the greater the strength of the new bond formed. Now, the bond broken is the –bond, and so this energy change will not affect the activation energy whichever way the free-radical addition occurs. Also, the strength of a C—H bond is not the order p > s > t; this order also applies to bonds other than C—H. hence, on this basis, it can be expected that the intermediate free radical will be preferably RCHCH₂X rather than RCHXCH₂^.. This, then, is possible explanation for free-radical occurring contrary to Markovnikov’s rule.

The abnormal reaction in the presence of peroxides can be prevented by the addition, in sufficient amount, of an inhibitor such as diphenylamine, catechol, etc. inhibitors combine with free radicals, and so prevent propagation of the chain-reaction. Thus an inhibitor produces (if present in small amount only) an induction period, i.e., a period when apparently no reaction is occurring. When the inhibitor is used up, it is than possible for the chain reaction to proceed in the normal fashion.