Haber-Willstatter chain reactions

However, although the Haber-Willstatter chain reactions have been assumed to occur in certain catalytic systems, notably the ferrous-ferric ion system (4), more recent evidence to be described subsequently, does not support this assumption. On the other hand, such reactions appear to offer the most plausible explanation for the photochemical decomposition of hydrogen peroxide, although even here a satisfactory analysis of the kinetics has yet to be made. 

All the individual steps in this have been considered previously by Haber and Willstatter or by Haber and Weiss. It will be seen, however, that step (2 ) of the Haber-Willstatter chain has been omitted but the other, step (2), retained. Barb et al. reject reaction (2 ) since this leads to values of n continuously increasing with H202 concentration which is contrary to their observations. Reaction (2) is supported by evidence obtained from experiments on the Fenton oxidation of dyestuffs. They find that the amount of dyestuff oxidized decreases as the hydrogen peroxide increases and interpret this to mean that the peroxide competes with dyestuff for hydroxyl radicals. 

As was the case for many other solution reactions the picture was clarified somewhat by the free radical concept introduced by Haber and Willstatter (3). They accounted for the rapid decomposition of hydrogen peroxide by the enzyme catalase in terms of a chain reaction involving the free radicals HO- and H02-... 

In 1931 Haber and Willstatter (1) suggested that the radicals OH and HO2 play an important role in the reactions of hydrogen peroxide and that these radicals could operate in a simple chain mechanism. Previously, Franck and Haber (2) had introduced these radicals into the mechanism of autoxidation processes in solution. 

---