Peroxide theory

The peroxide theory of Bach [20] and Engler [23] fixed the phenomenon of peroxide formation as the primary product of hydrocarbon oxidation by dioxygen. However, the problem of the mechanism of peroxide formation remained unsolved. The new stage of successful study of organic compound oxidation began after the discovery of free radicals as active intermediates of many chemical processes. 

In some cases the main portion of the final products is formed by decomposition of peroxides. This is the current aspect of the classic Bach-Engler peroxide theory with respect to the chain theory of oxidation processes. 

A discussion of this relation was contained in a paper presented in April 1951 (7). It was there pointed out that these facts are consistent with the free radical peroxidation theory of combustion, if it is assumed that the free radicals add at the carbon-to-carbon double bond. The assumption is justified on the basis of experimental evidence. 

Bone (9), in a lecture before the Chemical Society on October 19,1933, gave some consideration to the peroxidation theory. He said in part ... 

Within the past few years the experimental proof of the formation and degradation of organic peroxides which Bone felt so essential to the establishment of the peroxidation theory has been forthcoming in abundance. The problem is complex (10). The results vary not only with mixture ratio, temperature, flow rate, and pressure but also with the hydrocarbon structure and the size and shape of the reaction vessel. 

The years following Van t Hoff s publication [4] are known as a period of rapid progress in the study of multi-step chemical reactions. There appeared Ostwald s and Kistjakovskii s studies, Bach-Engler s peroxide theory, and Luther and Shilov s theory of conjugated reactions. The postulate claiming that "a reaction is not a single-act drama (Schonbein) had become a common belief. 

A notable exception is the discharged ion theory of the Kolbe reaction, dating back to 1891 (Brown and Walker, 1891). This remarkably farsighted suggestion is entirely in accordance with present-day theory of the Kolbe reaction, but was for a long time abandoned in favour of indirect mechanisms (e.g. the hydrogen peroxide theory). 

Further evidence in support of the peroxide theory has resulted from a study of the slow oxidation of pentene/5 This work is of more particular interest from the point of view of paraffin hydrocarbon oxidation as applied to knocking phenomena, however, and will not be discussed here. 

Although considerable controversy lias existed and still exists regarding the exact manner in which a hydrocarbon oxidizes, one fact stands out from the great mass of data that have accumulated and that is that aldehydes appear early and are prominent in the oxidation products. It has been recognized, however, that aldehydes are not the primary products of the encounter between oxygen and hydrocarbon molecules, and it has been proposed that the most probable initial product is peroxidic in type. While the protagonists of the hydroxylation theory concede the attractiveness of certain features of the peroxide theory, they consider that it does not contradict the evidence for the hydroxylation theory.9... 

The peroxide theory has developed largely from work in the liquid phase or at low temperatures conducted primarily in an attempt to solve some of the questions regarding knocking phenomena. It has been assumed that the evidence obtained in this manner would be directly applicable to the high temperature vapor phase oxidation. It has thus been assumed that the mechanism followed by the oxidation at relatively... 

Mardles 15 passed hexane-air mixtures through hot tubes and reported the presence of active oxygen in the products. He supports the peroxide theory of combustion for this reason and because it offers a better explanation of engine knocking than does the hydroxylation theory. 1-Ic has also proposed that the oxidation occurs in two steps, the first of which is peroxide formation followed by aldehyde formation through decomposition of the active molecule. 

According to Bach and Engler s peroxide theory [43], the coupling in oxidation reactions is explained in terms of intermediate formation of a peroxide. If of two substances, A and B, only one (e.g.. A) reacts with molecular oxygen to give a peroxide (AOj) (substance A is then called an inductor ), then substance B ( acceptor ) may be oxidized by this peroxide ... 

The peroxide theory considers reaction (III) as potential determining. This is based on the well-known experimental fact that on most electrodes reaction (III) is much more reversible than (I) or (II). Therefore, as soon as even a small amount of H2O2 accumulates close to the surface of the electrode due to cathodic reaction (III), the potential may be determined mainly by the O2-H2O2 equilibrium. 

After A. N. BaMi [1] and K. O. Engler [2] formulated the peroxide theory of oxidation, a large number of studies appeared in which the oxidation of a number of hydrocarbons and various other organic substances was studied. It was found that many oxidation reactions are autocatalytically accelerated and are characterized by well-defined induction periods. 

Hydrocarbons are oxidized without the introduction of a radical source but this oxidation occurs with autoacceleration. This autoacceleration was explained in the framework of the theory of degenerate-branched chain reactions by the formation of an intermediate product, initiator. It was proved in 1930-50 that these products are hydroperoxides (see above). The Bach-Engler peroxide theory was thus merged with Semenov s theory of degenerate branching. Soviet scientists made the decisive contribution to the development of this area. 

---