Hydrocarbon autoxidation, termination reaction

Hendry and Russell (15) have shown how polyarylmethanes and polyarylethylenes may retard autoxidations of other hydrocarbons at 60° to 90 °C. because the carbon radicals involved react relatively slowly and incompletely with oxygen, the free arylmethyl radicals contribute to a fast, crossed, termination reaction. A similar effect has been reported for benzyl radicals above 300°C. (11). These effects can usually be overcome by sufficient oxygen pressure hence, our restriction at the beginning of this section. 

There is excellent agreement between the decay constants obtained by ceric ion oxidation of secondary hydroperoxides and the rate constants for chain termination in hydrocarbon autoxidation determined by the rotating sector. The agreement suggests that secondary peroxy radicals do not undergo many nonterminating interactions, so that most self-reactions of secondary peroxy radicals must be chain terminating. 

From these results, it is clear that neither Equation A nor B represents the kinetics of the zinc diisopropyl dithiophosphate-inhibited autoxi-dation of cumene or Tetralin. This does not immediately indicate that the mechanism in Scheme 1 is wrong since it is highly idealized and takes no account of possible side reactions. A similar situation occurs in the inhibition of hydrocarbon autoxidation by phenols (AH), for which a basic mechanism similar to that in Scheme 1 is accepted. Termination occurs via Reactions 7 and 8 instead of Reactions 5 and 6. 

Soluble Co compounds are generally employed in the autoxidation of hydrocarbons, i.e., the oxidation with O2 as the oxidant. In neat hydrocarbons, low concentrations of Co compounds accelerate the autoxidation since the Co2+/Co3+ couple is excellent for decomposing alkyl hydroperoxides and thus initiates free radical chain reactions. However, at high conversions, the Co may be deactivated by formation of insoluble clusters with side products of the hydrocarbon autoxidation. Moreover, high concentrations of a Co compound may actually inhibit the reaction because Co also terminates radical chains by reaction with ROO radicals ... 

Termination of the radical chain reaction As the reaction proceeds, autoxidation is followed by an autoretardation stage, resulting in a standstill before the hydrocarbon is completely consumed. This autotermination is called the chain termination reaction and dominates in this final phase of the oxidation process such that degradation comes to a halt. Termination may be effected by the combination of radical species such as peroxy radicals to yield ketones and alcohols. Reaction sequence (4.14) ... 

The reaction of alkanes with a mixture of SO2 and O2 also occurs more readily than simple autoxidation of the hydrocarbon (reaction 30). Here the radical RSO2OO , formed as shown in Scheme 27, is apparently less prone to termination reactions than a simple alkylperoxy radicaF. ... 

In this mechanism, bromine radicals are key intermediates producing hydrocarbon radicals via hydrogen abstraction [32]. The subsequent reactions are those typical for hydrocarbon autoxidation [3], Bromine radicals may terminate to Br but the bromide is replenished via phase... 

Since the effects of heavy metals increase the amount of free radicals in the lipid phase, not only do the rates of initiation and propagation reactions increase, but also the rate of termination reaction increases. Heavy metals therefore also change the composition of the reaction products. At high concentrations of free radicals, the termination reaction may dominate and metals then act as the inhibitors of autoxidation. Autoxidation reaction can also be inhibited by metals when they are present at higher concentrations. It is assumed that the reason is the oxidation and reduction of free hydrocarbon radicals to anions and cations by ions of Fe and Cu and the formation of complexes of free radicals. Other complexes are also formed with Co. All these reactions interrupt the radical chain autoxidation reaction. Reactions with Fe ions are given as examples. 

Prior to 1957 the termination reaction for liquid-phase hydrocarbon autoxidation at oxygen pressures above oa. 100 torr was generally written as... 

Kinetic results were consistent with a bimolecular termination reaction whereas reaction products and mechanisms were something of a mystery. At that time it was known that the termination rate constant for autoxidation of cumene ( ) is about three orders of magnitude smaller than the termination rate constant for autoxidation of tetralin (7.). It was, however, generally accepted that the tennination rate constants for tertiary ( ) and secondary (9 ) alkylperoxy radicals are insensitive to the structure of the hydrocarbon residue in the radical. 

Bateman, Gee, Barnard, and others at the British Rubber Producers Research Association [6,7] developed a free radical chain reaction mechanism to explain the autoxidation of rubber which was later extended to other polymers and hydrocarbon compounds of technological importance [8,9]. Scheme 1 gives the main steps of the free radical chain reaction process involved in polymer oxidation and highlights the important role of hydroperoxides in the autoinitiation reaction, reaction lb and Ic. For most polymers, reaction le is rate determining and hence at normal oxygen pressures, the concentration of peroxyl radical (ROO ) is maximum and termination is favoured by reactions of ROO reactions If and Ig. 

This difficulty has now been overcome. Howard, Schwalm, and Ingold (24) show that the rate constant for reaction of any alkylperoxy radical with any hydrocarbon can be determined (by the sector method) by carrying out the autoxidation of the hydrocarbon in the presence of >0.1 M hydroperoxide corresponding to the chosen radical. All the absolute propagation and termination constants for the co-oxidation of cumene and Tetralin were thus determined. Our Tetralin-cumene work suggests that their results agree well with the best we have been able to get... 

The basic autoxidation theory of hydrocarbons, which involves a complex set of elementary reaction steps initiation, propagation, and termination, is similarly... 

Selectivity is an especially tough challenge in the case of autoxidation of small olefins in fluid phase because of diversion of the radical chain reaction by termination steps that result in the formation of oxy radicals. These are very reactive species that attack indiscriminately the starting hydrocarbon and primary products. The photochemical reaction in zeolites opens up much better controlled oxidation of low olefins, which are illustrated here briefly for the commercially important case of propylene oxidation. 

Uninhibited autoxidation of hydrocarbons in the absence of added initiators or terminators involves the following reactions (2) ... 

The radical chain mechanism outlined here avoids the ineffective direct reaction of molecular oxygen with the substrate hydrocarbon. The fast propagation reactions produce ROOH that in turn can initiate new radical chains. As the primary product of the reaction initiates new reactions, one ends up with an autocatalytic acceleration. The propagating peroxyl radicals can also mutually terminate and yield one molecule of alcohol and ketone in a one-to-one stoichiometry. The ratio between the rate of propagation and the rate of termination is referred to as the chain length and is of the order of 50-1000. As the desired chain products are more susceptible to oxidation, autoxidations are normally carried out at low conversions in order to keep the selectivity to an economically acceptable level. 

If in the chain initiated reaction when v,- = const the induction period is independent of the efficiency of retardation action of the inhibitor but is determined by its concentration, then during autoxidation the inhibitor is more slowly consumed when it more efficiently terminate chains because ROOM is more slowly accumulated and the retardation period increases. Then the initiated oxidation of hydrocarbons is retarded only by compounds terminating chains. Autoxidation is retarded by compounds decomposing hydroperoxides. This decomposition, if it is not accompanied by the formation of free radicals, decreases the concentration of the accumulated hydroperoxide and, hence, the autoxidation rate. Hydroperoxide decomposition is induced by compounds of sulfur, phosphorus and various metal complexes, for example, thiophosphate, thiocarbamates of zinc, nickel, and other metals. 

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