Cumenes, autoxidation hydrocarbons

The common initiators of this class are f-alkyl derivatives, for example, t-butyl hydroperoxide (59), Aamyl hydroperoxide (60), cumene hydroperoxide (61), and a range of peroxyketals (62). Hydroperoxides formed by hydrocarbon autoxidation have also been used as initiators of polymerization. 

Aryl phosphites inhibit the initiated oxidation of hydrocarbons and polymers by breaking chains on the reaction with peroxyl radicals (see Table 17.3). The low values of the inhibition coefficient / for aryl phosphites are explained by their capacity for chain autoxidation [14]. Quantitative investigations of the inhibited oxidation of tetralin and cumene at 338 K showed that with increasing concentration of phosphite /rises tending to 1 [27]. 

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... 

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. 

When a slow steady-state autoxidation of a suitable hydrocarbon is disturbed by adding either a small amount of inhibitor or initiatory a new stationary state is established in a short time. The change in velocity during the non-steady state can be followed with sensitive manometric apparatus. With the aid of integrated equations describing the nonsteady state the individual rate constants of the autoxidation reaction can be derived from the results. Scope and limitations of this method are discussed. Results obtained for cumene, cyclohexene, and Tetralin agree with literature data. 

Kropf and co-workers200-207b carried out detailed investigations of the autoxidation of alkyl aromatic hydrocarbons, such as cumene, catalyzed by metal phthalocyanines. They concluded that in the initial stages of reaction, in which the concentration of alkyl hydroperoxides is quite low, initiation occurred by an oxygen activation mechanism, e.g.,... 

It may be concluded from the preceding discussion that at this juncture there is no bona fide evidence for the initiation of autoxidations by direct hydrogen transfer between metal-dioxygen complexes and hydrocarbon substrates. Although such a process may eventually prove feasible, in catalytic systems it will often be readily masked by the facile reaction of the metal complex with hydroperoxide. The choice of cumene as substrate by many investigators is somewhat unfortunate for several reasons. Cumene readily undergoes free radical chain autoxidation under mild conditions and its hydroperoxide readily decomposes by both homolytic and heterolytic processes. 

Small but significant effects of solvent polarity were found in the autoxidation of a variety of alkenes and aralkyl hydrocarbons [216-220] (styrene [216, 218, 219], ethyl methyl ketone [217], cyclohexene [218], cumene [218, 219], tetralin [219], etc.). An extensive study on solvent effects in the azobisisobutyronitrile (AIBN)-initiated oxidation of tetralin in a great variety of solvents and binary solvent mixtures was made by Kamiya et al. [220],... 

The reactions of phosphites with peroxy radicals continue to attract attention because of the use of phosphites as anti-oxidants. The autoxidation of a variety of hydrocarbons, e.g. tetralin, cumene, styrene, and cyclohexane, is inhibited by zinc dialkyldithiophosphates (60). In order to assess the reactivity... 

We have developed an effective method for the selective autoxidation of alky-laromatic hydrocarbons to the corresponding benzylic hydroperoxides using 0.5 mol% NHPI as a catalyst and the hydroperoxide product as an initiator. Using this method we obtained high selectivities to the corresponding hydroperoxides, at commercially viable conversions, in the autoxidation of cyclohexylbenzene, cumene and ethylbenzene. The highly selective autoxidation of cyclohexylbenzene to the 1-hydroperoxide product provides the basis for a coproduct-free route to phenol and the observed inq)rovements in ethylbenzene hydroperoxide production provide a basis for in roving the selectivity of the SMPO process for styrene and propene oxide manufacture. 

It is known that manganese salts cause oxidation of hydrocarbons, like cumene, by initiating free radical chain reactions. However, this is normally done by catalytic decomposition of trace amounts of hydroperoxides found in the hydrocarbons. In our case, the catalyst does not seem to decompose CHP, as demonstrated in an independent experiment (see above). If it did, the rate of decomposition should increase in time as the reaction progresses leading to an increase in the autoxidation rate. While we do observe for cumene an initiation period up to the accumulation of 3-5% hydroperoxide, from that point on up to greater than 50% CHP accumulation, the oxidation rate is constant. This initiation period may be due to surface activation of the catalyst. 

There have been detailed studies on the relative reactivities of hydrocarbons in cobalt catalyzed oxidations. Relevant data have been collected by Sheldon and Kochi [3]. Remarkably, toluene is about 3 times more reactive than cumene in cobalt mediated oxidation. The situation is reversed in uncatalyzed autoxidation. 

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. 

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