Hydrocarbons, autoxidation

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. 

Variable valence transition metal ions, such as Co VCo and Mn /Mn are able to catalyze hydrocarbon autoxidations by increasing the rate of chain initiation. Thus, redox reactions of the metal ions with alkyl hydroperoxides produce chain initiating alkoxy and alkylperoxy radicals (Fig. 6). Interestingly, aromatic percarboxylic acids, which are key intermediates in the oxidation of methylaromatics, were shown by Jones (ref. 10) to oxidize Mn and Co, to the corresponding p-oxodimer of Mn or Co , via a heterolytic mechanism (Fig. 6). 

Peroxy radical recombination appears to be the most important source of the electronic excitation energy emitted during hydrocarbon autoxidation. In addition to the above-mentioned energetic considerations, this is clear from the following experimental facts the termination rate for secondary peroxy radicals is 103 times faster than for tertiary peroxy radicals due to their having no a-hydrogen 14> the termination rate constant decreases by 1.9 with a-deuteration 39 40>. 

The weak chemiluminescence of Grignard compounds in air has been known since 1906. A radical chain mechanism similar to that of hydrocarbon autoxidation appears to provide the excitation energy of the emitting product. Until recently the relations between constitution and chemiluminescence in Grignard compounds were rather obscure j>-chloro-phenylmagnesium chloride was found to be the most efficient compound. 

KI Ivanov. Intermediate Products and Reactions of Hydrocarbon Autoxidation. Moscow GNTI NGTL, 1949 [in Russian]. 

Influence of Reverse Micelles and Surfactant (SA) on the Hydrocarbon Autoxidation... 

Formulas for Kinetic Parameters of Hydrocarbon Autoxidation as Chain Reaction in Quasistationary Regime Chain Length v, Critical Concentration of Inhibitor [lnH]cr, and Quasistationary Concentration of Hydroperoxide [ROOH]s. The Following Symbols are Used / = k3/ka and V[0 is the Rate of Free Radical Generation on Reaction of RH with Dioxygen [33,38,45]... 

Hydrates, enthalpies of formation, 156, 157 Hydrazones, aldehyde detection, 670 Hydrocarbons autoxidation, 320... 

In the familiar hydroperoxide chain mechanism for hydrocarbon autoxidation, with propagation steps,... 

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. 

The inhibition of hydrocarbon autoxidation by zinc dialkyl dithiophosphates was first studied by Kennerly and Patterson (13) and later by Larson (14). In both cases the induction period preceding oxidation of a mineral oil at 155 °C. increased appreciably by adding a zinc dialkyl dithiophosphate. In particular, Larson (14) observed that zinc salts containing secondary alkyl groups were more efficient antioxidants than those containing primary groups. In these papers the inhibition mechanism was discussed only in terms of peroxide decomposition. 

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. 

Peroxide Decomposition Mechanism. Since virtually no work has been reported which concerns only the mechanism by which zinc dialkyl di-thiophosphates act as peroxide decomposers, it is pertinent to discuss metal dialkyl dithiophosphates as a whole. The mechanism has been studied both by investigating the products and the decomposition rates of hydroperoxides in the presence of metal dithiophosphates and by measuring the efficiency of these compounds as antioxidants in hydrocarbon autoxidation systems in which hydroperoxide initiation is significant. 

Effects of Different Metal Salicylaldimine Chelates. Varying the central metal profoundly affected catalytic and inhibitory properties. There were only small quantitative variations, however, between N-phenyl- and V-butylsalicylaldimines having the same central metal atom. The only other salicylaldimines where catalyst-inhibitor conversion could be demonstrated were those of copper (II). With copper (II) both the catalytic and the inhibitory effects are much less pronounced than for cobalt (II). Surprisingly nickel (II) complexes behaved like conventional catalysts for hydrocarbon autoxidation—i.e., the rate is proportional to... 

In the autoxidation of N-butylacetamide all the salicylaldimine chelates showed only inhibitory effects. We also found that two salicylaldimine chelates showed no significant catalytic properties and exhibited only inhibitory effects even in hydrocarbon autoxidation—viz., bis (N-butylsalicylaldimino) zinc and bis (N-butylsalicylaldimino) oxyvanadium-(IV). While there are some well known antioxidants containing zinc (e.g., zinc dialkythiophosphates or zinc dithiocarbamates), this is not a general property for zinc compounds. Zinc acetylacetonate, for example, had no inhibitory effect in the autoxidation of hydrocarbons or amides. 

Table I. 3,5 -Diisopropyl Salicylato Metal Chelates as Inhibitors of Hydrocarbon Autoxidation"...

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