Autoxidation liquid-phase reaction

C. Walling Rearrangements particularly of peroxy radicals have been mentioned recurrently in todays discussion of high temperature gas-phase autoxidations, and I will summarize briefly what is known about such processes in liquid-phase reactions run near room temperature. 

The catalysis of hydrocarbon liquid-phase autoxidation by transition metals is the result of the substitution of the slow bimolecular reaction (Ek 100 kJ mol-1, see Chapter 4)... 

Metals and metal oxides, as a rule, accelerate the liquid-phase oxidation of hydrocarbons. This acceleration is produced by the initiation of free radicals via catalytic decomposition of hydroperoxides or catalysis of the reaction of RH with dioxygen (see Chapter 10). In addition to the catalytic action, a solid powder of different compounds gives evidence of the inhibiting action [1-3]. Here are a few examples. The following metals in the form of a powder retard the autoxidation of a hydrocarbon mixture (fuel T-6, at T= 398 K) Mg, Mo, Ni, Nb V, W, and Zn [4,5]. The retarding action of the following compounds was described in the literature. 

Carlier fundamental studies of autoxidations of hydrocarbons have concentrated on liquid-phase oxidations below 100 °C., gas-phase oxidations above 200°C., and reactions of alkyl radicals with oxygen in the gas phase at 25°C. To investigate the transitions between these three regions, we have studied the oxidation of isobutane (2-methylpropane) between 50° and 155°C., emphasizing the kinetics and products. Isobutane was chosen because its oxidation has been studied in both the gas and liquid phases (9, 34, 36), and both the products and intermediate radicals are simple and known. Its physical properties make both gas- and liquid -phase studies feasible at 100°C. where primary oxidation products are stable and initiation and oxidation rates are convenient. 

Metal oxides have often been used as catalysts for the autoxidation of hydrocarbons.1 In many cases the metal probably dissolves in the reaction medium and catalysis involves homogeneous metal complexes. However, according to a recent report56 cerium oxide catalyzes the liquid phase oxidation of cyclohexanone in acetic acid (5-15 bar and 98-118°C) without dissolving in the reaction medium. 

Alkylperoxy radicals play vital roles in both propagation and termination processes. Hydroperoxides, R02H, are usually the primary products of liquid phase autoxidations [reaction (4)] and may be isolated in high yields in many cases. Much of the present knowledge of autoxidation mechanisms has resulted from studies of the reactions of alkylperoxy radicals30-33 and the parent hydroperoxides,348-d independently of autoxidation. Thus, the various modes of reaction of organic peroxides are now well-characterized.35 -39... 

A number of transition metals are now known147-156 to form stable dioxygen complexes, and many of these reactions are reversible. In the case of cobalt, numerous complexes have been shown to combine oxygen reversibly.157 158 Since cobalt compounds are also the most common catalysts for autoxidations, cobalt-oxygen complexes have often been implicated in chain initiation of liquid phase autoxidations. However, there is no unequivocal evidence for chain initiation of autoxidations via an oxygen activation mechanism. Theories are based on kinetic evidence alone, and many authors have failed to appreciate that conventional procedures for purifying substrate do not remove the last traces of alkyl hydroperoxides from many hydrocarbons. It is usually these trace amounts of alkyl hydroperoxide that are responsible for chain initiation during catalytic reaction with metal complexes. 

The reactions of metal catalysts with alkylperoxy radicals must be considered in liquid phase autoxidations, since peroxy radicals are the most abundant species in solution. The reduction of alkoxy radicals to the corresponding... 

In general, liquid phase autoxidations on hydrocarbons after the initial stages take place, may be considered as co-oxidations with aldehydes, alcohols, ketones, carboxylic acids, etc. Often aldehydes or ketones are deliberately added to hydrocarbon autoxidations in order to promote the reaction. For example, in the cobalt-catalyzed oxidations of alkylaromatics (see Section II.B.3.b), aldehydes, or methyl ethyl ketone are usually added in commercial processes in order to attain high rates and eliminate induction periods. 

The idea that the processes, which occur during the combustion of a fuel in an engine cylinder, take place by a chain reaction mechanism comparable to that associated with the liquid phase autoxidation of benzal-dehyde, has led to the proposal that knock suppressors act by destroying the chains and reducing the rate at which the flame front accelerates. It is known that surfaces or solid bodies suppress chain reactions, in fact one of the criteria for a chain reaction in gaseous combustion processes is the decrease in rate of reaction caused by the increase in surface exposed to the gases. However, the mode by which the chains are broken or... 

The liquid phase autoxidation of aldehydes by molecular oxygen is almost always a homogeneous reaction which is brought about by the intervention of active radical species. The initial kinetics for the different cases studied may be deduced from the general scheme of long kinetic chain radical oxidation already worked out for the oxidation of hydro-... 

Although liquid-phase oxidations of alkanes can be carried out even in the absence of any metal derivative (the role of an inihator of chain radical process can be played by a non-metal compound), derivahves of transition metals are often used in these reactions. Metal-catalyzed autoxidation will be considered in Chapter IX. 

The hypothesis of a bimolecular initiation reaction for liquid phase autoxida-tions was extended beyond cyclohexanone as a reaction partner. Also other substances featuring abstractable H-atoms are able to assist in this radical formation process. The initiation barrier was found to be linearly dependent on the C-H bond strength, ranging from 30 kcal/mol for cyclohexane to 5 kcal/mol for methyl linoleate [14, 15]. Substrates that yield autoxidation products that lack weaker C-H bonds than the substrate (e.g., ethylbenzene) do not show an exponential rate increase as the chain initiation rate is not product enhanced [16]. 

The compound Pd(PPh3)4 can be used as a liquid-phase autoxidation catalyst for cumene oxidation.It is again concluded that the role of the transition metal compound is in its reaction with preformed cumene hydroperoxides. 

The vast majority of liquid phase transition metal catalyzed oxidations of organic compounds fall into these three broad categories (a) free radical autoxidation reactions, (b) reactions involving nucleophilic attack on coordinated substrate such as the Wacker process, or (c) metal catalyzed reactions of organic substrates with hydroperoxides. Of these three classes of oxidations only the first represents the actual interaction of dioxygen with an organic substrate. The function of oxygen in the Wacker process is simply to re-oxidize the catalyst after each cycle [2]. 

Although some autoxidation reactions can be controlled in a useful way, organic substrates more often tend to oxidize in an unselective manner. Oxygen is, after all, a highly reactive molecule and many reaction pathways are open to it. It is imperative, however, that a reaction be selective if it is to have utility either as an economically attractive process or a convenient laboratory synthesis. Despite the rapid advances made during the past decade in the area of liquid phase oxidation, much is still to be learned concerning the efficient control of reactions of molecular oxygen. 

It has long been known that metal salts and complexes promote the reaction of olefins with oxygen in the liquid phase. Early work ([429] and references cited therein) established that during olefin oxidation in the presence of various copper, cobalt and manganese salts, free radicals arise via decomposition of a catalyst-hydroperoxide complex formed from allylic hydroperoxide generated in situ. Although the metal modifies the nature of the observed products in many cases, most homogeneous metal-catalyzed oxidations exhibit characteristics of free radical initiated autoxidations. 

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

Another distinction of polymer autoxidation fixim the oxidation of hydrocarbons concerns reactions involving alkyl radicals. Under conditions of liquid-phase oxidation, usually all alkyl radicals are transformed into peroxyl radicals (the reaction of R- with O2 occurs very rapidly, see Chapter 4). In solid polymer the alkyl macroradical reacts wifli O2 much more slowly (see Chapter 6). This results for polypropylene (PP) oxidation, for example, in the fact that the following two reactions compete with commensurable rates ... 

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