Autoxidation, of hydrocarbons

Other nonpolymeric radical-initiated processes include oxidation, autoxidation of hydrocarbons, chlorination, bromination, and other additions to double bonds. The same types of initiators are generally used for initiating polymerization and nonpolymerization reactions. Radical reactions are extensively discussed in the chemical Hterature (3—15). 

Free-radical chain inhibitors are of considerable economic importance. The term antioxidant is commonly appUed to inhibitors that retard the free-radical chain oxidations, termed autoxidations, that can cause relatively rapid deterioration of many commercial materials derived from organic molecules, including foodstuffs, petroleum products, and plastics. The chain mechanism for autoxidation of hydrocarbons is ... 

The sulfenic acids have been found to be extremely active radical scavengers showing rate constants of at least 107 m"1 s 1 for the reactions with peroxyl radicals at 333 K17. It has also been suggested that the main inhibiting action of dialkyl sulfoxides or related compounds in the autoxidation of hydrocarbon derives from their ability to form the transient sulfenic acids on thermal decomposition, i.e.17... 

Reich, L. and Stivala, S. (1969) Autoxidation of Hydrocarbons and Polyolefins, Marcel Dekker, New York. 

The autoxidation of ethers occurs with self-acceleration as autoxidation of hydrocarbons. The kinetics of such reactions was discussed earlier (see Chapter 2). The autoacceleration of ether oxidation occurs by the initiating activity of the formed hydroperoxide. The rate constants of initiation formed by hydroperoxides were estimated from the parabolic kinetic... 

This problem was first approached in the work of Denisov [59] dealing with the autoxidation of hydrocarbon in the presence of an inhibitor, which was able to break chains in reactions with peroxyl radicals, while the radicals produced failed to contribute to chain propagation (see Chapter 5). The kinetics of inhibitor consumption and hydroperoxide accumulation were elucidated by a computer-aided numerical solution of a set of differential equations. In full agreement with the experiment, the induction period increased with the efficiency of the inhibitor characterized by the ratio of rate constants [59], An initiated inhibited reaction (vi = vi0 = const.) transforms into the autoinitiated chain reaction (vi = vio + k3[ROOH] > vi0) if the following condition is satisfied. 

As described earlier, the mechanism of inhibited chain oxidation depends on the structural features of RH and InH, as well as on the reaction conditions (T, v,[RH], [InH], [O2], and [ROOH]). In this section we present data illustrating this approach with reference to the autoxidation of hydrocarbons inhibited by sterically nonhindered phenols of group A. 

It was noted by Ohto et al. (1977b) that PBN inhibits the AIBN-initiated autoxidation of hydrocarbons, and that in the course of these inhibited... 

Table II. Rate Constants and Kinetic Parameters for Chain Termination in Autoxidation of Hydrocarbons as Determined with the Rotating Sector (25, 26, 27, 28) (Neat Hydrocarbon or Hydrocarbon Diluted with Chlorobenzene)...

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. 

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. 

Autoxidation of Hydrocarbons Catalyzed by Cobalt and Bromide Ions... 

The autoxidation of hydrocarbons catalyzed by cobalt salts of carboxylic acid and bromide ions was kinetically studied. The rate of hydrocarbon oxidation with secondary hydrogen is exactly first order with respect to both hydrocarbon and cobalt concentration. For toluene the rate is second order with respect to cobalt and first order with respect to hydrocarbon concentration, but it is independent of hydrocarbon concentration for a long time during the oxidation. The oxidation rate increases as the carbon number of fatty acid solvent as well as of cobalt anion salt are decreased. It was suggested that the cobalt salt not only initiates the oxidation by decomposing hydroperoxide but also is responsible for the propagation step in the presence of bromide ion. 

T he rate of metal salt-catalyzed autoxidation of hydrocarbons reaches - a maximum at a certain catalyst concentration (1, 7, 13), and any further increases in this concentration do not accelerate the rate. However, when bromide ion is added to the solution of hydrocarbon and fatty acid with metal salts, the oxidation rate increases over the maximum value of k32(RH)2/2k6 as a function of metal concentration. 

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. 

The complexes [Cu(S2CNEt2]2] and [Cu S2P(OPr )2 2] have been shown to be extremely effective scavengers for peroxy radicals and can be used to inhibit the autoxidation of hydrocarbons.99 Poly(2,6-dimethyl-1,4-phenylene oxide) can be effectively stabilized against thermal degradation by the bistriazene complex (41).100 The stabilizing action is thought to involve quenching of thermally excited states and the decomposition of hydroperoxides by the complex. 

On the other hand dehydrochlorinated polyvinylchloride li > and polimethyl-jS-chlorvinyl-ketone 74> catalyze the autoxidation of hydrocarbons, and the activities are related to the semiconductive properties of the catalysts. Recently it has been shown that entirely inert polymers like polyethylene, polypropylene and polyftetrafluoro) ethylene are rather efficient catalysts for the oxidation of te-tralin 75>. 

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 ... 

Many liquid phase oxidations, hereafter known as autoxidations, occur virtually spontaneously under relatively mild conditions of temperature and oxygen pressure. They are frequently subject to autocatalysis by products (i.e., hydroperoxides, peracids, etc.). The liquid phase autoxidation of hydrocarbons has been studied extensively and is the subject of several monographs and reviews.17 29 With few exceptions, the majority of liquid phase autoxidations... 

---