Initiation, rate, autoxidation

The concentrations of the products change during oxidation and, consequently, the absolute value as well as the ratio of partial initiation rates increases. The following table provides the example of dynamics of free radical generation in the autoxidation of 2-methylbutane at T=412K [130],... 

For initiated oxidation, the inhibitory criterion could be defined as the ratio v0/v or (v0/ v — v/v0), where v0 and v are the rates of initiated oxidation in the absence and presence of the fixed concentration of an inhibitor, respectively. Another criterion could be defined as the ratio of the inhibition coefficient of the combined action of a few antioxidants / to the sum of the inhibition coefficients of individual antioxidants when the conditions of oxidation are fixed (fx = IfiXi where f, and x, are the inhibition coefficient and molar fraction of z th antioxidant terminating the chain). It should, however, be noted that synergism during initiated oxidation seldom takes place and is typical of autoxidation, where the main source of radicals is formed hydroperoxide. It is virtually impossible to measure the initial rate in the presence of inhibitors in such experiments. Hence, inhibitory effects of individual inhibitors and their mixtures are usually evaluated from the duration of retardation (induction period), which equals the span of time elapsed from the onset of experiment to the moment of consumption of a certain amount of oxygen or attainment of a certain, well-measurable rate of oxidation. Then three aforementioned cases of autoxidation response to inhibitors can be described by the following inequalities (r is the induction period of a mixture of antioxidants). 

The quantity and quality of experimental information determined by the new techniques call for the use of comprehensive data treatment and evaluation methods. In earlier literature, quite often kinetic studies were simplified by using pseudo-first-order conditions, the steady-state approach or initial rate methods. In some cases, these simplifications were fully justified but sometimes the approximations led to distorted results. Autoxidation reactions are particularly vulnerable to this problem because of strong kinetic coupling between the individual steps and feed-back reactions. It was demonstrated in many cases, that these reactions are very sensitive to the conditions applied and their kinetic profiles and stoichiometries may be significantly altered by changing the pH, the absolute concentrations and concentration ratios of the reactants, and also by the presence of trace amounts of impurities which may act either as catalysts and/or inhibitors. 

Initiation, polymer autoxidation, 3 102,103 Initiation rate constants ( ), in VDC... 

In the examples above, one or both of the reaction centers are already attached to the metal center. In many cases, the reactants are free before reaction occurs. If a metal ion or complex is to promote reaction between A and B, it is obvious that at least one species must coordinate to the metal for an effect. It is far from obvious whether both A and B enter the coordination sphere of the metal in a particular instance. A number of metal-oxygen complexes can oxygenate a variety of substrates (SOj, CO, NO, NO2, phosphines) in mild conditions. Probably the substrate and O2 are present in the coordination sphere of the metal during these so-called autoxidations. In the reaction of oxygen with transition metal phosphine complexes, oxidation of metal, of phosphine or of both, may result. The initial rate of reaction of O2 with Co(Et3P)2Cl2 in tertiary butylbenzene. 

Let us now consider what we mean by the reactivity of a hydrocarbon in autoxidation. One measure is how fast it oxidizes by itself at unit concentration and unit rate of initiation. (Rates of thermal oxidation at unknown rates of initiation are not useful enough to be considered.) The first two columns of figures in Table VII give such comparisons in terms of kp/(2kt)l/l at 30° and 60°C. and determine the order in which hydrocarbons are listed in the table. The results of Ingold and Sajus agree fairly well the orders of reactivity are identical except for a trivial difference with the xylenes. The stated quotients at 60°C. are uniformly 2 to 3 times as large as at 30°C. for sec-butylbenzene and more reactive hydrocarbons but 3 to 6 times as large for less reactive hydrocarbons. 

Initiated Oxidation. The initial rates of oxidation of chloroprene, initiated with 2,2 -azobisisobutyronitrile, were measured in the range 20° to 40°C. at a total pressure of 700 mm. of Hg. The difficulty with these measurements was that chloroprene autoxidizes so readily that even when the initiator is used at the fairly massive concentration of 0.462M, the rate of oxidation is constant for only a few minutes before acceleration, resulting from a contribution to initiation from chloroprene peroxide. 

AIBN is, however, virtually unaffected by the presence of dithio-phosphates (Table II). Further, with specific reference to the oxidation of the disulfide in Table I, which has no effect on the rate of AIBN-initi-ated autoxidation of cumene (6), it is unlikely that the efficiency of radical production from AIBN increases since this would produce a prooxidant effect in cumene. Thus, the zinc salt inhibitor is being oxidized in competition with the main chain reaction. 

If this mechanism is strictly followed the chain length and hence the value of 02-uptake (see below) increases linearly with the substrate concentration and (initiation rate)"1/2 (i.e., in radiolytic studies the dose rate) and in charged polymers also on the pH (cf. Ulanski et al. 1996a). In polymers, the chain reaction may mainly proceed intramolecularly (Ulanski et al. 1996a Janik et al. 2000). An example for an efficient intramolecular autoxidation is poly(acrylic acid) [reactions (34)-(36) Ulanski et al. 1996a], In these autoxidation reactions, hydroperoxides are formed which, in some cases, are quite unstable [e.g. reaction (37) see also Leitzke et al. 2001],... 

The experimental data of Lamb and Elder (96), however, are not in agreement with this predicted rate expression for they find the initial rate proportional to the square of the ferrous ion concentration and directly proportional to the oxygen pressure. This has recently been confirmed by the author (97), and it would appear that either the autoxidation is subject to a true catalysis by trace impurities (an induced reaction is excluded by the total ferrous ion oxidized being large, about M/20) or the actual mechanism is different from that suggested by Weiss. 

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 mechanism of hydroquinone autoxidation likely proceeds by a radical chain pathway. Kinetic studies carried out under relevant reaction conditions support a second-order rate law for the reaction, rate =Ar[QH2] [Oj], with an apparent activation energy of = 15 kcal/mol [21]. Based on these kinetic findings, as well as DFT studies [22], anthrahydroquinone autoxidation has been proposed to occur through initial, rate-limiting, direct H-atom abstraction from the hydroquinone species by O2 (Eq. (14.2)). The semiquinone species then react readily with triplet O2 (Eq. (14.3)), and hydroperoxy radical, HO2, has been proposed to act as a radical chain carrier (Eq. (14.4)). 

One possible way of overcoming this problem is to introduce into the reaction mixture a compound that decomposes at a constant rate to free radicals (X ) capable of extracting a hydrogen atom from the PUFAs (RH) and consequently initiating the autoxidation process. The compounds most frequently used for this are the so-called azo-initiators (X-N=N=X), which thermally decompose to highly reactive carbon-centered radicals. ... 

Reactions (22) and (23) require metal ions which can exist in two oxidation states with a suitable redox potential. Reaction (22) is usually much more rapid than reaction (23), and the metal is converted mostly in its most oxidized state, so that the rate of chain initiation depends on reaction (22). In the presence of metals, these reactions thus initiate additional autoxidation chains, which accelerate the rates of lipid oxidation. 

Aqueous dispersions of charged copolymer latexes are active supports of cobalt catalysts for autoxidations of tetralin by cobalt-pyridine complexes and of 2,6-di-rm-butylphenol and 1-decanethiol by CoPcTs. Since these reactants are insoluble in water, a simple explanation of the catalytic activity is that the organic polymer serves to solubilize the reactants in the phase that contains the cobalt catalyst. All three reactions have initial rates that are independent of substrate concentration, as determined by absorption of dioxygen from a gas buret. All three reactions appear to proceed by different mechanisms. Tetralin autoxidation is a free radical chain process promoted by the CoPy complex, whereas the CoPcTs reactions are not free radical chain processes. The thiol autoxidation is reported to involve hydrogen peroxide, whereas the 2,6-di-re/t-butylphenol autoxidation apparently does not. 

If oxidation is carried out without an initiator, that is, in the autoxidation regime, it occurs with self-acceleration due to the initiation rate increasing during the reaction. It is very important that the temp of acceleration depends on the rate of chain oxidation, i.e., there is a positive feedback between the processes of autoinitiation and chain oxidation of RH. This relationship is also manifested in the inhibition oxidation of organic compounds. 

Note, Added in Proof-. In their study of the autoxidation of 2-butyl-isoindoline, Kochi and Singleton showed that 2-butylisoindole is formed and is converted by further oxidation to 2-butylphthalimide and 2-butylphthalimidine. The rate of oxidation of 2-butylisoindoline to the isoindole was found to be markedly dependent on hydrogen donor ability of the solvent and was shoivn to involve a free radical chain process. Autoxidation of 2-butylisoindole also appears to be a radical process since it can initiate autoxidation of 2-butylisoindoline. 

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). 

We can see that the rate of initiation increases during autoxidation from 10 7 to 10 4 mol L 1 s-1 (about 1000 times). Due to increasing concentrations of 2-methylbutyl alcohol and acetic acid, the latter becomes a very important reactant in the reactions of free radical generation. 

---