Autoxidation inhibitors

By-products formed during their preparation (e.g., ethylbenzene and divinyl-benzenes in styrene acetaldehyde in vinyl acetate) added stabilizers (inhibitors) autoxidation and decomposition products of the monomers (e.g., perox-... 

By-products formed during their preparation (e.g., ethylbenzene and divin-ylbenzenes in styrene acetaldehyde in vinyl acetate) added stabilizers (inhibitors) autoxidation and decomposition products of the monomers (e.g., peroxides in dienes, benzaldehyde in styrene, hydrogen cyanide in acrylonitrile) impurities that derive from the method of storage of the monomer (e.g., traces of metal or alkali from the vessels, tap grease etc.) dimers, trimers, and polymers that are generally soluble in the monomer, but sometimes precipitate, for example, polyac-rylOTiitrile from acrylonitrile. Likewise, in polycondensation reactions it is important to remove reactive impurities because they can cause considerable interference during the polyreaction. 

Chemical Properties. Without inhibitors, tetrahydrofurfuryl alcohol is susceptible to autoxidation, developing color and carbonyl functionality. In the absence of air, however, no observable changes occur even after several years storage. In the presence of air, if a stabilizer such as Naugard is added, tetrahydrofurfuryl alcohol remains colorless after protracted periods of storage. Peroxide accumulation is low, not dangerous, and readily dischargeable on redistillation. 

Another method for producing petoxycatboxyhc acids is by autoxidation of aldehydes (168). The reaction is a free-radical chain process, initiated by organic peroxides, uv irradiation, o2one, and various metal salts. It is terrninated by free-radical inhibitors (181,183). In certain cases, the petoxycatboxyhc acid forms an adduct with the aldehyde from which the petoxycatboxyhc acid can be hberated by heating or by acid hydrolysis. If the petoxycatboxyhc acid remains in contact with excess aldehyde, a redox disproportionation reaction occurs that forms a catboxyhc acid ... 

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 ... 

It should be pointed out that not all benzoin derivatives (75) are suitable for use as photoinitialors. Benzoin esters (75, R=aeyl) undergo a side reaction leading to furan derivatives. Aryl ethers (75, R=aryl) undergo (3-seission to give a phenoxy radical (an inhibitor) in competition with a-scission (Scheme 3.54). Benzoin derivatives with a-hydrogens (75 R-Il) are readily autoxidized and consequently can have poor shelf lives. 

Complexes such as [Ni(16)] are known to stoichiometrically interact with 02 to give 1 1 adducts and subsequently the autoxidized Ni111 species (compare Section 6.3.4.10.2(v)). 5 Such systems have been tested for the NiIII-catalyzed cleavage of DNA (see Section 6.3.4.10.2(v)). It has been suggested that Ni11 macrocycle complexes with rather low Nin/Nim reduction potentials can be active inhibitors of aldehyde autoxidation.156... 

The autoxidation of aldehydes, and of other organic compounds, may be lessened considerably by very careful purification—removal of existing peroxides, trace metal ions, etc.—but much more readily and effectively by the addition of suitable radical inhibitors, referred to in this context as anti-oxidants. The best of these are phenols and aromatic amines which have a readily abstractable H atom, the resultant radical is of relatively low reactivity, being able to act as a good chain terminator (by reaction with another radical) but only as a poor initiator (by reaction with a new substrate molecule). 

Vinyl acetate is normally inhibited with hydroquinone to prevent polymerisation. A combination of too low a level of inhibitor and warm, moist storage conditions may lead to spontaneous polymerisation. This process involves autoxidation of acetaldehyde (a normal impurity produced by hy droly sis of the monomer) to a peroxide which initiates exothermic polymerisation as it decomposes. In bulk, this may accelerate to a dangerous extent. Other peroxides or radical sources will initiate the exothermic polymerisation. 

Numerous autoxidation reactions of aliphatic and araliphatic hydrocarbons, ketones, and esters have been found to be accompanied by chemiluminescence (for reviews see D, p. 19 14>) generally of low intensity and quantum yield. This weak chemiluminescence can be measured by means of modern equipment, especially when fluorescers are used to transform the electronic excitation energy of the triplet carbonyl compounds formed as primary reaction products. It is therefore possible to use it for analytical purposes 35>, e.g. to measure the efficiency of inhibitors as well as initiators in autoxidation of polymer hydrocarbons 14), and in mechanistic studies of radical chain reactions. 

The duration of the inhibition period of a chain-breaking inhibitor of autoxidation is proportional to its efficiency. Indeed, with an increasing rate of chain termination, the rates of hydroperoxide formation and, hence, chain initiation decrease, which results in the lengthening of the induction period (this problem will be considered in a more detailed manner later). It should be noted that when initiated oxidation occurs as a straight chain reaction, the induction period depends on the concentration of the inhibitor, its inhibitory capacity, and the rate of initiation, but does not depend on the inhibitor efficiency. 

As shown above (see earlier) for straight chain reactions, the inhibitor is consumed at a constant rate v-Jf Similarly, during the inhibited autoxidation of RH, the inhibitor is initially consumed at a constant rate vi0/f, but then the rate of inhibitor consumption drastically increases [57,58], which leads to a rapid accumulation of hydroperoxide and the enhancement of initiation (see Figure 14.1). 

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. 

The mechanisms responsible for inhibited oxidation depend on the experimental conditions and particular properties of RH and antioxidant (see earlier). Let us assume that hydroperoxide is relatively stable, so that it virtually does not decompose during the induction period (kdr -c 1). Actually, this means that the rate of ROOH formation is much higher than the rate of its decomposition, / 2[RH] [RO]2 ] 3> d[ROOH]. For each of the mechanisms of inhibited autoxidation, there is a relationship between the amounts of the inhibitor consumed and hydroperoxide produced (see Tablel4.2). For example, for mechanism V with key reactions (2), (7), (—7), and (8), we can get (by dividing the oxidation rate v into the rate of inhibitor consumption) the following equation ... 

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]... 

Table 14.9 summarizes respective formulae for kq of optimal inhibitors as functions of T, [InH]0,/, and k3. At V = const, the kq value of optimal inhibitor decreases with increasing temperature. But during autoxidation, kq and T change unidirectionally. Such an inconsistency is due to an inverse relation between the efficiency of inhibitor and the temperature dependence of zyo. The temperature-dependent rate constant k3 may also contribute to this inconsistency, with the contribution depending on the ratio k3/( 1 + /)[InH]0. 

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). 

Nitroxyl radicals as alkyl radical acceptors are known to be very weak antioxidants due to the extremely fast addition of dioxygen to alkyl radicals (see Chapter 2). They retard the oxidation of solid polymers due to specific features of free radical reactions in the solid polymer matrix (see Chapter 19). However, the combination of two inhibitors, one is the peroxyl radical acceptor (phenol, aromatic amine) and another is the alkyl radical acceptor (nitroxyl radical) showed the synergistic action [44-46]. The results of testing the combination of nitroxyl radical (>NO ) (2,2,6,6-tetramethyl-4-benzoylpiperidine-l-oxyl) + amine (phenol) in the autoxidation of nonene-1 at 393 K are given here ([>NO ]o + [InH]o = 1.5 x 10 4mol L 1 p02 98 kPa) [44]. 

The effects of flavonoids on in vitro and in vivo lipid peroxidation have been thoroughly studied [123]. Torel et al. [124] found that the inhibitory effects of flavonoids on autoxidation of linoleic acid increased in the order fustin < catechin < quercetin < rutin = luteolin < kaempferol < morin. Robak and Gryglewski [109] determined /50 values for the inhibition of ascorbate-stimulated lipid peroxidation of boiled rat liver microsomes. All the flavonoids studied were very effective inhibitors of lipid peroxidation in model system, with I50 values changing from 1.4 pmol l-1 for myricetin to 71.9 pmol I 1 for rutin. However, as seen below, these /50 values differed significantly from those determined in other in vitro systems. Terao et al. [125] described the protective effect of epicatechin, epicatechin gallate, and quercetin on lipid peroxidation of phospholipid bilayers. 

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. 

H. S. Olcott (University of California, Berkeley, Calif.) We have studied the effects of aliphatic amines on the autoxidation of a fish oil and squalene in air at moderate temperatures. There was little protection unless phenolic-type inhibitors were also added, in which case secondary amines were more effective than primary or secondary amines. However, at 70 °C. trioctylamine alone protected the fish oil, whereas at lower temperatures it did not (2). Further study revealed that peroxides react with trioctylamine to yield some dioctylhydroxylamine which has antioxidant properties (1). These and other observations (3) indicate that... 

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. 

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. 

In our method reproducible non-stationary states are effected as follows the low stationary-state rate of an autoxidizing hydrocarbon is decreased by a factor of 2 to 5 by adding an appropriately small amount of inhibitor. Under the conditions outlined below, the time required to establish the new stationary state at the inhibited rate is not immeasurably small, as it would be in conventional measurements, but of the order of 100-300 sec. With sufficiently sensitive apparatus a number of determinations of the decreasing velocity can be made, which delineate the course of the non-steady state. Similarly a non-steady state with an increasing velocity can be realized by introducing a small amount of initiator. 

For experimental reasons (discussed below) the relative rate of decrease should be smaller than 0.01 sec."1. Since kA is of the order of 104 liter per mole per sec., [AH] must be of the order of 10 6 mole per liter. This low concentration of inhibitor will only have an appreciable effect on the rate of autoxidation if the rate of initiation—and consequently the rate of oxidation—is low. To measure the decrease in rate during the short-lived non-steady state, one must be able to determine these low velocities within short periods of time. From the usual inhibition formulas one can compute, for instance, that in order to obtain a ratio of original to inhibited rate of about 5 with [AH] = 10 6 mole per... 

The stable free radical diphenylpicrylhydrazinyl (39) was first prepared by Goldschmidt in 1922,and this " and galvinoxyl (40) have found application as radical scavengers in kinetic studies. " Davies and Roberts used galvinoxyl as a radical inhibitor to show the radical nature of the autoxidation of 1-phenylethylboronic acid. " ... 

CyHeO, Mr 106.12, Z />101.3kPa 178.1 °C, d] 1.0415, Wp 1.5463, is the main, characteristic component of bitter almond oil. It occurs in many other essential oils and is a colorless liquid with a bitter almond odor. In the absence of inhibitors, benzaldehyde undergoes autoxidation to perbenzoic acid, which reacts with a second molecule of benzaldehyde to benzoic acid. 

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