Sulfur compounds, hydroperoxide decomposition

Kinetic Order of the Reaction. The decomposition of the hydroperoxide had an initial slow reaction or induction period followed by a faster main reaction. The induction period was unaffected by the addition of dilauryl sulfinyl dipropionate or by carrying out the reaction in an atmosphere of nitrogen but was eliminated by the addition of acetic acid. The length of the induction period decreased as the initial concentration of both hydroperoxide or sulfur compound increased. 

The final possible mode of action for an antioxidant is as a peroxide decomposer. In the sequences that lead to photodegradation of a polymer the ready fragmentation of the hydroperoxide groups to free radicals is the important step. If this step is interfered with because the peroxide has undergone an alternative decomposition this major source of initiation is removed. The additives which act by decomposing hydroperoxide groups include compounds containing either divalent sulfur or trivalent phosphorus. The mechanism involves... 

Although zinc dialkyl dithiophosphates, [(RO)2PS2]2Zn, have been used as antioxidants for many years, the detailed mechanism of their action is still not known. However, it is certain that they are efficient peroxide decomposers. The effect of a number of organic sulfur compounds, including a zinc dithiophosphate, on the rate of decomposition of cumene hydroperoxide in white mineral oil at 150°C. was investigated by Kennerly and Patterson (13). Each compound accelerated the hydroperoxide decomposition, the zinc salt being far superior in its activity to the others. Further, in each case the principal decomposition product... 

Kennerly and Patterson (13) studied the effect of several organic sulfur compounds, including thiols, sulfides, a disulfide, sulfonic acids, and a zinc dialkyl dithiophosphate, on the decomposition rate of cumene hydroperoxide in white mineral oil at 150 °C. In each case they found phenol as the major product. They suggested that the most attractive mechanism by which to explain these results involves ionic rearrangement catalyzed by acids or other electrophilic reagents (10) as... 

Some organosulfur compounds can function as fuel antioxidants by acting to decompose hydroperoxides. Organosulfides are believed to react with hydroperoxides to form sulfoxides. The sulfoxides then further react with hydroperoxides to form other more acidic compounds. These newly formed acids continue the process of decomposing and reaction with hydroperoxides. Thus, organosulfur compounds function in the process oxidation inhibition through hydroperoxide decomposition. However, in most fuel applications, sulfur-containing antioxidants are not utilized. 

Scheme 2), which acts as a catalyst for the ionic decomposition of hydroperoxides (B-80MI11504, B-81MI11502). Other sulfur compounds known to be active peroxide decomposers are the nickel dialkyldithiocarbamates (3) (B-80MI11505) and the thiol (4) (B-81MI11502). 

Two types of antioxidants are used One type—amines and phenolics —reacts with the peroxy radicals to form more stable free radicals. The second type—sulfur compounds and phosphites—decomposes the hydroperoxides without formation of free radicals. The effect of sulfur compounds, such as dialkyl dithiocarbamates and alkyl thiols on the hydroperoxide decomposition has been investigated by Marshall. 

The object of this work was to gain some insight into the mechanism by which sulfur compounds function as stabilizers by studying the kinetics of the decomposition of hydroperoxides in the presence of dilauryl thio-dipropionate—one of the most widely used sulfur compounds in the stabilization of polyolefins. 

A first-order plot of logi0 [Hydroperoxide] vs. time (Figure 1) was linear, at least until the amount of hydroperoxide decomposed approached a value equal to the initial concentration of sulfur compound. After this point the rate of decomposition increased. Confirmation that the reaction was first order with respect to hydroperoxide was obtained by measuring the initial slopes of the rates of decomposition of varying amounts of... 

The mechanism proposed so far takes account of the induction period and initial stages of the reaction only, and it is difficult to see how it can account for the large amount of hydroperoxide decomposed by the sulfur compound. However, Tetralin hydroperoxide is decomposed catalytically by acids (5). Although in the absence of dilauryl thiodipropionate the decomposition of Tetralin hydroperoxide in the presence of acetic acid at 70 °C. was very slow, if the acid species is a much stronger acid than acetic—e.g., a sulfonic acid as seems likely from the nature of the products of the reaction, the rate of acid-induced decomposition may be comparable with the rate of decomposition by the sulfur compound. Some evidence that acid-induced decomposition does occur at some stage in the over-all reaction is found in the presence of an ortho substituted aromatic compound in the solid product of the reaction. The acid catalyzed decomposition of Tetralin hydroperoxide follows the path of Reaction 14 (5) to give y-(o-hydroxyphenyl)butyraldehyde. This forms a brown resin which is mainly the aldol of this aldehyde (cfthe resin obtained in this work). 

Instead of either an acid-induced decomposition or acid-catalyzed sulfur compound—hydroperoxide decomposition reaction occurring— formation of alternative active species which catalytically decompose the hydroperoxide is possible, such as SCL, as suggested by Hawkins and Sautter (4). However, under conditions of this work where vigorous drying of solvents was not used, SOL could be converted into the acid which would then induce the acid-catalyzed decomposition of the peroxide. 

Preventive antioxidants (sometimes referred to as secondary antioxidants) act by interrupting the secondary oxidation cycle to prevent or inhibit the generation of free radicals. The most important preventive mechanism is the nonradical hydroperoxide decomposition, PD. Phosphite esters and sulfur-containing compounds, e.g., AOs 15-24, Table 1, are the most important groups of peroxide decomposers. Phosphite esters are known as stoichiometric peroxide decomposers (PD-S) they reduce hydroperoxides to... 

The efficient decomposition of hydroperoxides by a non-radical pathway can greatly increase the stabilizing efficiency of a chain-breaking antioxidant. This generally occurs by an ionic reaction mechanism. Typical additives are sulfur compounds and phosphite esters. These are able to compete with the decomposition reactions (either unimolecular or bimolecular) that produce the reactive alkoxy, hydroxy and peroxy radicals and reduce the peroxide to the alcohol. This is shown in the first reaction in Scheme 1.69 for the behaviour of a triaryl phosphite, P(OAr)3 in reducing ROOH to ROH while itself being oxidized to the phosphate. 

There are two ways in which stabilizers can function to retard autoxidation and the resultant degradation of polymers. Preventive antioxidants reduce the rate of initiation, e.g., by converting hydroperoxide to nonradical products. Chain-breaking antioxidants terminate the kinetic chain by reacting with the chain-propagating free radicals. Both mechanisms are discussed and illustrated. Current studies on the role of certain organic sulfur compounds as preventive antioxidants are also described. Sulfenic acids, RSOH, from the decomposition of sulfoxides have been reported to exhibit both prooxidant effects and chain-breaking antioxidant activity in addition to their preventive antioxidant activity as peroxide decomposers. 

Recently we reported a study of the peroxide decomposing activity of sulfoxides, sulfenic acids, thiolsulfinates, and their oxidation or decomposition products (2,15). A benzene solution of the sulfenic acid reacted rapidly with both tert-butyl hydroperoxide and cumene hydroperoxide, consuming two moles of hydroperoxide per mole of sulfur compound. A slower catalytic process destroyed many additional moles of hydroperoxide per mole of sulfur compound. A similar solution of cumene hydroperoxide in benzene with thiolsulfinate present in a ratio of 10 moles of hydroperoxide per mole of sulfur compound showed no change for 22 hr at 25°C. At that time a catalytic decomposition started which destroyed many moles of hydroperoxide per mole of sulfur compound. 

These experiments were repeated in the presence of CaC03 to see if the base would affect the activity of the sulfur compounds as peroxide decomposers. The initial reaction of sulfenic acid with the hydroperoxide was slowed, as shown in Figure 2, but ultimately consumed two moles of ROOH per mole of RSOH. The subsequent catalytic decomposition was almost completely stopped with excess of base consistent with neutralization of an acid catalyst, presumed to be the sulfonic acid formed by oxidation of the sulfenic acid ... 

The products from cumene hydroperoxide decomposition induced by organic sulfur compounds were determined by quantitative NMR except for phenol by high-pressure liquid chromatography and cumene hydroperoxide by iodometric titration (16). Cumyl alcohol is produced in the initial oxidation of sulfenic acid to sulfonic acid, and subsequently most of it is converted to a-methylstyrene as shown in Table II. The major products (40-45%) are phenol and acetone consistent with an acid-catalyzed decomposition of cumene hydroperoxide. Considerable... 

Table II. Products from Cumene Hydroperoxide Decomposition Induced by Organic Sulfur Compounds (16)...

The polar nature of the catalytic decomposition of cumene hydroperoxide with added terf-butyl terf-butanethiolsulfinate has been clearly established (I). The thiolsulfinate is converted into an active peroxide decomposer capable of destroying many moles of hydroperoxide per mole of sulfur compound. The acidic character of the active species was demonstrated by its effective neutralization with the added base calcium carbonate. Formation of the active peroxide decomposer may be envisaged as involving one or more of the following three reaction types concerted process, ionic processes, and free-radical processes. 

Radical involvement is indicated also by the inhibitory effect of radical trapping agents and product analysis, since 10-15% of the products from cumene hydroperoxide decomposition induced by the organic sulfur compounds result from free-radical processes (2). Acids will... 

When air or oxygen is bubbled through or otherwise contacted with liquid cumene at temperatures in the range of 100-130 C, oxidation occurs with resultant formation of cumene hydroperoxide, which is comparatively stable under these conditions. The Usual oxidation catalysts, such as salts of the transition metals, cannot be used for the reaction ance they tend to cause decomposition of the cumene hydroperoxide. Purity of the charge material is important since small amounts of certain impurities such as sulfur compounds, phenols, aniline, unsaturated hydrocarbons, and the like act as inhibitors to break the chain reaction and thereby slow down the reaction. The maximum reaction rate is attained after a portion of hydroperoxide is formed in fresh cumene charge. ... 

However, hydroperoxide decomposers may act by much more complicated mechanisms. Many sulfur compounds, like the thiodipropionate esters (DRTPs) or the metal dialkyldithiocarbamates (MRDCs) are oxidised to sulfur acids (sulfinic, sulfonic and SO3) which are ionic catalysts for the non-radical decomposition of hydroperoxides. The MRDCs are particularly important since, unlike the phosphites, they also contain complex transition metal ions and when M is a transition metal ion e.g. Ni) they are also UVAs. 

Undoubtedly more important than the rate of decomposition of hydroperoxides is the type of product obtained. Table 5 lists the major products obtained by completely decomposing TBHP with various sulfur compounds. With nickel and zinc dithiocarbamates... 

Substances capable of reacting with hydroperoxides without creating radical decomposition products are called secondary antioxidants. These are in particular phosphites and phosphonites as well as sulfur costabilizers. Sulfur compounds are already very active at room temperature, while phosphites are active only at increased temperatures. The active application range of phosphites/phosphonites is therefore essentially limited to common processing temperatures. 

If in the chain initiated reaction when v,- = const the induction period is independent of the efficiency of retardation action of the inhibitor but is determined by its concentration, then during autoxidation the inhibitor is more slowly consumed when it more efficiently terminate chains because ROOM is more slowly accumulated and the retardation period increases. Then the initiated oxidation of hydrocarbons is retarded only by compounds terminating chains. Autoxidation is retarded by compounds decomposing hydroperoxides. This decomposition, if it is not accompanied by the formation of free radicals, decreases the concentration of the accumulated hydroperoxide and, hence, the autoxidation rate. Hydroperoxide decomposition is induced by compounds of sulfur, phosphorus and various metal complexes, for example, thiophosphate, thiocarbamates of zinc, nickel, and other metals. 

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