Hydroperoxide dithiophosphates

When the operating temperature exceeds ca 93°C, the catalytic effects of metals become an important factor in promoting oil oxidation. Inhibitors that reduce this catalytic effect usually react with the surfaces of the metals to form protective coatings (see Metal surface treatments). Typical metal deactivators are the zinc dithiophosphates which also decompose hydroperoxides at temperatures above 93°C. Other metal deactivators include triazole and thiodiazole derivatives. Some copper salts intentionally put into lubricants counteract or reduce the catalytic effect of metals. 

Zinc dithiophosphates act as anti-oxidants by promoting the decomposition of hydroperoxides. The mechanism of this reaction is complicated involving hydroperoxides and peroxy radicals192,193 and is also affected by the other additives present in the lubricant oil.194 However the first step is thought to be a rapid initial reaction of the zinc dithiophosphate and hydroperoxide to give a basic compound [Zn4(/i4-0)(S2P(0R)2)6] (Equation 88 Figure 9).141... 

Metal dialkyl dithiocarbamates inhibit the oxidation of hydrocarbons and polymers [25,28,30,76 79]. Like metal dithiophosphates, they are reactive toward hydroperoxides. At room temperature, the reactions of metal dialkyl dithiocarbamates with hydroperoxides occur with an induction period, during which the reaction products are formed that catalyze the breakdown of hydroperoxide [78]. At higher temperatures, the reaction is bimolecular and occurs with the rate v = k[ROOH][inhibitor]. The reaction of hydroperoxide with dialkyl dithiocarbamate is accompanied by the formation of radicals [30,76,78]. The bulk yield of radicals in the reaction of nickel diethyl dithiocarbamate with cumyl hydroperoxide is 0.2 at... 

A combined addition of a chain-breaking inhibitor and a hydroperoxide-breaking substance is widely used to induce a more efficient inhibition of oxidative processes in polyalkenes, rubbers, lubricants, and other materials [3 8]. Kennerly and Patterson [12] were the first to study the combined action of a mixture, phenol (aromatic amine) + zinc dithiophosphate, on the oxidation of mineral oil. Various phenols and aromatic amines can well serve as peroxyl radical scavengers (see Chapter 15), while arylphosphites, thiopropionic ethers, dialkylthio-propionates, zinc and nickel thiophosphates, and other compounds are used to break down hydroperoxide (see Chapter 17). Efficient inhibitory blends are usually prepared empirically, by choosing such blend compositions that induce maximal inhibitory periods [13],... 

Shopov, D., Yordanov, N. D. Interaction of copper(II) dithiophosphates and coppeifll) dithiooarbamates with organic hydroperoxides-EPR study. Proc. XIVth Intern. Conf. Chem. Toronto 1972, 236. 

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... 

More recently it has been shown (6, 7) that zinc dialkyl dithiophosphates also act as chain-breaking inhibitors. Colclough and Cunneen (7) reported that zinc isopropyl xanthate, zinc dibutyl dithiocarbamate, and zinc diisopropyl dithiophosphate all substantially lowered the rate of azobisisobutyronitrile-initiated oxidation of squalene at 60°C. Under these conditions, hydroperoxide chain initiation is negligible, and it was therefore concluded that inhibition resulted from removal of chain-propagating peroxy radicals. Also, consideration of the structure of these zinc dithioates led to the conclusion that no suitably activated hydrogen atom was available, and it was suggested that inhibition could be accounted for by an electron-transfer process as follows ... 

C Zinc diisopropyl dithiophosphate, 0.0013M, plus cumene hydroperoxide, 0.017M... 

Peroxide Decomposition Mechanism. Since virtually no work has been reported which concerns only the mechanism by which zinc dialkyl di-thiophosphates act as peroxide decomposers, it is pertinent to discuss metal dialkyl dithiophosphates as a whole. The mechanism has been studied both by investigating the products and the decomposition rates of hydroperoxides in the presence of metal dithiophosphates and by measuring the efficiency of these compounds as antioxidants in hydrocarbon autoxidation systems in which hydroperoxide initiation is significant. 

We have carried out a limited study of the effect of metal dialkyl dithiophosphates on a hydroperoxide-autocatalyzed oxidation system. Table III summarizes induction periods for the oxidation of squalane at 140 °C. These results do not unambiguously reflect the peroxide-decomposing property of each dithiophosphate radical capture also occurs. 

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... 

Shopov and his co-workers have recently published two papers on hydroperoxide decomposition by barium dialkyl dithiophosphates. The decomposition rate of cumene hydroperoxide at 140 °C. in the presence of barium dibenzyl dithiophosphate was found (20) not to be described by Equation H. A mechanism, similar to that of Kennerly and Patterson (13) but slightly more detailed was proposed as follows ... 

The catalytic nature of the action of metal dialkyl dithiophosphates in the decomposition of cumene hydroperoxide at room temperature has been clearly shown by Holdsworth, Scott, and Williams (11) They... 

No readily acceptable mechanism has been advanced in reasonable detail to account for the decomposition of hydroperoxides by metal dialkyl dithiophosphates. Our limited results on the antioxidant efficiency of these compounds indicate that the metal plays an important role in the mechanism. So far it seems, at least for the catalytic decpmposition of cumene hydroperoxide on which practically all the work has been done, that the mechanism involves electrophilic attack and rearrangement as shown in Scheme 4. This requires, as commonly proposed, that the dithiophosphate is first converted to an active form. It does seem possible, on the other hand, that the original dithiophosphate could catalyze peroxide decomposition since nucleophilic attack could, in principle, lead to the same chain-carrying intermediate as in Scheme 4 thus,... 

The zinc dithiophosphates (7) appear to be active antioxidants they produce sulfur acids by reduction of hydroperoxides and also act as radical scavengers (B-79MI11502). 

Decomposition of ZDDP takes place in the presence of oxygen, either coming from oxygen dissolved in engine oil or from peroxy radicals and hydroperoxides. Solution studies of the oxidation of zinc dialkyl dithiophosphates by peroxy radicals have shown that disulfides are major reaction products (Paddy et al., 1990 Rossi and Imparato, 1971 Willermet, 1998 Willermet et al., 1983 Willermetand Kandah, 1984). 

Besides hindered amines (see Section 9.3.4), there are compounds that are capable of functioning as long-term hydroperoxide decomposers. These include alkyl and aryl phosphites, and organosulfur compounds such as dialkyl dithio-carbamates, dithiophosphates, and dithioalkyl propionates (see Chart 9.14). 

Applying the steady-state approximation to the radicals R and OH- a rate law of the form above may be derived where [Cu J ofCu L j. The inverse dependence on [L l indicates that [Cu L] does not react with HgOj at a significant rate. A chain reaction has also been postulated in the oxidation of copper(n) dialkyldithiophos-phates with alkyl hydroperoxides. Initially, the redox process yields alkoxy and alkyl peroxy radicals. The kinetics of the initial reaction are second-order. For the dithiophosphate, the reaction is first-order with respect to each reagent, whereas for the corresponding dithiocarbamate complex there is a zero-order dependence on the metal species and a second-order dependence on the hydroperoxide concentration. 

Metal complexes, first of all, dialkyl dithiophosphates and dialkyl dithiocarba-mates of such metals as Zn, Ni, Ba, and Ca, are widely used for the stabilization of polymers and lubricants. Inhibitors of this type are inferior to phenols in efficiency at moderate temperatures (350-400 K) but exceed them at higher temperatures (430-480 K). The mechanism of action of inhibitors of this type is complicated. The reaction of these inhibitors with hydroperoxide plays a very important role in the complex mechanism of inhibition. 

Figure 9 Formation and decay of products formed in the reaction between cumene hydroperoxide (CHP) and nickel dibutyl dithiophosphate (NiDBP) at molar ratio [CHP]/[NiDBP] = 110 in chlorobenzene at 100°C (numbers on curves are vibrational frequencies in cm" ). The decay of CHP is superimposed (reproduced from ref. 110 by permission of Butterworth...

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