Hydroperoxide decomposer

An important descriptor of a chain reaction is the kinetic chain length, ie, the number of cycles of the propagation steps (eqs. 2 and 3) for each new radical introduced into the system. The chain length for a hydroperoxide reaction is given by equation (10) where HPE = efficiency to hydroperoxide, %, and 2/ = number of effective radicals generated per mol of hydroperoxide decomposed. For 100% radical generation efficiency, / = 1. For 90% efficiency to hydroperoxide, the minimum chain length (/ = 1) is 14. 

This reaction is one example of several possible radical transition-metal ion interactions. The significance of this and similar reactions is that radicals are destroyed and are no longer available for initiation of useful radical reactions. Consequentiy, the optimum use levels of transition metals are very low. Although the hydroperoxide decomposes quickly when excess transition metal is employed, the efficiency of radical generation is poor. 

In the presence of strong acid catalysts many commonly used commercial alkyl hydroperoxides decompose to acetone to some extent. Consequendy, the diperoxyketals derived from other ketones and alkyl hydroperoxides are often contaminated with small amounts of diperoxyketals derived from acetone (1, X = OOR, = methyl, R = R = tert — alkyl). 

Metal-Catalyzed Oxidation. Trace quantities of transition metal ions catalyze the decomposition of hydroperoxides to radical species and greatiy accelerate the rate of oxidation. Most effective are those metal ions that undergo one-electron transfer reactions, eg, copper, iron, cobalt, and manganese ions (9). The metal catalyst is an active hydroperoxide decomposer in both its higher and its lower oxidation states. In the overall reaction, two molecules of hydroperoxide decompose to peroxy and alkoxy radicals (eq. 5). 

The synergistic effect of a hydroperoxide decomposer, eg, dilauryl thiodipropionate [123-28-4] (34), and a radical scavenger, eg, tetrakis[methylene(3,5-di-/ f2 butyl-4-hydroxyhydrocinnamate)]methane (9), ia protecting polypropylene duting an oxygen-uptake test at 140°C is shown ia Table 3. 

The foUowing scheme iUustrates some of the many reactions that occur during cross-linking. NaturaUy present hydroperoxides decompose to form free radicals ... 

Another method for slowing oxidation of rubber adhesives is to add a compound which destroys the hydroperoxides formed in step 3, before they can decompose into radicals and start the degradation of new polymer chains. These materials are called hydroperoxide decomposers, preventive antioxidants or secondary antioxidants. Phosphites (phosphite esters, organophosphite chelators, dibasic lead phosphite) and sulphides (i.e. thiopropionate esters, metal dithiolates) are typical secondary antioxidants. Phosphite esters decompose hydroperoxides to yield phosphates and alcohols. Sulphur compounds, however, decompose hydroperoxides catalytically. 

Organic peroxides and hydroperoxides decompose in part by a self-induced radical chain mechanism whereby radicals released in spontaneous decomposition attack other molecules of the peroxide.The attacking radical combines with one part of the peroxide molecule and simultaneously releases another radical. The net result is the wastage of a molecule of peroxide since the number of primary radicals available for initiation is unchanged. The velocity constant ka we require refers to the spontaneous decomposition only and not to the total decomposition rate which includes the contribution of the chain, or induced, decomposition. Induced decomposition usually is indicated by deviation of the decomposition process from first-order kinetics and by a dependence of the rate on the solvent, especially when it consists of a polymerizable monomer. The constant kd may be separately evaluated through kinetic measurements carried out in the presence of inhibitors which destroy the radical chain carriers. The aliphatic azo-bis-nitriles offer a real advantage over benzoyl peroxide in that they are not susceptible to induced decomposition. 

This hydroperoxide decomposes slowly, avoiding accumulation. However, if the conditions are ideal for peroxidation (heat, prolonged time exposure to air, solar light), the hydroperoxide converts into extremely dangerous peroxides. Phenolic antioxidants inhibit this peroxidation efficiently. If tetrahydrofuran is peroxidised, it is not possible to destroy peroxides with ferrous salts or sulphites since tetrahydrofuran dissolves in water. Alumina or active carbon (passing over an alumina column or activated carbon at 20-66 C with a contact period of two minutes) are used, or by stirring in the presence of cuprous chloride. 

An unsubstituted hydroxylamine is a powerful hydroperoxide decomposer and peroxyl radical scavenger, and could play an important role in photo-stabilization even if present at only a low concentration after dark intervals. 

The next step was therefore to study the interaction of TMP derivatives with peroxides and with the products arising from them during photooxidation. A direct hydroperoxide decomposing effect of the HALS derivatives studied was not observed. 

A 9 g sample of the freshly prepared hydroperoxide decomposed after 20 min at ambient temperature, bursting the 20 ml glass container. A 30% solution of the hydroperoxide in ethylbenzene is stable. 

Synergism based on the mixture of a chain-breaking antioxidant (sterically hindered phenol) and a hydroperoxide decomposer (organic sulfides or phosphites). 

In special cases allylic hydroperoxides decompose directly to a,P-un-saturated ketones. The allylic hydroperoxide formed in the photooxydation of calarene gives the three types of products discussed above (6.16) 623). 

Synergism of Chain Termination and Hydroperoxide Decomposing the Antioxidants... 

The kinetic study of cumyl hydroperoxide decomposition in emulsion showed that (a) hydroperoxide decomposes in emulsion by 2.5 times more rapidly than in cumene (368 K, [RH] [H20] = 2 3 (v/v), 0.1 N Na2C03) and (b) the yield of radicals from the cage in emulsion is higher and close to unity [19]. The activation energy of ROOH decomposition in cumene is Ed = 105 kJ mol-1 and in emulsion it is lower and equals Ed 74 kJ mol 1 [17]. 

Oxidation of organic compounds occurs by the chain mechanism via alternating reactions of alkyl and peroxyl radicals (see Chapter 2). The accumulated hydroperoxide decomposes into radicals, thereby increasing the rate of oxidation (see Chapter 4). The oxidation of an organic compound may be retarded by one of the following three ways ... 

Hydroperoxide decomposing antioxidants. These are compounds that react with hydroperoxides without forming free radicals sulfides, phosphites, arsenites, thiophosphates, carbamates, and some metal complexes. Reactions with hydroperoxides can be either stoichiometric (typical of, for example, sulfides and phosphites) or catalytic (typical of chelate metal complexes). 

At high temperatures or in the presence of catalysts, hydroperoxide decomposes at a high rate, so that, after t t, inhibited oxidation becomes a quasi-stationary process with balanced rates of ROOH formation and decomposition. In this case, kdr 1, where kd is the overall specific rate of ROOH decomposition with allowance made for its decomposition... 

Linear chain termination is not, however, a necessary condition for the critical behavior. Indeed, with mechanisms V and XII, chain termination is quadratic (v v,172), but critical transition does take place because hydroperoxide decomposes into radicals that contribute to chain propagation. As a result, v (v [ROOH])1/2 v, [ROOH]172, and v [ROOH] (see Equation (14.11)) which explains the critical behavior. 

SYNERGISM OF CHAIN TERMINATION AND HYDROPEROXIDE DECOMPOSING THE ANTIOXIDANTS... 

In the presence of a hydroperoxide decomposer, InH is consumed with the rate ... 

Hydroperoxides decompose in a bimolecular reaction with the formation of water. The activation energy of the peroxide decomposition reaction could be reduced by using some activators, i.e., Fe2+, Cu2+ and sodium hyposulphite, etc. 

Scheme 10). The hydroperoxide decomposes to yield p-tolualdehyde selectively [81,109]. 

Note A decomposer for hydroperoxides is termed a hydroperoxide decomposer. 

It is generally agreed that alkenyl hydroperoxides are primary products in the liquid-phase oxidation of olefins. Kamneva and Panfilova (8) believe the dimeric and trimeric dialkyl peroxides they obtained from the oxidation of cyclohexene at 35° to 40° to be secondary products resulting from cyclohexene hydroperoxide. But Van Sickle and co-workers (20) report that, The abstraction/addition ratio is nearly independent of temperature in oxidation of isobutylene and cycloheptene and of solvent changes in oxidations of cyclopentene, tetramethylethylene, and cyclooctene. They interpret these results to support a branching mechanism which gives rise to alkenyl hydroperoxide and polymeric dialkyl peroxide, both as primary oxidation products. This interpretation has been well accepted (7, 13). Brill s (4) and our results show that acyclic alkenyl hydroperoxides decompose extensively at temperatures above 100°C. to complicate the reaction kinetics and mechanistic interpretations. A simplified reaction scheme is outlined below. 

The observed half life at 100°C. of 23 hours for a dilute solution of hydroperoxide in benzene indicates that significant decomposition may occur in the autoxidation of butene, depending on reaction conditions. No reliable evaluation can be made because of the known complications introduced on hydroperoxide decomposition by the effect of the solvent, the hydroperoxide concentration (2), the presence of oxygen (12), and the possibility of a strong acceleration in rate in the presence of oxidizing olefin, observed in at least one system (8). However, using the data reported by Bateman for a benzene solvent at 100 °C. in the presence of air (2), l-butene-3-hydroperoxide decomposes 13 times faster than cyclohexene hydroperoxide, a product which may be formed in extremely high yield by the oxidation of cyclohexene. 

---