Dialkyl peroxides, decomposition

The rate constants of dialkyl peroxide decomposition (kA and /c, = 2ekd) are presented in... 

Some attention has been given to the effect of substituents upon the kinetics of dialkyl peroxide decomposition. The data are presented in Table 67. A linear enthalpy-entropy of activation correlation was made for the decomposition of alkyl peroxides (exclusive of the hydroxyalkyl peroxides) using data in solution and in the gas phase. The isokinetic temperature was found to be 483 °K (210 °C) . No rational explanation was advanced for the substituent effects in solution or the gas phase . However, the discussion of the effect of a chain reaction upon the activation parameters, given in the section on gas phase reactions, should be consulted. The large differences in and log A between the alkyl and the hydroxyalkyl peroxides suggests a change in mechanism. This is supported by the products from the hydroxyalkyl peroxides. A cyclic activated complex was suggested , viz. 

Most likely singlet oxygen is also responsible for the red chemiluminescence observed in the reaction of pyrogaHol with formaldehyde and hydrogen peroxide in aqueous alkaU (152). It is also involved in chemiluminescence from the decomposition of secondary dialkyl peroxides and hydroperoxides (153), although triplet carbonyl products appear to be the emitting species (132). 

In the preparation of hydroperoxides from hydrogen peroxide, dialkyl peroxides usually form as by-products from the alkylation of the hydroperoxide in the reaction mixture. The reactivity of the substrate (olefin or RX) with hydrogen peroxide is the principal restriction in the process. If elevated temperatures or strongly acidic or strongly basic conditions are required, extensive decomposition of the hydrogen peroxide and the hydroperoxide can occur. 

Primary and secondary dialkyl peroxides undergo thermal decompositions more rapidly than expected owing to radical-induced decompositions (73). Such radical-induced peroxide decompositions result in inefficient generation of free radicals. 

Decomposition products from primary and secondary dialkyl peroxides include aldehydes, ketones, alcohols, hydrogen, hydrocarbons, carbon monoxide, and carbon dioxide (44). 

Because di-/ fZ-alkyl peroxides are less susceptible to radical-induced decompositions, they are safer and more efficient radical generators than primary or secondary dialkyl peroxides. They are the preferred dialkyl peroxides for generating free radicals for commercial appHcations. Without reactive substrates present, di-/ fZ-alkyl peroxides decompose to generate alcohols, ketones, hydrocarbons, and minor amounts of ethers, epoxides, and carbon monoxide. Photolysis of di-/ fZ-butyl peroxide generates / fZ-butoxy radicals at low temperatures (75), whereas thermolysis at high temperatures generates methyl radicals by P-scission (44). 

Thermal or photo-induced decompositions of dialkyl peroxides in the presence of suitable substrates yield various products. For example, with nitric oxides, alkyl nitrites or nitrates are formed and, with carbon monoxide, Z fZ-alkyl esters are obtained (44) ... 

A slow rate of p-scission also means that the main cage recombination process will be cage return to reform the peroxydicarbonate. Dialkyl peroxides are typically not found amongst the products of peroxydicarbonate decomposition. In these circumstances, cage recombination is unlikely to be a factor in reducing initiator efficiency. 

To add to the silver salt (Eq. 19) and peroxymercuration (Eq. 34-36) methods of preparing dialkyl peroxides, a third mild alkylation procedure has been developed that involves the use of alkyl trifluoromethane sulphonates (triflates)55). The peroxide transfer reaction between bistributyltin peroxide and the bistriflate 58 of cis-1,3-cyclopentanediol provided one of the first syntheses of 2,3-dioxabicyclo[2.2.1]heptane 9 (Eq. 44, H for D Tf = 02SCF3)56). Because of the sensitivity of 9, it was necessary to carry out the reaction in vacuo with rapid transfer of the volatile products to a cold trap to avoid decomposition a yield of 22% was achieved. 

Thermal decomposition of dialkyl peroxides, diacyl peroxides, hydroperoxides and peracids depending on the structure of the peroxidic compound occurs in a measurable rate usually above 60°C. Diacyl peroxides and peracids are considerably less stable than dialkyl peroxides and hydroperoxides. 

Dialkyl (C12-C18) dimethylammonium chloride, 24 147 hair cleaner ingredient, 7 850t Dialkyldimethylammonium quaternary higher alcohols, 2 21 Dialkylgold(III) halides, 12 708 Dialkylhydrazines, unsymmetrical, 13 571 Dialkyl peroxides, 14 281 18 434, 436-448 a-oxygen-substituted, 18 448-460 chemical properties of, 18 439-444 decompositions of, 18 441 physical properties of, 18 436-439 primary and secondary, 18 439,... 

At increased temperatures, the peroxide bond is cleaved homolytically, giving radicals. Dibenzoyl peroxide is a diacyl peroxide and cleaves rather more readily than the dialkyl peroxide, but further decomposition then occurs in which carbon dioxide... 

Methods for detecting whether peroxy compound have been used for cross-linking elastomers have been reviewed. An important application of dialkyl peroxides is as initiators of cross-linking and graft polymerization processes. The success of both processes depends on the ability of the peroxide to produce free radicals and the ability of the free radicals for H-abstraction from a relevant donor substrate. A method for evaluating this ability consists of inducing thermal decomposition of the peroxide dissolved in a mixture of cyclohexane and MSD (225). The free radical X" derived from the... 

Radiation-induced decomposition, 5,6-dihydrothymine, 930 Radiation stress, polymers, 685 Radical polymerization dialkyl peroxides, 707 peroxycarboxylic esters, 697 Radicals... 

The alkenyl hydroperoxides and polymeric dialkyl peroxides are fairly stable at ambient temperature but decompose appreciably at the reaction temperatures studied. Thermal stabilities of the alkenyl hydroperoxides and dialkyl peroxides in the olefin solution were determined by heating the solution at 110°C. under nitrogen. The peroxide numbers were plotted vs. time to estimate the half-lives in solution. The thermal decomposition half-lives of these alkenyl hydroperoxides are compared with values from the literature for acyclic and cyclic hydroperoxides in Table IV. Secondary acyclic alkenyl hydroperoxides appear to be less... 

The experimental activation energies given in the last column of Table II are in the anticipated order of magnitudes. The activation energy of 24.0 kcal. per mole for the oxidation of 1-hexadecene to hydroperoxide is close to the value of 25.3 kcal. per mole recently reported for the constant velocity of peroxide accumulation. .. for butene-1 (9). The activation energy for the alkenyl hydroperoxide decomposition is reasonable. The activation energy of 48.1 kcal. per mole for the decomposition polymeric dialkyl peroxide is considerably higher than the value of about 37 kcal. per mole for tert-butyl peroxide decomposition. The... 

At Van Sickle s conditions of low temperatures and low conversions, branching routes A and B appear to be dominant since there is little alkenyl hydroperoxide decomposition. In our work above 100°C., the branching routes are supported by the nearly linear initial portions at low conversions for alkenyl hydroperoxide and polymeric dialkyl peroxide curves (see Figures 2, 3, and 4). The polymeric dialkyl peroxides formed under our reaction conditions include those formed by the branching mechanism postulated by Van Sickle (routes A and B) and those formed by the reaction of the alkenoxy and hydroxy radicals from alkenyl hydroperoxide thermal decomposition reacting further and alternately with olefin and oxygen (step C). The importance and kinetic fit of the sequential route A to C appears to increase with temperature and extent of olefin conversion owing to the extensive thermal decomposition of the alkenyl hydroperoxides above 100°C. 

The inhibition of hydrocarbon autoxidation by zinc dialkyl dithiophosphates was first studied by Kennerly and Patterson (13) and later by Larson (14). In both cases the induction period preceding oxidation of a mineral oil at 155 °C. increased appreciably by adding a zinc dialkyl dithiophosphate. In particular, Larson (14) observed that zinc salts containing secondary alkyl groups were more efficient antioxidants than those containing primary groups. In these papers the inhibition mechanism was discussed only in terms of peroxide decomposition. 

One possible problem peculiar to a quantitative study of the inhibition of oxidation of aromatic hydrocarbons by zinc dialkyl dithiophos-phates is that peroxide decomposition could yield a phenol during the initial-rate measurement. Rate curves for the zinc diisopropyl dithio-phosphate-inhibited oxidation of cumene are shown in Figure 7. In the initial presence of hydroperoxide the uninhibited rate is never reached, and the reaction soon exhibits autoinhibition, presumably caused by the... 

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. 

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,... 

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