Thermal decomposition of acetyl propionyl peroxide in acetone-d 6

Abstract

The kinetics of thermolysis of acetyl propinyl peroxide in acetone-d 6 in the temperature range 3233373 K was studied using NMR spectroscopy and the effect of chemically induced nuclear polarization. The peroxide decomposes in acetone at rates comparable with the rates of thermolysis in alcohols, yielding numerous products. In the examined temperature range, the solvent molecules act as efficient donors of deuterium atoms, forming acetylmethyl-d 5 radicals which recombine to a significant extent with the peroxide radicals. A scheme of the processes involved in decomposition of the peroxide was suggested. The parameters of the Arrhenius equation for the peroxide decomposition were determined.Diacyl peroxide are widely used as initiators of polymerization and other free-radical reactions in chemical and petrochemical industries. Therefore, it seems important to analyze the products of decomposition of diacyl peroxides in cheap and available solvents commercially produced in large amounts. One of such solvents is acetone. Data on decomposition of diacyl peroxides in acetone are scarce. Bagdasar'yan and Mulyutinskaya [1, 2] studied only the rate of thermolysis of a 0.185 solution of benzoyl peroxide at 348 K and determined the yields of benzoic acid and carbon dioxide. Leffler et al. studied the effect of methoxy and nitro substituents on the contributions of the radical and nonradical mechanisms in thermolysis of benzoyl peroxide [3] and determined the yield of certain products in decomposition of unstable bis(o-iodophenylacetyl) peroxide [4]. Dutka et al. [5] measured the rates of thermal decomposition of a series of diacyl peroxides and determined the contribution of the induced decomposition to the apparent constant of the process.For aliphatic diacyl peroxides, we found only a single paper [6] in which the product yields in decomposition of acetyl peroxide in a mixture of acetone with carbon tetrachloride were reported.Our goals were to study the effects of chemically induced nuclear polarization, perform kinetic measurements, and analyze, on the basis of the results obtained, the mechanism of thermolysis of one of the simplest unsymmetrical aliphatic diacyl peroxides, acetyl propionyl peroxide I, in acetone-d 6 . Figure 1 shows the 1 H NMR spectrum of a 0.5 M solution of I in the course of thermolysis. Positively polarized are the protons of ethene [d 5.39 ppm, s (4H)], methylene protons of 2-pentanone-d 5 CH 3 CH 2 . CD 2 COCD 3 [1.59 ppm, m (2H)], and methyl protons of butanone-d 5 CH 3 CD 2 COCD 3 [0.98 ppm, quintet (3H)] and propane [0.92 ppm, t (3H)]. Negatively polarized are the methylene protons of ethyl ethanoate [4.10 ppm, q (2H)] and propane [1.39 ppm, m (2H)] and methyl protons of 2-pentanone-d 5 [0.89 ppm, t (3H)] and methanes [0.19 ppm, s (CH 4 ); 0.17 ppm, t (CH 3 D)]. Also, protons of propane and 2-pentanoned 5 have an insignificant contribution of the multiplet 5 4 3 2 1 0 d, ppmFig. 1. 1 H NMR spectrum of a 0.5 M solution of peroxide I in acetone-d 6 at 368 K in the course of thermolysis.

Exceptionally high decarboxylation rate of a primary aliphatic acyloxy radical determined by radical product yield analysis and quantitative 1H‐CIDNP spectroscopy

“…However, previous CIDNP investigations [43] failed to detect any such enhancements leading to a common assumption that these and other primary aliphatic acyloxy radicals, such as long-chain 1d, decarboxylate at rates at least an order of magnitude higher than that of 1b. Indeed, the most recent quantitative CIDNP and product yield studies of the thermolysis of acetyl propanoyl peroxide [36][37][38] appear to confirm that assumption (estimated decarboxylation rate of 1.0-10 Â 10 10 s À1 for 1c at 80 8C). In fact, failure to detect net CIDNP enhancements during thermal decomposition of 2 and other primary diacyl peroxides [28,43] was often taken as evidence against the existence of a discrete primary acyloxy radical intermediate such as 1c or 1d.…”

“…[36][37][38] For example, the wide range of the decarboxylation rate values for the propanoyloxy radical 1c is a direct result of the typically small values of the decarboxylation activation energy (7 AE 3 kJ/mol), and correspondingly large errors in the its estimate. [36][37][38] The error of this magnitude is also due to product yields reported in Refs. [36][37][38] being obtained exclusively from decomposition of very high peroxide concentrations (0.1-1.0 M) in reactive solvents such as methanol and acetone.…”

“…[36][37][38] The error of this magnitude is also due to product yields reported in Refs. [36][37][38] being obtained exclusively from decomposition of very high peroxide concentrations (0.1-1.0 M) in reactive solvents such as methanol and acetone. It is likely that such conditions cause artificial variation of the product yields due to the bimolecular side reactions between the peroxide and the escaped radicals and/or cage products.…”

“…Estimates of the decarboxylation rate of 1d from quantitative 1 H-CIDNP studies 1 H-CIDNP spectroscopy can, in principle, also be used to quantitatively estimate the decarboxylation rates of acyloxy radicals 7 as a crosscheck on lifetime estimates obtained from cage product yields. There has been a number of previous studies that employed CIDNP spectroscopy to quantify the lifetimes of benzoyloxy, [27,28] acetyloxyl [11] and propanoy-loxy [36][37][38] radicals. With very few exceptions, these studies used a semi-quantitative approach that requires independent estimates of the relaxation times of products, whose polarization is observed and simulated numerically.…”

“…[34] In general, the long chain primary acyloxy radical 1d, usually generated by thermolysis of a primary symmetrical diacyl peroxide 2 (lauroyl peroxide; RCO 2 CO 2 R; R ¼ XCH 2 CH 2 ; X ¼ n-C 9 H 19 ), is often used as a simple model for primary acyloxy radicals. [10,11,[36][37][38] This is largely due to the ready availability of 2 as a commercial product, Alperox, and ease of purifying it through repeated recrystallization from methanol/ CHCl 3 . We base this work on our previous detection of the disproportionation and combination products from short-lived acyloxy-alkyl radical pairs, Cage II in Scheme 1, in thermolysis of very low (10 À3 -10 À4 M) concentrations of 2 and related primary peroxides.…”