Radical mechanism pyrolysis

Free-radical mechanisms are mostly found in pyrolyses of polyhalides and of primary monohalides,though they also have been postulated in pyrolysis of certain carboxylic esters/ Much less is known about these mechanisms and we shall not consider them further. Free-radical eliminations in solution are also known but are rare. ... 

The two polymer substrates investigated as part of the study of DBDPO mixtures were polypropylene (PP) and linear high density polyethylene (HDPE). while both PP and HDPE decompose by similar random chain scission, radical mechanisms, chain transfer occurs much more teadily during the pyrolysis of PP because of the presence of the tertiary hydrogens. In addition, only primary chain end radicals are formed when the HDPE chain cleaves homolytically. Therefore, a comparison of the PP/DBDPO and the HDPE/DBDPO mixtures volatile product distributions was undertaken. 

The radical mechanism was confirmed by matrix isolation of the pyrolysis products of 2s.102 Flash vacuum pyrolysis of 2s with subsequent trapping of the products in argon at 10 K produces methyl radicals, which are easily identified by IR spectroscopy. 

The fact that most alkylated benzenes show the same tendency to soot is also consistent with a mechanism that requires the presence of phenyl radicals, concentrations of acetylene that arise from the pyrolysis of the ring, and the formation of a fused-ring structure. As mentioned, acetylene is a major pyrolysis product of benzene and all alkylated aromatics. The observation that 1-methylnaphthalene is one of the most prolific sooting compounds is likely explained by the immediate presence of the naphthalene radical during pyrolysis (see Fig. 8.23). 

For a review of free radical mechanisms involving peroxides in solution, see Howard, in Patai The Chemistry of Peroxides Wiley New York, 1983, pp. 235-258. For a review of pyrolysis of peroxides in the gas phase, see Batt Liu. in the same volume, pp. 685-710. See also Chateauneuf Lusztyk Ingold J. Am. Chem. Soc. 1988,110. 2877. 2886. 

Dehydrohalogenation is generally carried out in solution, with a base, and the mechanism is usually E2, though the El mechanism has been demonstrated in some cases. However, elimination of HX can be accomplished by pyrolysis of the halide, in which case the mechanism is Ei (p. 1006) or, in some instances, the free-radical mechanism (p. 1008). Pyrolysis is normally performed without a catalyst at about 400°C. The pyrolysis reaction is not generally useful synthetically, because of its reversibility. Less work has been done on pyrolysis with a catalyst245 (usually a metallic oxide or salt), but the mechanisms here are probably El or E2. 

Thus, as predicted by the orbital symmetry rules, this thermal suprafacial [1,3] sigmatropic reaction took place with complete inversion at C-7. Similar results have been obtained in a number of other cases.426 However, similar studies of the pyrolysis of the parent hydrocarbon of 103, labeled with D at C-6 and C-7, showed that while most of the product was formed with inversion at C-7, a significant fraction (11 to 29%) was formed with retention.427 Other cases of lack of complete inversion are also known.428 A diradical mechanism has been invoked to explain such cases.429 There is strong evidence for a radical mechanism for some [1,3] sigmatropic rearrangements.430 Photochemical suprafacial [1,3] migrations of carbon have been shown to proceed with retention, as predicted.431... 

The well-known Elbs reaction has been applied to thiophene syntheses, but the reaction may involve some isomerization. Thus pyrolysis of 3-o-toluoylbenzo[6]thiophene (347) for 3 hours at 340-360 °C gave naphtho[2,1 -b]benzothiophene (348) in 35% crude yield, instead of the expected naphtho[2,3-Z>]benzothiophene. A radical mechanism (Scheme 26) was proposed to explain this result (56JCS3435). 

The products of the thermolysis of 3-phenyl-5-(arylamino)-l,2,4-oxadiazoles and thiazoles have been accounted for by a radical mechanism.266 Flash vacuum pyrolysis of 1,3-dithiolane-1-oxides has led to thiocarbonyl compounds, but the transformation is not general.267 hi an ongoing study of silacyclobutane pyrolysis, CASSF(4,4), MR-CI and CASSCF(4,4)+MP2 calculations using the 3-21G and 6-31G basis sets have modelled the reaction between silenes and ethylene, suggesting a cyclic transition state from which silacyclobutane or a trcins-biradical are formed.268 An AMI study of the thermolysis of 1,3,3-trinitroazacyclobutane and its derivatives has identified gem-dinitro C—N bond homolysis as the initial reaction.269 Similar AMI analysis has determined the activation energy of die formation of NCh from methyl nitrate.270 Thermal decomposition of nitromethane in a shock tube (1050-1400 K, 0.2-40 atm) was studied spectrophotometrically, allowing determination of rate constants.271... 

Radical mechanisms are postulated for all these pyrolysis reactions. Modification of the radical mechanism by the reaction conditions, would explain the variable results. 

Since an insertion reaction into alkyl or aryl silicon bonds seems to be difficult, a radical mechanism usually takes place in the pyrolysis of such compounds. On the other hand, at Si—H, Si—Hal, Si—OR and Si—N bonds, a silylene mechanism occurs because the insertion reaction has only a small activation energy. 

Sundaram, K.M. and G.F. Fioment, Modeling of Thermal Cracking kinetics. 3. Radical Mechanisms for the Pyrolysis of Simple Paraffins, Olefins, and Their Mixtures., Ind. and Eng. Chem. Fund., 17,174—182,1978. 

These two goals will be discussed separately with reference to pyrolysis technology on the one hand and to free radical mechanisms and processes on the other. 

The steam pyrolysis of LPG follows the same pathway of that for ethane, namely by a complex branching chain free radical mechanism. This can be divided into initiation, chain propagation and chain termination reactions. This gives rise to a large number of intermediates and products. As with ethane, products of higher carbon number than the feed are formed. 

Many of these reactions have been studied before in the section on NaOa and so will not be discussed again here. In excess NO, the rate becomes nearly first-order over most of the decomposition with a rate constant which is itself a function of the total pressure. NO2 is an inhibitor for the decomposition, and in consequence the reaction in the absence of added NO shows a steady fall in apparent first-order rate constant with continuing decomposition. In this respect the nitrates and nitrites all seem to have in common the feature that the pyrolysis products inhibit the rate of decomposition. Tliis is to be expected in systems decomposing via radical mechanisms when the products of the reaction include such efficient radical traps as NO and NO2. It is unfortunate that quantitative data on these systems are at present so sparse and in many cases disparate. This is to be expected for systems that are so complex and show such sensitivity to surface reactions. The free radical chemistry of these systems is, however, a very interesting and important one, and efforts to elucidate it will eventually turn out to be quite rewarding. 

The observed pyrolysis product distribution of PP is developed by a free radical mechanism [28, 29] which begins with the homolytic breakage of the polymer chain drawn in Scheme 12.3 where X represent a methyl group in this case. Primary macroradical 1 is formed only in the initial step, thus its decomposition plays a minor role in PP. Secondary macroradicals may decompose to propene by depolymerization reproducing 2, or... 

Although it has been generally agreed since the early 1930 s that hydrocarbon pyrolyses occur mainly by free-radical mechanisms, there has been considerable controversy about whether purely molecular mechanisms play any significant role in these processes. A full discussion of this problem involves a study of the effects of inhibitors on pyrolysis, a matter that is briefly dealt with in Section 7. Here we merely summarize the main lines of argument. 

It was proved by a separate experiment that isotope mixing in a mixture of methane and methane-i proceeds very slowly even above 600 °C. Thus, it must be concluded that, in the pyrolysis, the formation of the partially deuterated methanes is a result of free radical reactions and not of the secondary exchange of the methanes. Consequently, these results support the free radical mechanism of the acetaldehyde decomposition. 

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