Radical formation pyrolysis

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). 

Thus, the presence of 03 as a background gas provides new sources for OH radical formation during sonolysis. The direct pyrolysis in the cavitation bubbles of volatile intermediates generated during ozonation also explains the improvement in the overall efficiency. In addition to these direct chemical effects, sonication also increases mass transfer coefficients of ozone from the bubbles to the solution, where the direct reaction of 03 with the substrates, or further radical formation, takes place. 

Thermochemical data for alkyl amino radicals show for alkylhydrazines a trend in D (N-N) for these compounds in which the N-N bond was strengthened by increasing the degree of substitution by methyl in NH2 NH2 . From these values it was possible to determine values for the enthalpies of formation of the alkylamino radicals , and to confirm these by electron impact studies. The latter values were found to be in agreement with those obtained from pyrolysis studies. Hydrazine has often been used as a source of amino radicals by pyrolysis flame decomposition shock tube decomposition , electrodeless discharge and microwave discharge , viz. 

Eliminations and other reactions do not necessarily take place only on the polymeric chain or only on the side groups. Combined reactions may take place, either with a cyclic transition state or with free radical formation. The free radicals formed during polymeric chain scission or during the side chain reactions can certainly interact with any other part of the molecule. Particularly in the case of natural organic polymers, the products of pyrolysis and the reactions that occur can be of extreme diversity. A common result in the pyrolysis of polymers is, for example, the carbonization. The carbonization is the result of a sequence of reactions of different types. This type of process occurs frequently, mainly for natural polymers. An example of combined reactions is shown below for an idealized structure of pectin. Only three units of monosaccharide are shown for idealized pectin, two of galacturonic acid and one of methylated galacturonic acid ... 

In conclusion, the initiation reaction caused by heat can be a complex process, and more than one type of free radical formation can occur during pyrolysis. This adds compiexity to the pyroiysis process, which may generate a significant number of different pyroiysis products even from a polymer with a simple structure. 

The results showed that the -factor proposal alone is too simplistic, and other processes during the radical formation and transfer may play a more important role than termination alone. The rate of degradation of the host poly(styrene) was found to be enhanced and not diminished. However, the degradation rates of the added blend polymer (the acrylates) were reduced by a factor of about eight. Several other studies using the kinetics information on the pyrolysis process were used for the elucidation of the structure of polymeric samples [32-34]. 

Scheme HI. Formation of the phenalenyl radical during pyrolysis of acenaphthylene and dihydronaphthalene.

This has already been commented on in connection with rule 3 in so far as excessive temperatures are concerned, but it may be added that the extent of the pyrolysis is more than would be predicted from data obtained by passing the pure chlorides through a hot reactor in the absence of chlorine. This result would be expected on the basis of free radical formation, since the radicals may decompose as follows ... 

The practical, real-world aspects of bond cleavage and radical formation are explored. Pyrolysis is a process that often gives mixtures of many products, and methods have been developed (mostly within the petroleum industry) to control this reaction somewhat. We will frequently explore the issue of reaction control the modification of conditions under which a chemical transformation is carried out in order to give a desired molecule as a major or exclusive product. 

The major conclusion of this work so far is that the trace inorganics Al Fe and Cu organometallic additives will increase the pyrolysis yield without affecting the mesophase microstructure. Addition of organometallics can either have inhibitory or promotional effects. The prosiotional effect (e.g. Cu) is perhaps due to the termination of radical formation whereas the inhibitory effect (e.g. V and Ni) is due to the initiation of radical formation. This will be studied further using the temperature dependence of electron spin resonance (ESR) studies. 

The pyrolysis of a diluted mixture of equimolar CH4 and CD4 was performed in a shock tube between 1500 and 1600 K and the products were quantitatively determined. From the hydrogen isotopic distribution of the products, methyl radical formation was confirmed as the initiation step of CH4 pyrolysis ... 

Clearly propylene and ethylene are Important during pyrolysis In the production of coke. The high reactivity of ethylene as compared to propylene Is thought to be caused primarily by the lower stability of vinyl radicals, as compared to that of allyl radicals. Formation of the Cg - Cg Intermediates possibly occurs by dimerization and/or trlmerization of these radicals. 

The first two precursors in this category to be discovered (see Table 5), 112 and 113, contain the relatively weak C-I and C-Hg bonds which undergo either photolysis or thermolysis to yield benzyne (1). The latter process is noteworthy as one of the first generations of this intermediate in the gas phase. Similar o-iodoorganometallic intermediates may be involved in the formation of arynes from o-diiodobenzene derivatives in the presence of zinc or copper. Both direct thermolysis and photolysis of 114 also lead to arynes, presumably via a stepwise mechanism involving o-iodophenyl radicals (47). The extent to which the latter go on to arynes depends not only on the nature of the aryl residue but, as mentioned in Section ILl.C, on the source of the radical center. Pyrolysis of dibromo (115), iodonitro (116), nitrobromo (117), and dinitro (118) aromatic compounds in a mass spectrometer also produces the corresponding arynes and didehydro compounds. 

In connection with the participation of metals in the preparation of organometallic compounds, attention might be called to a reaction involving metallic thallium as a product. We have presented evidence for the formation of the phenylthallium radical by pyrolysis of triphenylthallium in xylene. 

From an examination of the products formed from the pyrolysis of high-density polyethylene and polymethylene, Tsuchiya and Sumi [3,4] proposed that the major hydrogen-abstraction reaction is due to an intramolecular cyclisation. They proposed that, following initial radical formation at Cj, successive intramolecular hydrogen abstractions occur along the chain resulting in the formation of new radicals at C5, C9 and Ci3 as shown in Equations 6.2 to 6.5. 

Combustion chemistry in diffusion flames is not as simple as is assumed in most theoretical models. Evidence obtained by adsorption and emission spectroscopy (37) and by sampling (38) shows that hydrocarbon fuels undergo appreciable pyrolysis in the fuel jet before oxidation occurs. Eurther evidence for the existence of pyrolysis is provided by sampling of diffusion flames (39). In general, the preflame pyrolysis reactions may not be very important in terms of the gross features of the flame, particularly flame height, but they may account for the formation of carbon while the presence of OH radicals may provide a path for NO formation, particularly on the oxidant side of the flame (39). 

The use of free-radical reactions for this mode of ring formation has received rather more attention. The preparation of benzo[Z)]thiophenes by pyrolysis of styryl sulfoxides or styryl sulfides undoubtedly proceeds via formation of styrylthiyl radicals and their subsequent intramolecular substitution (Scheme 18a) (75CC704). An analogous example involving an amino radical is provided by the conversion of iV-chloro-iV-methylphenylethylamine to iV-methylindoline on treatment with iron(II) sulfate in concentrated sulfuric acid (Scheme 18b)(66TL2531). 

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