Pyrolytic reaction pathway

Figure 1. Generic pyrolytic reaction pathway (Rebick, 1983) 1.2. HEAVY OIL PYROLYSIS MECHANISMS...

Each of these reactions and their reverse reactions may, in fact, be involved in major reaction pathways in coal conversion. Reaction 10, for instance, is expected to be a key step in H-transfer and aromatization, reaction 11 can lead to crosslinking and polymerization while reaction -11 breaks up complex molecules, and reaction 12 should provide a steady source of free radicals in many pyrolytic systems even in the absence of weak covalent bonds (vide infra). Furthermore, these free radicals contain weak C-H bonds that may rupture to yield H atoms, which can in turn lead to breaking of C-C or C-0 bonds through aromatic displacement reactions (reaction 5). 

The experiments illustrated in Figure 21.4 however, were carried out with 4 g of material because, as was mentioned before, the aim was not to elucidate the reaction pathway or the kinetics parameters of the pyrolytic reaction, but to provide know how about the microwave pyrolysis process. Therefore as can be seen in the figure, the fastest degradation was achieved with the laminate because of its smaller thickness (plastic layer 90-150 p,m) in comparison with the average diameter of the HOPE powder (150 p,m) and pellets (3 mm diameter, 1 mm high). 

Pyrolytic reactions often occur in the gas phase, without the addition of another reagent. For example, heating carboxylate esters readily produces an alkene and a carboxylic acid. There is a requirement for a c/s-(3-hydrogen in order for this reaction pathway to proceed, because the reaction proceeds via a six-membered cyclic transition state. This cyclic pyrolytic elimination is labelled the Ei mechanism, which stands for intramolecular, or internal, elimination. In those versions of the Ei mechanism that involve a four- or five-membered cyclic transition state, there is a requirement that all the atoms are co-planar or virtually so. 

Since pyrolytic reactions can proceed through alternative pathways, 1,4 reaction does not exclude the simultaneous and direct participation of HO-C(6). 1,2- and 1,4-anhydro... 

As described above, models for l pin-derived biofuels typically utilize derivatives of PPEs (PhCH2CH20Ph) or similar molecules as surrogates for the more compHcated rmits in hgnin (Figure 7). The mechanistic studies in this area focus on the compHcated pyrolytic decomposition pathways for the surrogate ethers. A strong temperature dependence to the reaction charmels is observed with a competition between homolytic bond fission reactions and a concerted retro-ene reaction. 5-Scissions following H atom abstractions, H atom transfer reactions, and the influence of... 

Studies of lignoceUulosic biofuel model compounds have likewise increased significantly in the past decade. Reaction pathways with implications for soot, aldehydic, and emissions from oxygenated and otherwise-functionalized fuels have been examined. Several key intermediates in pyrolytic pathways have been identified, including furanic carbenes, furanylmethyl radicals, dihydrofurans, and unsaturated ketones and aldehydes. Low-temperature combustion reactions available to these compounds are largely unexplored and, where modeled, involve highly functionalized per-oxy radicals with the capacity for novel reactions. As cycHc oxygenated species break down into smaller acyclic species, some potential arises for overlap with extant mechanisms for HC combustion. Composite ab initio and DFT calculations have been proved to be particularly useful in recent computational explorations of these compounds and their reaction pathways. 

Retro-Diels-Alder reactions can be used to regenerate dienes or alkenes from Diels-Alder protected cyclohexene derivatives under pyrolytic conditions144. Most of the synthetic utility of this reaction comes from releasing the alkene by diene-deprotection. However, tetralin undergoes cycloreversion via the retro-Diels-Alder pathway to generate o-quinodimethane under laser photolysis (equation 89)145. A precursor of lysergic acid has been obtained by deprotection of the conjugated double bond and intramolecular Diels Alder reaction (equation 90)146. 

While most studies have focused on the pyrolytic unimolecular decomposition of these monoheteroaromatic compounds, our group has explored their oxidative decomposition. As with benzene, where phenylperoxy radical plays a major role in dictating oxidation pathways, we hypothesize that the peroxy radicals derived from heteroaromatic rings are reactive species of considerable interest for combustion and atmospheric reactions. 

The concerted nature of these reactions is also supported by the pyrolytic cleavage of cis-2,3-dimethylcyclobutanone and rra i-2,3-dimethylcyclobutanone at a temperature of 325 °C under a pressure of 10 Torr, from which approximately 99% retention of configuration in the resulting but-2-enes is observed.92,93 Moreover, the results obtained from the thermolysis of cis- and m -2,4-dimethylcyclobutanone also lend support to the concertedness of this reaction.92,93 However, as substantiated by Arrhenius parameters, a diradical pathway is the most likely competing reaction in the pyrolysis of 2-chlorocyclobutanone.94 On the other hand, the kinetics of the gas-phase pyrolysis of bicyclo[3.2.0]hept-2-en-6-one (14),95 bicyclo[3.2.0]heptan-6-one (IS)96 and 2,2-dimethyl-3-ethoxycyclobutanone (16)97 are commensurate with a concerted cycloreversion mechanism involving four-center auasi-zwitterionic transition structures. 

In the gas phase, when the reaction is normally initiated by heat, and so is called pyrolysis, there are two common pathways. The first is via a cyclic transition state and the second involves a free radical pathway. These two pyrolytic eliminations are rather different in nature from those that occur in solution, and so we will discuss them separately. 

This is an example of a five-membered ring version of the Ei mechanism. Moreover, it is an example of a synthetic reaction that proceeds via two different pathways depending upon the exact conditions. These pyrolytic elimination reactions are valuable synthetically, because there is normally no opportunity for the starting material to rearrange, which is always a possibility when the elimination occurs via the El route, because in that pathway the reaction proceeds via a carbonium ion intermediate. 

Transient monomeric intermediates may be formed during the pyrolytic synthesis of borazines. The pathway shown in Eq. (x) has been postulated for the reaction of ammonia with diborane ... 

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