Pyrolysis rate-determining step

Chemical reactivity differences may be calculated if for the transition state of a rate-determining step of a reaction a structural model can be given which is describable by a force field with known constants. We give only two examples. Schleyer and coworkers were able to interpret quantitatively a multitude of carbonium-ion reactivities (63, 111) in this way. Adams and Kovacic studied the pyrolysis of 3-homoadamantylacetate (I) at 550 °C and considered as transition state models the two bridgehead olefins II and III (112). From kinetic data they estimated II to be about 2 kcal mole-1 more favourable than III. 

The pyrolysis of methyl phenyl sulfide using the toluene flow carrier technique in the range 550-706 °C leads to the formation of CH4, PhSH and dibenzyl the rate-determining step is probably... 

It has been suggested that the depolymerization of cellulose to levoglucosan is the first, and rate-determining, step. General mechanisms involved in the pyrolysis of cellulose have been discussed, - and some of these have invoked the formation of 1,2-anhydro-a-D-glucopyranose as an essential intermediate - The effect of the pr ence of salts on this mechanism has also been discussed. " - ... 

Even though the pyrolysis of polypropylene is mechanistically complicated (vide infra), the kinetics is first order because the rate determining step is the homolysis of the C—C bond describable by a well-defined rate constant. We found the activation energy for pyrolysis to be 51 and 56 kcal mol"1, respectively, for IPP and APP. Other literature values are 60 kcal mol"1 measured at 350°-400°C by Wall and Straus (10), 58 kcal mol 1 in the range 336°-366°C reported by Madorsky and Straus (12) and 55 kcal mol"1 found by Moissev et al. (13) in the temperature range 320°-420°C. 

Competitive hydrogen abstraction reactions by phenoxy and benzyl radicals on the a-or )8-carbon of PPE are the core reactions of the radical chain propagation (Scheme 7.2) in the thermal degradation of PPE and are the only reactions needed in the analytical kinetic model derived in Section 7.3.2.3 to calculate overall a/)8-selectivities. Because hydrogen abstractions are the rate-determining steps, effects of substituents on hydrogen abstraction have the most impact on overall product distributions and pyrolysis rates. 

This first limiting case implies that the radical process 3 is definitely more difficult than radical process 2, It follows that, in the pyrolysis of pure yH, process 3 is the rate determining step of chain propagation one has ( y. ) ( 3. ) and the chain termination chiefly involves process 33,... 

From a fundamental point of view, the reactions studied in the presence of these additives have permitted to point out without cimbiguity the elementary processes which are the rate determining steps of the pyrolysis. Such a method can be a useful means to determine the details of a reaction mechanism. We think that it might be useful when investigating the mechanisms of olefin thermal decompositions which are usually more complex than those of alksuie pyrolyses. Investigations on olefin pyro-lyses in the presence of H S have been performed at Nancy (31) and in the U. S. A. (32),... 

A semi-polar cyclic five-membered transition state has been proposed to account for the unimolecular pyrolysis kinetics of primary, secondary, and tertiary 2-phenoxycarboxylic acids in the gas phase reaction rates increase in the order 2-phenoxyacetic, 2-phenoxypropionic, 2-phenoxybutyric, and 2-phenoxyisobutyric acid. Phenol formation in the rate-determining step is followed by formation and subsequent decompositon of an intermediate lactone to give carbon monoxide and the corresponding carbonyl compound however, in the case of 2-phenoxyisobutyric acid, a parallel elimination reaction gives phenol and methacrylic acid." ... 

However, many reactions, although their mechanism may be quite complex, do conform to simple first or second-order rate equations. This is because the rate of the overall reaction is limited by just one of the elementary reactions which is then said to be rate-determining. The kinetics of the overall reaction thus reflect the kinetics of this particular step. An example is the pyrolysis of ethane(4> which is important industrially as a source of ethylene01 (see also Section 1.7.1 Example 1.4). The main overall reaction is ... 

---