Hydrogen atom transfer reactions, pressure

The kinetics of the photolysis is much more complex at lower temperatures than at around 300 °C. The role of rate-determining step, i.e. the hydrogen atom transfer reaction (20) at high temperatures, is taken over by the decomposition of the acetyl radical as the temperature decreases. At the highest temperatures, the chains are terminated almost exclusively by the recombination of the methyl radicals, while at medium and low temperatures the disproportionation step (26) as well as self combination of the formyl and acetyl radicals are dominant. The first-order wall reaction of the radicals, such as reactions (22) and (31), may also play an important role, especially at low light intensities and pressures. On account of the aforesaid, it seems almost impossible to attempt a general discussion of the kinetics of the reaction. Instead, only selected questions will be dealt with in detail. 

The mechanistic proposal of rate-limiting hydrogen atom transfer and radical recombination is based on the observed rate law, the lack of influence of CO pressure, kinetic isotope effects [studied with DMn(CO)s] and CIDNP evidence. In all known cases, exclusive formation of the overall 1,4-addition product has been observed for reaction of butadiene, isoprene and 2,3-dimethyl-l,3-butadiene. The preferred trapping of allyl radicals at the less substituted side by other radicals has actually been so convincing that its observation has been taken as a mechanistic probe78. 

The HCo(CO)4 complex is therefore inferred to be involved in initial hydrogen transfer to carbon monoxide. This step was initially proposed to comprise rate-determining hydrogen atom transfer from HCo(CO)4 to free CO, affording a formyl radical, HtO subsequent reaction with further HCo(CO)4 would lead to the observed products (35). However, kinetic observations (the zero-order dependence on CO partial pressure) were later made which are inconsistent with such a process (36). 

Vapors of mercury, cadmium and zinc sensitize photochemical alkane transformations [21], Thus, irradiation of propane with light of = 307.6 nm at 633 K and pressure 67 0000 Pa in the presence of zinc vapor gives rise to the formation of hydrogen, methane, ethylene and dimethylbutane [21a]. The first step in the reaction is a hydrogen atom transfer from the alkane to the excited zinc atom ... 

The present paper concentrates on the first of these topics, because the need to measure the rate coefficients of many elementary reactions to an accuracy of 10% has established some unpredicted features such as the pressure dependence of the second-order rate coefficients for a number of bimolecular reactions involving hydrogen atom transfer. However, we will consider first the second topic which is important to the background and development of the main theme. It sdso emphasizes our current inability to predict the rates of elementary bimolecular transfer reactions even to an order of magnitude. 

Using either of the above approaches we have measured the thermal rate constants for some 40 hydrogen atom and proton transfer reactions. The results are tabulated in Table II where the thermal rate constants are compared with the rate constants obtained at 10.5 volt cm.-1 (3.7 e.v. exit energy) either by the usual method of pressure variation or for concurrent reactions by the ratio-plot technique outlined in previous publications (14, 17, 36). The ion source temperature during these measurements was about 310°K. Table II also includes the thermal rate constants measured by others (12, 13, 33, 39) using similar pulsing techniques. 

The thermal alkylation of ethylene-isobutane mixtures at high pressures in the gas phase has been studied in the presence and absence of HCl, and it has been found that HCl can (a) dramatically increase the total yield of alkylate, (b) increase the fraction of the alkylate which is C6 rather than C8> and (c) both increase and decrease the ratio of 2-methyl pentane to 2J2-dimethylbutane in the C6 fraction of the alkylate, this latter depending on the amount of HCl used. All of these effects can be explained readily in terms of the generally accepted free radical mechanism of thermal alkylation, provided one assumes that HCl acts as a catalyst for those reaction steps that involve transfer of a hydrogen atom between a free radical and a hydrocarbon. 

The generally accepted mechanism for the formation of short branching in polyethylene involves a backbiting intramolecular transfer reaction in which a radical at the end of the polymer chain abstracts a hydrogen atom from a methylene unit in the same chain (Fig. 6.9). This is a very important process in the free-radical, high-pressure polymerization of this monomer. Branched polyethylene from this process has lower crystallinity than linear polyethylene produced by a low-pressure process and as a consequence it tends to be less rigid and tougher and form clearer films than the latter. 

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