M. Szwarc

The C–C Bond Energy in Ethylbenzene

The pyrolysis of ethylbenzene in the presence of excess of toluene has been shown to be a unimolecular process, the rate‐determining step being C6H5·CH2·CH3→C6H5·CH2·+CH3·. The methyl radicals were removed by a reaction with toluene, yielding CH4 and benzyl radicals, and the quantity of methane obtained was a measure of the extent of the decomposition of ethylbenzene. The energy of activation and the frequency factor were estimated experimentally at 63.2 kcal./mole and 1.0·1013 sec.−1, respectively. Taking the D(C–C)C6H5·C2H5 at 63.2 kcal./mole and using this value in conjunction with the value reported previously for D(C–H)C6H5·CH3 and the relevant thermochemical data, we calculated the D(C–H)CH4 at 102.9 kcal./mole.The results of the thermal decomposition of ethylbenzene in the absence of toluene have been investigated, but it has been found unsuitable for kinetics studies because of a complicating chain process.A short discussion on the frequency factors of these unimolecular reactions is given.

The C–Br Bond Dissociation Energy in Benzyl Bromide and Allyl Bromide

The pyrolysis of organic bromides in a stream of toluene is described as a method for the determination of the C–Br bond dissociation energies. The technique used makes it possible to discriminate between two mechanisms of decomposition: R.Br→R·+Br R.Br→olefin+HBr. It was found that both benzyl and allyl bromide decomposed according to mechanism (a), the bromine atoms reacting readily with toluene to give hydrobromic acid. The rate of the primary dissociation process was measured by the rate of formation of HBr. It was proved that the thermal decompositions of benzyl and allyl bromide were homogeneous gas reactions obeying first order kinetics. The activation energies were calculated at 50.5±2 kcal./mole and 47.5±2 kcal./mole, and identified with D(C6H5.CH2–Br) and D(CH2:CH.CH2–Br), respectively.The fate of the allyl radical is discussed, and a rough estimate of the activation energy for the reaction CH2:CH. CH2·+C6H5. CH3→CH2:CH. CH3+C6H5CH2· leads to a value of 14–17 kcal./mole.The problem of ionic cont...

The Kinetics of the Thermal Decomposition of Propylene

The thermal decomposition of propylene was investigated for temperatures ranging from 680°C up to 870°C and with percentages of decomposition from 0.01 percent up to about 2 percent. The reaction was shown to be a homogenous gas reaction of the first order, the first‐order constant being given by an expression 1.1·1013 exp—(72,000/RT). Two mechanisms are discussed, both of which account for the observed kinetics and products of decomposition. It is demonstrated that the first step in the thermal decomposition of propylene is the splitting of the C–H bond leading to the formation of H atoms and allyl radicals. The problem of the value of D(C–H) in propylene is discussed in the light of the two suggested mechanisms. An attempt is made to account for the variety of the reactions between H atoms and propylene molecules as observed by various investigators.

The Kinetics of the Thermal Decomposition of Isobutene

The thermal decomposition of isobutene was found to be a homogeneous gas reaction of the first order. The first‐order constant is 0.5·1013 exp(67,000/RT). The products of the decomposition included H2, CH4, and allene. The experimental results are explained in terms of a chain mechanism, the initiating step being the decomposition of the isobutene molecule into an H atom and a [Complex chemical formula] radical. The most probable value of the D(C–H) in isobutene is 76 kcal./mole. This estimate is based on certain assumptions concerning the frequency factors.