The Pyrolysis of n-Butyl-Benzene and the Heat of Formation of n-Propyl Radical

Abstract

It has been shown that the pyrolysis of n-butyl-benzene results in its dissociating into benzyl and n-propyl radicals, C6H5·CH2·CH2·CH2·CH3→C6H5·CH2·+·CH2·CH2·CH3,and the propyl radicals subsequently decompose to give ethylene and methyl radicals, CH3·CH2·CH2·→CH3·+C2H4. By the use of the toluene-carrier technique, it has been possible to measure the rate of reaction (1) by measuring the rate of formation of ethylene. The activation energy of reaction (1) has been thus determined at 65 kcal/mole. The latter value has been identified with the C6H5·CH2–C3H7 bond dissociation energy, i.e., D(C6H5·CH2–CH2·CH2·CH3)=65 kcal/mole. Since D(C6H5·CH2–C3H7)=ΔHf(C6H5·CH2)+ΔHf(n−C3H7)−ΔHf(n·C6H5·C4H9) and ΔHf(C6H5·CH2)=37.5 kcal/mole, one may compute the unknown value for ΔHf(n—C3H7), namely, ΔHf(CH3·CH2·CH2·)≈24 kcal/mole. Having ΔHf(n—C3H7), we calculated the dissociation energy of the primary C–H bond in propane at D(C3H7–H)=ΔHf(n−C3H7)+ΔHf(H)−ΔHf(C3H8)≈100 kcal/mole. This value for D(CH3·CH2·CH2–H) is compared with the findings of other workers. It agrees well with the recent findings of Stevenson, although the existing indirect chemical evidence seems to require a lower value.