Thermolysis of model compounds for coal. 3. Radical chain decomposition of 1,3-diphenylpropane and 1,4-diphenylbutane

Abstract

Cracking of 1,3-diphenylpropane (1) at 345-425/sup 0/C to give toluene and styrene proceeded much more rapidly than expected from the strength of its weakest C-C bond. Styrene was rapidly consumed in secondary reactions, one of which was addition to 1 to form 1,3,5-triphenylpentane. Toluene product from 1-1,1,3,3-d/sub 4/ contained no detectable deuterium in the aromatic ring. Combination of these experimental data with thermochemical kinetic estimation procedures demonstrates that the rate ''acceleration'' results from a free-radical chain decomposition mechanism involving steps 5 and 6, in which k/sub 6/>>k/sub 5/(1), rather than from a concerted retro-ene cleavage. The failure of such a chain to develop for 1,2-diphenylethane is a consequence of the dependence of ..delta..H/sup 0/ for radical ..beta.. scission on structure. Cracking of 1,4-diphenylbutane (2) gave both toluene plus allylbenzene and ethylbenzene plus styrene. The ratio between these competitive pathways increased nonlinearly with concentration from 0.18/sub 5/ at 100-110 kPa to 0.82 in the neat liquid. The key step is the interconversion of 1,4-diphenyl-1-butyl radical and its 2-isomer by hydrogen abstraction from 2, a process which is competitive with ..beta.. scission. Thermochemical kinetic estimates lead to a set of rate constants which are consistent with the observed product ratio behavior. Thesemore » ..cap alpha..,..omega..-diphenylalkanes, Ph(CH/sub 2/)/sub n/Ph, serve as models for aliphatic bridges between aromatic units in coal. The implications of the demonstrated chain character of their thermolysis when n..-->..>3 for modeling the thermal decomposition of coal are discussed.« less