Thermolysis of Model Compounds for Coal. 3. Radical Chain Decomposition of 1,3-Diphenylpropane and 1,4-Diphenylbutane

Article abstract of DOI:10.1021/jo00146a017

Cracking of 1,3-diphenylpropane (1) at 345-425 deg 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 from 1,3,5-triphenylpentane.The kinetic order at low conversion over a 10³ variation in initial concentration from neat liquid through solutions in biphenyl to gas at 20 kPa was 1.59 +/- 0.03.The activation energy for the neat liquid was 52.3 +/- 1.3 kcal*mol^-1.Toluene product from 1-1,1,3,3-d₄ 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 k₆ >>k₅<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 ΔH⁰ for radical β 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 0.18₅ at 100-110 kPa to 0.82 in the neat liquid.This behavior is interpreted in terms of chain reaction steps 22-26.The key step is the interconversion of 1,4-diphenyl-1-butyl radical (6) and its 2-isomer (7) by hydrogen abstraction from 2, a process which is competitive with β scission.Thermochemical kinetic estimates lead to a set of rate constants which are consistent with the observed product ratio behavior.Use of the hydrogen atom donor tetralin as the solvent had only minor effects on the rates of thermolysis of 1 and 2 but dampened the product ratio dependence from 2.Tetralyl radicals are too reactive as hydrogen abstractors at 350-400 deg C to serve as chain-inhibiting species.These α,ω-diphenylalkanes, Ph(CH2)nPh, serve as models for aliphatic bridges between aromatic units in coal.The implications of the demonstrated chain character of their thermolysis when n is more or equal to 3 for modeling the thermal decomposition of coal are discussed.