THERMOLYSIS OF PHENETHYL PHENYL ETHER - A MODEL FOR ETHER LINKAGES IN LIGNIN AND LOW-RANK COAL

The thermolysis of phenethyl phenyl ether (PPE) was studied at 330-425 degrees C to resolve the discrepancies in the reported mechanisms of this important model of the beta-ether linkage found in lignin and low rank coal. Cracking of PPE proceeded by two competitive pathways that produced styrene plus phenol and two previously undetected products, benzaldehyde plus toluene. The ratio of these pathways, defined as the alpha/beta selectivity, was 3.1 +/- 0.3 at 375 degrees C and independent of the PPE concentration. The kinetic order over ca. 10(3) variation in the initial concentration from the neat liquid, in solutions with biphenyl, and in the gas phase was 1.29 +/- 0.02. The rate expression for the decomposition in the liquid phase was log (k/M(-0.29) s(-1)) = (11.4 +/- 0.1) - (46.4 +/- 1.0)/2.303RT. The reaction could be accelerated by the addition of a free-radical initiator or a hydrogen bonding solvent, such as p-phenylphenol, but the product composition was altered with the latter. Thermolysis of PPE in tetralin, a model hydrogen donor solvent, increased the alpha/beta selectivity to 7 and accelerated the formation of secondary products. All the data was consistent with a free-radical chain mechanism for the decomposition of PPE. Styrene and phenol are produced by hydrogen abstraction at the alpha-carbon, beta-scission to form styrene and the phenoxy radical, followed by hydrogen abstraction. Benzaldehyde and toluene are formed by hydrogen abstraction at the beta-carbon, 1,2-phenyl migration from oxygen to carbon, beta-scission to form benzaldehyde, and the benzyl radical, followed by hydrogen abstraction. Thermochemical kinetic estimates indicate that product formation is controlled by the relative rate of hydrogen abstraction at the alpha- and beta-carbons by the phenoxy radical (dominant) and benzyl radical (minor) since beta-scission and 1,2-phenyl migration are fast relative to hydrogen abstraction. The electrophilic phenoxy radical has an inherently lower alpha/beta selectivity than the nonpolar benzyl radical because it benefits from the polar effects of the alpha-oxygen at the beta-carbon. The rate of the 1,2-phenyl migration was much faster than interconversion of 1-phenoxy-2-phenyl-1-ethyl radical and 1-phenoxy-2-phenyl-2-ethyl radical, and an activation barrier of <18 kcal mol(-1) was estimated for the 1,2-phenyl migration. Thermolysis of PhCD(2)CH(2)OPh and PhCH(2)CD(2)OPh was consistent with the previous results, indicating that there was no significant contribution of a concerted retro-ene pathway to the thermolysis of PPE.