Michael Govek

Date of Award

1-1-1980

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Abstract

High temperature - high pressure batch reactions were done with bibenzyl and several other compounds to study the mechanism of C-C bond cleavage and hydrogen transfer under coal liquefaction type conditions. Reactions were run in a pair of 250-mL Hastelloy "C" autoclaves and in a 12-mL tubing reactor (microreactor) designed in our Laboratory. Temperatures and pressures approximated those of coal liquefaction, i.e., 425-450(DEGREES)C and 1500 psi (initial).In the study of bibenzyl, the mechanism of both (alpha)- and (beta)-cleavage were studied. The initial reaction bibenzyl under high temperatures is the formation of benzyl radicals through the (beta)-cleavage, with the subsequent formation of toluene. If no hydrogen donor is added, bibenzyl Will act as its own hydrogen donor, forming the 1,2-diphenylethyl radical and, ultimately, trans-stilbene. Bibenzyl Will also cleave at the (alpha)-position to form benzene and ethylbenzene and Will also undergo secondary reactions.In this study, three approaches were taken to study the reactions of bibenzyl: product analysis of bibenzyl and related reactions run at high temperatures, the study of the fate of radicals known to be present in bibenzyl decomposition, and deuterium labeling experiments to trace the path of hydrogen transfers during the reaction.All of the products found in the reactions of bibenzyl in this study have been previously reported. The rate of (alpha)-cleavage was low under essentially non-reducing conditions in this study, i.e., argon, CO-H(,2)O and CO-D(,2)O. In this study, the mixture of carbon monoxide and water was found to be essentially non-reducing. However, under reducing conditions, using H(,2), the rate of (alpha)-cleavage is greatly enhanced. This result supports the result found by Vernon (99).(,)The presence of bibenzyl with other substituted benzenes under molecular hydrogen causes the (alpha)-cleavage of the substituted benzene to increase. This increased activity in (alpha)-cleavage has also been reported elsewhere (98,99,106). At the same time, the rate of (alpha)-cleavage of bibenzyl decreases. The intermediate is suggested as being the hydrogen atom formed by the reaction of a radical with molecular hydrogen. This intermediate is the rate limiting species.Relative rates of hydrogen transfer are noted. As the concentration of molecular hydrogen increases, the yield of trans-stilbene decreases. In the presence of tetralin, the trans-stilbene yield decreases. When deuterium is present in the benzyl position of bibenzyl the trans-stilbene yield decreases.In reactions of molecular deuterium with bibenzyl, deuterium exchanges both into the benzyl and aromatic positions of bibenzyl and its products. Exchange into the aromatic positions appears to be random. When no source of hydrogen is present except molecular hydrogen and bibenzyl, the bibenzyl acts in a significant way as a hydrogen donor. The benzyl radical Will abstract hydrogen from both the aliphatic and aromatic positions on the bibenzyl or its products. The same is true for phenyl and 2-phenylethyl radicals. It was found that benzene is formed predominantly through the phenyl radical. Ethylbenzene is formed, at least to some extent through the 2-phenylethyl radical.When deuterium is substituted for hydrogen gas, an isotope effect of about three is found for the formation of (alpha)-cleavage products. This deuterium isotope effect suggests that a C-D or D-D is broken in the rate determining step.The fate of the 1,2-diphenylethyl radical was studied under high temperature conditions by two methods: peroxide initiator and pyrolysis of azobibenzyl. The significant formation of biphenyl in the reaction of azobibenzyl in benzene solvent gives evidence for the abstraction of aromatic hydrogens. Evidence for aromatic hydrogen abstraction was also found in the pyrolysis of tetraphenyllead in benzene-d(,6).

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