Energy & Fuels 1989, 3, 562-565
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Acid-Catalyzed Reactions Accelerated by Polycyclic Aromatic Hydrocarbons and Coal. 1. Isomerization of 5,6,7,8 -Tetrahydro- 1-naphtholt Malvina Farcasiu* and Charlene Smith1 Pittsburgh Energy Technology Center, US.Department of Energy, P.O. Box
10940,
Pittsburgh, Pennsylvania 15236 Received March 14, 1989. Revised Manuscript Received May 8, 1989
In the presence of trifluoromethanesulfonic acid (triflic acid) 5,6,7,8-tetrahydro-l-naphthol undergoes isomerization to 5,6,7,&tetrahydro-2-naphthol. The rate of the isomerization is increased dramatically by addition of certain polycyclic aromatic hydrocarbons, in particular alkyl-substituted naphthalenes and those aromatics containing more than two rings, and addition of coal. Naphthalene accelerates the isomerization after pretreatment with triflic acid. A mechanism for the rate acceleration is proposed that involves hydrocarbon cation-radical intermediates. These results have relevance to strongacid-catalyzed reactions as well as to coal-oil coprocessing. Introduction In previous studies of strong-acid-catalyzedreactions of aromatic hydrocarbons, we observed some peculiarities of these reactions that could not be explained by a simple carbocation mechanism. We refer in particular to the following findings: (a) transalkylation reactions between an alkylpolyaromatic hydrocarbon (donor) and a monoaromatic hydrocarbon (acceptor) catalyzed by trifluoromethanesulfonic acid (triflic acid) are second order in donor,' and (b) the rate of the transalkylation reaction between durene and o-xylene catalyzed by triflic acid is increased by the presence of small quantities of polycyclic aromatic hydrocarbons such as naphthalene and anthracene.2 Recently, during an exploratory study of acid-catalyzed reactions with model compounds relevant to coal-oil coprocessing, we observed that triflic acid induces the isomto 5,6,7,8-tetraerization of 5,6,7,8-tetrahydro-l-naphthol hydro-2-naphthol. Surprisingly, this isomerization is accelerated dramatically by the presence of small amounts of polycyclic aromatic hydrocarbons or coal. The present paper presents our results and mechanistic rationalization of this strong-acid-catalyzed isomerization reaction. Our observations contribute to the understanding of acid-catalyzed reactions of coal and coal-petroleum mixtures. Results and Discussion Isomerization of 5,6,7,8-Tetrahydro-l-naphthol. In the course of our studies on the transalkylation reactions catalyzed by triflic acid, we found that reaction of 5,6,7,8-tetrahydro-l-naphthol(5-hydroxytetralin, 1)with HO
hydroxytetralin, 2;eq 1). Under mild conditions, the interconversion of 1 and 2 is the only reaction observed. The equilibrium mixture obtained from either 1 or 2 contains roughly equal amounts of the two isomers. The isomerization reaction proceeds cleanly when either 1 or 2 is treated with triflic acid in refluxing cyclohexane. The rate of isomerization increases with the acid concentration. As established for closely related cases,3 the reaction mechanism most likely involves ring carbon atom protonation, 1,2-shift to a hydroxyspiroarenium ion intermediate, and another shift to the isomerized product (eq 2). The mechanism illustrated is supported by literature HO
HO
data concerning preference for ring protonation over oxygen protonation in phenols and phenol ethers4 and by solvolysis studies conducted on arenes~lfonates~ in which spiroarenium compounds are formed as intermediates. Effect of Polycyclic Aromatics on the Rate of Isomerization of 5-Hydroxytetralin. The rate of interconversion of 1 and 2 was found to be influenced by the nature of the added aromatic compound. For purposes of comparison, in the following discussions the reaction with no added polycyclic aromatic hydrocarbon (eq 1)will be referred to as the base case. A number of aromatic hydrocarbon compounds of different basicities6 and structures (Table I) were screened in order to understand the role that the polycyclic aromatic compounds play in the isomerization process. These re(1) Farcasiu, M.; Forbus, T. R.; Rubin, B. R. J. Energy and Fuels 1987, 1
OD
1 , 10.
polycyclic alkylaromatics is accompanied by a reversible (6isomerization of 1 to 5,6,7,8-tetrahydro-2-naphthol 'Reference in the paper to any specific commercial product, process, or service is to facilitate understanding and does not necessarily imply ita endorsement or favoring by the United States Department of Energy. Oak Ridge Associated Universities Appointee, Postgraduate Research Training Program.
*
0887-0624f 8912503-0562$01.50/0
(2) Farcasiu, M.; Forbus, T. R. U.S.Patent No. 4,454,364 June, 12, 1984. (3) Dewar, M. J. S. In MoZecular Rearrangements, Part 1 ; de Mayo, P., Ed.; Wiley-Interscience: New York, 1963; Chapter 3, pp 302-303. (4) Birchall, T.; Bourns, A. N.; Gillespie, R. J.; Smith, P. J. Can. J. Chem. 1964,42, 1433. (5) Heck, R.; Winstein, S. J. Am. Chem. SOC.1957, 79, 3105. Heck, R.; Winstein, S. J . Am. Chem. SOC.1957, 79, 3114. Jackman, L. M.; Haddon, V. R. J. Am. Chem. SOC.1974,96,5130. Gates, M.; Frank, D. L.; von Felten, W. C. J . Am. Chem. SOC.1974,96, 5138.
(6) Mackor, E.L.;Hofstra, A.; Van der Waals,J. H. Trans. Faraday
SOC.1958,54,66. Mackor, E. L.; Hofstra, A.; Van der Waals,J. H. "ram. Faraday SOC.1958,54, 186.
1989 American Chemical Society
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