The gas-phase thermal chemistry of tetralin and related model systems

Malandra, James
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In the gas-phase thermal decomposition of tetralin (1), the relative amounts of dehydrogenation products, 1,2-dihydronaphthalene (2) and naphthalene (5), to ethylene loss products, benzocyclobutene (3) and styrene (6), vary with the conditions of the experiment. The multiphoton dissociation (MPD) of 1 produces mostly ethylene loss while the pulsed and continuous wave (cw) laser-sensitized decomposition of 1 produces mostly dehydrogenation. The relative amounts of dehydrogenation to ethylene loss in the flash vacuum pyrolysis (FVP) of 1 are dependent on the system pressure and the sample temperature. When 1 is pyrolyzed under flow conditions, where sample molecules are diluted with a large excess of argon, ethylene loss exceeds dehydrogenation. We conclude that the lowest energy unimolecular gas-phase decomposition channel is ethylene loss. A bimolecular dehydrogenation reaction is responsible for greater amounts of hydrogen-loss products observed under some conditions. In none of our pyrolysis experiments did we find any evidence of heterogeneous catalytic reactions on surfaces;We have observed the facile transformation of o-allyltoluene (4) to 2-methylindan (7), which we propose occurs through an intramolecular hydrogen atom transfer from the benzylic methyl group to the double bond of 4. Loss of a methyl from 7 leads to the formation of indene (8). We have identified the transformation 4 to 7 to 8 as the major source of 8 in the gas-phase thermal decomposition of 1;The FVP and photolysis of ketone 3-benzocycloheptenone and the FVP of sulfone 1,3,4,5-tetrahydro-2-benzothiepin-2,2-dioxide were studied to produce the 1,6-diradical formed by cleavage of the benzylic bond of 1. An unusual aspect of the ketone pyrolysis is the formation of 1-methylnaphthalene and 2-methylnaphthalene, products formed by a net loss of water from 3-benzocycloheptenone. Bis(o-allylbenzyl) oxalate was pyrolyzed to observe the gas-phase thermal chemistry of o-allylbenzyl radical. The FVP of 1,4-diphenylbutane produces 3-phenylpropyl radical. The FVP of o-(3-butenyl)toluene and o-(4-pentenyl)toluene were also studied.

Chemistry, Organic chemistry