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Bornadiene rearranges cleanly to 3,7,7-trimethyl-l,3,5-cycloheptatriene and p-cymene by parallel, unimolecular, first order processes. The Arrhenius expressions over the temperature range 233.6 - 270.6[degrees]C for formation of trimethyltropilidene and p-cymene are k = 10[raised 13.97[plus-minus].15] exp(-40,610[plus-minus]l,990/RT) and k = 10[raised 14.12[plus-minus]20] exp(-42,400[plus-minus]3,430/RT) respectively. The stereochemistry of the bornadiene-trimethyltropilidene rearrangement is probed utilizing optically active 2,5-bornadiene-9-d[lowered 1] of known enantiomeric purity. Stereochemical analysis employing deuterium decoupling of the 100 MHz nmr spectrum of 3-deuterio-7,7-dimethyl-l,3,5-cycloheptatriene formed by pyrolysis of the optically bornadiene showed that a racemic mixture was formed. A reaction coordinate involving an intermediate diradical is presented for the thermal reorganization of bornadiene. In an optional mechanistic description, a set of likely molecular motions for rearrangement is shown to pass through a quantum mechanical degenerate state which is racemic. Therefore, bornadiene must form racemic trimethyltropilidene even if the reaction is concerted. The present results are critically compared with the thermal chemistry of 3,7,7-trimethyl-l,3,5-cycloheptatriene.