The thermal isomerization of [alpha]-pinene

The kinetic and thermodynamic properties of the thermal isomerization of [alpha]-pinene have been examined. A stereospecifically deuterated [alpha]-pinene was synthesized and pyrolyzed. The positions of the label in the products were determined, and this information was used in formulating a mechanism for the pinene rearrangements. Arrhenius expressions for the thermal rearrangement of [alpha]-pinene to dipentene and alloocimene have been determined for rate data obtained from a stirred flow reactor. The new rates have been compared, in detail, to those determined in earlier studies and found to be similar, but not identical. The pinene reaction is unimolecular and produces dipentene and alloocimene as primary products. It is not possible to construct a completely detailed mechanism from a review of all the available data. It has been found that the two primary products are not formed in two simultaneous independent reactions. The report that pinene undergoes a thermal racemization has been disproved for temperatures less than 275[degrees]C.