The ozonolysis reactions of 1,3- and 1,4-cyclohexadiene have been studied using a combination of matrix isolation, infrared spectroscopy and theoretical calculations.  Experimental and theoretical results demonstrate that these reactions predominantly do not follow the long-accepted Criegee mechanism.  Rather, the reaction of O3 with 1,4-cyclohexadiene leads to the essentially barrierless formation of benzene, C6H6, and H2O3.  These two species are then trapped in the same argon matrix cage and weakly interact to form a molecular complex.  There is also evidence for the formation of the primary ozonide as a quite minor product, formed through a transition state that is slightly higher in energy.  The reaction of O3 with 1,3-cyclohexadiene follows two pathways, one of which is the Criegee mechanism through a low energy transition state leading to formation of the primary ozonide.  In addition, with a similar barrier, ozone abstracts a single hydrogen from C5 while adding to C1, forming a hydroperoxy intermediate.  This study presents two of the rare cases in which the Criegee mechanism is not the dominant pathway for the ozonolysis of an alkene as well as the first evidence for dehydrogenation of an alkene by ozone.

Matrix isolation study of the ozonolysis of 1,3- and 1,4-cyclohexadiene

246th National Meeting (2013)

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