Relative Reactivities of Alkylbenzenes and Related Compounds toward Ozone. The Mechanism of Ozonation at Benzylic Positions

Article abstract of DOI:10.1021/jo00188a012

Four monoalkylbenzenes and selected polyphenylalkanes, fluorenes, and partially reduced anthracenes and phenanthrenes were ozonized at 25 deg C in dichlormethane, and relative reactivities were determinated.These reactivities can be divided into the percent of ring ozonation plus the percent of reaction at the benzylic position (which yields hydrotrioxides) by comparison with related systems that undergo only the former reaction. (For example, the relative reactivities of toluene and tert-butylbenzene can be compared to determine the relative reactivity of toluene's benzylic position.) Introduction of either one or two methyl or phenyl group at the exocyclic position of toluene markedly increases benzylic reactivity.Although 9,10-dihydroanthracene derivatives are structurally similar to diphenylmethane, their benzylic reactivity toward ozone is some 50 times greater on a per hydrogen basis; this is explicable in terms of the greater ease of achieving a planar transition state in the former case and the resultant greater delocalization in the benzylic indermediate.The relatively modest amount of benzylic reaction for fluorene leads us to propose that the intermediate involved in hydrotrioxide formation is a carbocationic species, rather than a free radical, in agreement with the recent suggestion of Nangia and Benson.