The ozonolysis of <i>ortho</i>-xylene and its relevance to the history of the benzene problem

Abstract

Using B3LYP density functional theory in conjunction with the 6-311++G** basis set, the relative rates of the initial rate-determining attack of ozone upon the carbon—carbon bonds of o-xylene are found to be 4,5- 3,4- > 2,3- > 1,2-. With the assumption that the final products glyoxal (G), methylglyoxal (M), and dimethylglyoxal (D) are determined by the site of the initial attack, and taking statistical factors into account, the ratio G:M:D is calculated to be 3:2:1. This is close to the value found experimentally, and identical to the result predicted by the contention that two nearly equivalent Kekulé–Pauling valence bond (resonance) structures of o-xylene exist and each localized carbon—carbon double bond of these structures is oxidized at the same rate. Although Frontier Molecular Orbital theory also predicts a 3:2:1 ratio of G:M:D, this theory incorrectly predicts that the initial attack of ozone will take place at the 1,2- and 4,5- bonds, with 1,2-attack slightly preferred. These results are discussed in relation to a recent historical analysis of the benzene problem, and it is concluded that since the products of ozonolysis of o-xylene are determined by the relative energies of the transition states leading to the four possible primary ozonides, these transition states should be the focus of theory.Key words: Kekulé structure, resonance, frontier orbital theory, density functional theory, kinetics.