Coming 'round to old views of benzene.

Coming 'round to old views of benzene

A new view of the electronic structure of benzene comes nearly full circle to a 121-year-old vision of the molecule. In 1858 Friedrich August Kekule introduced his valence bond theory of chemical combination that he used in 1865 when he suggested a chemical structure for benzene. He argued that the then anomalous chemical properties of benzene were somehow due to the ability of benzene's double bonds to oscillate between two six-carbon ring structures that were identical save for a shifting of the three double bonds.

Had Kekule known in his time about quantum theory's concept of molecular orbitals introduced in the late 1920s, he would have melted his two structures into what is now called a "resonance hybrid." Molecular orbital theory, which accounts for chemical bonding by describing where electrons are likely to be found around a molecule, is presented in today's general chemistry courses as a modern refinement of Kekule's valence bond theory, which describes how electrons localized around atoms interact to form bonds. Modern theory draws a picture of benzene in which delocalized electrons dash about in doughnut-shaped orbitals above and below the entire ring of six carbon atoms.

Now, using a revised valence bond theory that incorporates electronic spin, new calculations of benzene's electronic structure reported in the Oct. 23 NATURE by David L. Cooper of the University of Liverpool, Joseph Gerratt of the University of Bristol and Mario Raimondi of the University of Milan indicate that the "delocalized" electrons of molecular orbital theory are actually localized in deformed orbits about the individual carbon atoms. They comment in their article that Kekule's 19th century views on the matter come closer to this newest picture of benzene than to the benzene of molecular orbital theory.