Alternatives to the Kekulé Formula for Benzene: The Ladenburg Formula

The disposition of the fourth valence of the carbon atoms in benzene has caused extensive discussion and speculation. Following Kekulé's formula of 1865, a variety of formulas for benzene was proposed. Two trends were evident: the desire to arrive at the actual structure and the desire to devise formulas which were faithful to the functional behavior and broadly indicative of the structural relationship of the constituent elements. In 1869, Ladenburg criticized Kekulé's formula and suggested alternatives, one of which was the prism formula, which for a time was a serious rival of the hexagon. The strengths and weaknesses of the Ladenburg formula relative to the Kekulé formula are assessed. Recent laboratory studies by Viehe and co-workers have renewed interest in the prism structure.