Kekulé’s Benzene Structure: A Case Study of Teaching Usefulness of Symmetry

Benzene is one of the basic building blocks of organic molecules. One of the reasons for benzene’s ubiquity is its unusual ring structure first discovered by Kekulé in 1865. In this paper, we show that a simple symmetry-based analysis can narrow down possible benzene structures to three ring ones, including the Kekulé’s ring. Thus, Kekulé’s benzene structure provides a good pedagogical example on which one can explain usefulness of symmetries. 1 Kekule’s Benzene Structure: Reminder The importance of benzene. Hydrocarbons are extremely important molecules: they form the basis of oil, our civilization’s current main source of energy; they form the basic of energy processes in biological systems, etc. From the chemical viewpoint, most hydrocarbons are composed of one or several benzene rings. Thus, benzene C6H6 itself – the simplest of such molecules – can be viewed as one of the most basic of the hydrocarbons. How the chemical structure of benzene was determined. The importance of benzene was recognized already in the 19th century. Because of this importance, several researchers proposed possible schemes for benzene’s chemical structure; see, e.g., [1, 2, 3, 11, 12, 13]. A breakthrough was achieved in 1865, when a German chemist Friedrich August Kekulé proposed the currently accepted ring structure [7]; see also [8, 9, 10]. In this structure, six Carbon atoms form a ring, and a Hydrogen atom is attached to each of these Carbon atoms.