ABSTRACT

Benzene The study of the class of compounds now referred to as aromatics began in 1825 with the isolation of a compound, now called benzene, by Michael Faraday. At this time the molecular formula of benzene, C6H6, was thought quite unusual due to the low ratio of hydrogen to carbon atoms. Within a very short time the unusual properties of benzene and related compounds began to emerge. During this period, for a compound to be classified as aromatic it simply needed to have a low carbon to hydrogen ratio and to be fragrant; most of the early aromatic compounds were obtained from balsams, resins or essential oils. It was sometime later before Kekulé and coworkers recognized that these compounds all contained a six-carbon unit that remained unchanged during a range of chemical transformations. Benzene was eventually recognized as being the parent for

this new class of compound. In 1865 Kekulé proposed a structure for benzene; a sixmembered ring with three alternating double bonds (Figure 1). However, if such a structure were correct then the addition of two bromine atoms to adjacent carbons would result in the formation of two isomers of 1,2-dibromobenzene (Figure 1). Only one compound has ever been found. To account for this apparent anomaly Kekulé suggested that these isomers were in a state of rapid equilibrium (Figure 1). This is now known to be incorrect, and the Kekulé structures do not actually represent the true structure of benzene. Nevertheless, they are still routinely used, particularly when illustrating reaction mechanisms involving such compounds.