Kekulé von Stradonitz , Friedrich August

(1829–1896) German chemist

Kekulé was born in Darmstadt. As a youth he showed considerable skill in drawing and was consequently encouraged to be an architect. Although he began as a student of architecture at Giessen he soon switched, despite family opposition, to the study of chemistry, which he continued abroad. He first went to Paris in the period 1851–52 where he studied under Jean Dumas and Charles Gerhardt who influenced him greatly. He worked in Switzerland for a while before taking a post in England in 1854–55 as a laboratory assistant at St. Bartholomew's Hospital, London. While in London he met and was influenced by Alexander Williamson and William Odling. He accepted an unsalaried post at the University of Heidelberg before his appointment to the chair of chemistry at Ghent in 1858. He then moved to the chemistry chair at the University of Bonn in 1867, where he remained for the rest of his life.

Kekulé's main work was done on the structure of the carbon atom and its compounds. It has often been claimed that he had changed his career from the architecture of buildings to the architecture of molecules. Certainly, after Kekulé it was much easier to visualize the form of atoms and their combinations. In 1852 Edward Frankland had pointed out that each kind of atom can combine with only so many other atoms. Thus hydrogen can combine with only one other atom at a time, oxygen could combine with two, nitrogen with three, and carbon with four. Such combining power soon became known as the valency (valence) of an atom. Each atom would be either uni-, bi-, tri-, quadrivalent, or some higher figure.

In 1858 both Kekulé and Archibald Couper saw how to use this insight of Frankland to revolutionize organic chemistry. They both assumed that carbon was quadrivalent and that one of the four bonds of the carbon atom could be used to join with another carbon atom. He published his results in 1858 in his paper The Constitution and the Metamorphoses of Chemical Compounds and the Chemical Nature of Carbon and in the first volume of hisLehrbuch der organische Chemie (1859; Textbook of Organic Chemistry).

The diagrams of carbon compounds used today come not from Kekulé but from Alexander Crum Brown in 1865. Kekulé's own notation, known as ‘Kekulé sausages’, in which atoms were represented by a cumbersome system of circles, was soon dropped. The gains from such representations were immediate. It can be seen why two molecules could have the same number of atoms of each element and yet differ in properties. Thus C₂H₆O represents both ethanol and dimethyl ether. If the rules of valence are observed these are the only two ways in which two carbon, six hydrogen, and one oxygen atom can be combined and indeed these are the only two compounds of the formula ever observed.

While Kekulé had dramatic success demonstrating how organic compounds could be constructed from carbon chains, one set of compounds, the aromatics, resisted all such treatment. Benzene, discovered by Michael Faraday in 1825, had the formula C₆H₆, which, on the assumption of a quadrivalent carbon atom, just could not be represented as any kind of chain. The best that could be done with alternating single and double carbon bonds would still violate the valence rules, for at the end of the chain the carbon atoms will both have an unfilled bond. Kekulé has left a description of how the solution of the puzzle came to him. In 1890 he recalled that while working on his textbook in 1865, “I dozed off. Again the atoms danced before my eyes. This time the smaller groups remained in the background. My inner eye … now distinguished bigger forms of manifold configurations. Long rows, more densely joined; everything in motion, contorting and turning like snakes. And behold what was that? One of the snakes took hold of its own tail and whirled derisively before my eyes. I woke up as though I had been struck by lightning; again I spent the rest of the night working out the consequences.”

The snake with its tail in its mouth is in fact an ancient alchemical symbol and is named Ouroboros but, to Kekulé, it meant a more prosaic image, that of a ring. For if the two ends of the benzene chain are joined to each other then benzene will have been shown to have a ring structure in which the valence rules have all been observed. Again the rewards in understanding were immediate. It was now obvious why substitution for one of benzene's hydrogen atoms always produced the same compound. The mono-substituted derivative C₆H₅X was completely symmetrical whichever H atom it replaced. Each of the hydrogen atoms were replaced by NH₂ and in each case the same compound, aniline C₆H₅.NH₂, was obtained.

Such was the revolution in organic chemistry initiated by Kekulé. Together with new methods introduced by Stanislao Cannizzaro at Karlsruhe in 1860 for the determination of atomic weight, a new age of chemistry was about to dawn in which the conflicts and uncertainties of the first half of the 19th century would be replaced by a unified chemical theory, notation, and practice. After this it comes as something of a shock to discover that Kekulé had no firm belief in the existence of atoms. Whether they exist he argued in 1867 “has but little significance from a chemical point of view; its discussion belongs rather to metaphysics. In chemistry we have only to decide whether the assumption of atoms is an hypothesis adapted to the explanation of chemical phenomena.”