Johann von Baeyer | Robert Bunsen | August Wilhelm von Hofmann
Friedrich August Kekulé was one of the most brilliant and imaginative German chemists of the 19th Century. He is best known for his elucidation of the structure of benzene.
Kekulé was born in Darmstadt on 7 September, 1829. At the age of 18, he matriculated from the University of Gießen as a student of architecture.
While at Gießen he attended Justus von Liebig's lectures on chemistry, which he found so attractive he decided to abandon his architectural studies in favour of chemistry. Kekulé's training was completed by a year studying in Paris. Thereafter, he held private research posts in Switzerland and London.
In 1856 he returned to Germany as Privatdozent1 at Heidelberg. Then, in 1858, Kekulé was appointed to the chair of chemistry at Ghent, Belgium. In 1867 he became professor of chemistry at Bonn, where his health became compromised by overwork. He died on 13 July, 1896.
To understand Kekulé's contribution to our understanding of molecular structure, and not overlook the Scottish chemist Archibald Scott Couper's (1831 - 1892) very significant contributions to this field, it's necessary to put it into context with what was known at the time.
Combining Power of Atoms-Historical Background
During the mid-19th Century, confusion raged over the 'combining power' of atoms. It had long been known, and quantitatively expressed in the 'Law of multiple proportions' (Berzelius), that atoms may combine in more than one proportion. Thus, in 1834, Jean-Baptiste Dumas had shown that, while one atom of chlorine replaces one of hydrogen, one atom of oxygen replaces two atoms of hydrogen. Moreover, in the type formulae it was clear that, whereas one hydrogen atom or one chlorine atom could be linked with only one radical, one oxygen atom could be linked with two radicals, as in diethylether (ethoxyethane). Furthermore, it was known that nitrogen could be linked with three radicals, as in triethylamine, or with five elementary or compound radicals, as in tetramethylammonium iodide: (CH3)4NI.
The concept of a 'combining capacity', and the idea that the combining capacity of any given atom is limited, was first clearly expressed by the English chemist Edward Frankland in a paper entitled On a New Series of Organic Bodies containing Metals. Frankland wrote:
When the formula of inorganic chemical compounds are considered, even a superficial observer is struck with the general symmetry of their construction; the compounds of nitrogenphosphorus, antimony and arsenic especially exhibit the tendency of these elements to form compounds containing three or five equivalents of other elements, and it is in these proportions that their affinities are best satisfied... This hypothesis that atoms have a variable but limited combining power came to form the basis of the doctrine of atomicity, quantivalence, valency or valence.
For a number of years the development of the doctrine of valency2 was held back because there was disagreement over the values of the atomic weights of certain elements. For example, some accepted the formula of water as being HO, which would give oxygen an atomic weight of eight units. Others preferred the formula H2O, making oxygen's atomic weight 16. Carbon could be rated as six or 12, causing disagreement over the representation of the formulae of organic compounds. These issues were resolved by Stanislau Cannizzaro's revival of the Avogadro hypothesis, which established O = 16 as the correct value.
Patterns of Molecular Structure
There was now the question of how the atoms in molecules were arranged in space. Berzelius had pondered how pairs of compounds with the same composition could have different physical and chemical properties, and coined the term 'isomerism' for this phenomenon. Several chemists, including Joseph Louis Gay-Lussac and Michel Eugène Chevreul, had suggested that such pairs of compounds contained the same atoms arranged in different patterns. The nature of these patterns, however, remained obscure.
Archibald Scott Couper
Couper was aware that the combining power of carbon (later to be called its valency) was usually four. He was also aware that it could sometimes exhibit a combining power of two - eg, in carbon dioxide. Couper had the radical insight that carbon 'could enter into chemical union with itself' - that is, its atoms could join together in long chains. Examples of this, which he cited, included propyl and butyl alcohols (propanol and butanol), respectively.
In modern terms, their formulae are C3H8O and C4H10O. But, because Couper took the atomic weight of oxygen as eight rather than 16, he thought they were C3H8O2 and C4H9O2. Nevertheless, this conveniently explained the existence of numerous series of carbon compounds where each succeeding member differs from the previous one in the sequence by CH2.
Later, in an updated version of this paper, published in 1858, Couper introduced a new idea: that some carbon-nitrogen compounds including cyanuric acid (2,4,6-trihydroxy-1,3,5-triazine) could have ring structures. From here, it would have been a short step to working out the configuration of benzene. But Couper never got this far. Kekulé made it seven years later.
Kekulé's Contribution to Modern Structural Chemistry
Both Couper and Kekulé were working on two main postulates: the quadrivalency of carbon, and the capacity of carbon atoms to catenate, or form long chains, as realised by Couper. From these, Kekulé and Couper showed how the molecular constitution or mutual linking together of atoms in a compound could be represented diagrammatically, such that the relations between atoms could be made intelligible.
In his classic paper On a New Chemical Theory, Couper had shown how the molecular constitution of compounds could be represented by means of graphic formulae in which the valencies of atoms are represented by lines. Thus, except for the fact that he retained the atomic weight of oxygen as being eight rather than 16 and wrote a double-oxygen atom, O-O, in place of single atoms, his formulae are similar to those in use today. On the other hand, in his Textbook of Organic Chemistry (1861), Kekulé used a graphic form of representation in which univalent atoms were represented by circles and multivalent atoms by figures formed by merging two, three or four circles to give rather unwieldy 'sausage formulae'. Kekulé described how he had a dream from which he developed his sausage formulae:
I was returning [to my lodgings] by the last omnibus, 'outside' as usual, through the deserted streets of the metropolis, which are at other times so full of life. I fell into a reverie and lo! the atoms were gambolling before my eyes. Whenever, hitherto, these diminutive beings had appeared to me, they had always been in motion; but up to that time I had never been able to discern the nature of their motion. Now, however, I saw how, frequently, two smaller atoms united to form a pair; how a larger one embraced two smaller ones; how still larger ones kept hold of three or even four of the smaller; whilst the whole kept whirling a giddy dance. I saw how the larger ones formed a chain... I spent part of the night putting on paper at least sketches of these dream forms.
The Structure of Benzene
How, precisely, the six carbon atoms known to be present in benzene were arranged continued to mystify chemists for some time. Carbon was known to have four valencies, while hydrogen had one. Furthermore, it was known that the benzene molecule contained six atoms of each of these elements. So, how did they link up? Six carbon atoms ought to be able to link up with 14 hydrogen atoms, given the fact that Couper had shown carbon atoms could link up in chains.
It was Kekulé who provided the answer. One evening in 1865, when he was professor of chemistry at Ghent, Kekulé was writing his textbook, but his thoughts were elsewhere and he had another dream. He wrote:
I turned my chair to the fire and dozed. Again the atoms were gambolling before my eyes. This time the smaller groups kept modestly in the background. My mental eye, rendered more acute by repeated visions of this kind, could now distinguish larger structures, of manifold conformation; long rows, sometimes more closely fitted together; all twining and twisting in snake-like motion. But look! What was that? One of the snakes had seized hold of its own tail, and the form whirled mockingly before my eyes. As if by a flash of lightning I awoke.
From these dream forms, Kekulé's graphical formulae for benzene were developed. He suggested a closed ring of six carbon atoms in a regular hexagon, to each of which a hydrogen atom was attached.
To harmonise the hexagonal formula for benzene with the universal quadrivalence of carbon, as postulated by Kekulé, it was necessary for the carbon atoms to be linked by alternate single and double bonds. However, there was a difficulty with this formula: it would indicate the possibility of two distinct ortho-derivatives.3 Hence Kekulé (1872) made the ad hoc suggestion that there's an oscillation of the bonds, giving rise to two alternating ring structures.
In this system, the two ortho-positions (and the two meta-positions4) become identical. This dynamic formula for benzene recognised for the first time what came to be called 'desmotropic change'5. It may have been suggested in Kekulé's vision in which 'rows' of carbon atoms danced before his eyes, and then one of the rows coiled itself like a snake and bit its tail, forming a ring.
This theory of the molecular constitution of benzene (and other 'aromatic'6 compounds) wasn't immediately and universally accepted. But it was justified over the following decades by experimental investigations designed to test the theory, and the predictions that the theory enabled scientists to make.
The basic ring structure of benzene, as proposed by Kekulé, is still accepted by scientists today, albeit with much modification to account for the unreactivity (termed 'aromaticity') of benzene. For example, as a system of alternating single and double bonds, it could be termed 'cyclohexatriene'. However, it doesn't undergo the addition reactions typical of alkenes. Furthermore, X-ray spectrometry has shown that all the carbon-carbon bonds are the same length, intermediate between that of single and double bonds.
The contribution which Kekulé made to chemistry and the chemical industry is almost incalculable. The structural representations developed independently by Couper and Kekulé form the basis on which the whole of organic chemistry, defined by Charles Frédéric Gerhardt as 'the chemistry of the carbon compounds', has been built. Furthermore, the theory of the structure of benzene underpins such diverse industries as aniline dyes and other synthetic dye-stuffs, synthetic drugs, plastics, photographic materials, and so on. It's shown itself capable of an almost limitless development, so as to include compounds with multiple rings of atoms which can even be fused together (eg, as in naphthalene and anthracene). Besides forming homocyclic rings, composed solely of carbon, it's possible to have heterocyclic rings containing such elements as sulphur, nitrogen, or oxygen as well as carbon.
In addition to receiving many honours for his work, Kekulé was ennobled by the German emperor, at which point he dropped the accent on the final 'é' of his name and became Kekule von Stradonitz.