Friedrich August Kekulé, later Friedrich August Kekule von Stradonitz ( , ; 7 September 1829 – 13 July 1896), was a German
organic chemist
Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic matter, organic materials, i.e., matter in its various forms that contain ...
. From the 1850s until his death, Kekulé was one of the most prominent chemists in Europe, especially in the field of
theoretical chemistry
Theoretical chemistry is the branch of chemistry which develops theoretical generalizations that are part of the theoretical arsenal of modern chemistry: for example, the concepts of chemical bonding, chemical reaction, valence, the surface ...
. He was the principal founder of the theory of
chemical structure
A chemical structure of a molecule is a spatial arrangement of its atoms and their chemical bonds. Its determination includes a chemist's specifying the molecular geometry and, when feasible and necessary, the electronic structure of the target m ...
and in particular the
Kekulé structure of
benzene
Benzene is an Organic compound, organic chemical compound with the Chemical formula#Molecular formula, molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar hexagonal Ring (chemistry), ring with one hyd ...
.
Name
Kekulé never used his first given name; he was known throughout his life as August Kekulé. He did however say it once in his work. After he was ennobled by the Kaiser in 1895, he adopted the name August Kekule von Stradonitz, without the French acute accent over the second "e". The French accent had apparently been added to the name by Kekulé's father during the Napoleonic occupation of Hesse by France, to ensure that
French-speakers pronounced the third syllable.
Early years
The son of a civil servant, Kekulé was born in
Darmstadt
Darmstadt () is a city in the States of Germany, state of Hesse in Germany, located in the southern part of the Frankfurt Rhine Main Area, Rhine-Main-Area (Frankfurt Metropolitan Region). Darmstadt has around 160,000 inhabitants, making it the ...
, the capital of the
Grand Duchy of Hesse
The Grand Duchy of Hesse and by Rhine () was a grand duchy in western Germany that existed from 1806 to 1918. The grand duchy originally formed from the Landgraviate of Hesse-Darmstadt in 1806 as the Grand Duchy of Hesse (). It assumed the name ...
. After graduating from secondary school (the Grand Ducal Gymnasium in Darmstadt), in the fall of 1847 he entered the
University of Giessen
University of Giessen, official name Justus Liebig University Giessen (), is a large public research university in Giessen, Hesse, Germany. It is one of the oldest institutions of higher education in the German-speaking world. It is named afte ...
, with the intention of studying architecture.
After hearing the lectures of
Justus von Liebig
Justus ''Freiherr'' von Liebig (12 May 1803 – 18 April 1873) was a Germans, German scientist who made major contributions to the theory, practice, and pedagogy of chemistry, as well as to agricultural and biology, biological chemistry; he is ...
in his first semester, he decided to study chemistry.
[ Following four years of study in ]Giessen
Giessen, spelled in German (), is a town in the Germany, German States of Germany, state () of Hesse, capital of both the Giessen (district), district of Giessen and the Giessen (region), administrative region of Giessen. The population is appro ...
and a brief compulsory military service, he took temporary assistantships in Paris (1851–52), in Chur
''
Chur (locally) or ; ; ; ; ; ; or ; , and . is the capital and largest List of towns in Switzerland, town of the Switzerland, Swiss Cantons of Switzerland, canton of the Grisons and lies in the Alpine Rhine, Grisonian Rhine Valley, where ...
, Switzerland (1852–53), and in London (1853–55), where he was decisively influenced by Alexander Williamson. His Giessen doctoral degree was awarded in the summer of 1852.
Theory of chemical structure
In 1856, Kekulé became Privatdozent
''Privatdozent'' (for men) or ''Privatdozentin'' (for women), abbreviated PD, P.D. or Priv.-Doz., is an academic title conferred at some European universities, especially in German-speaking countries, to someone who holds certain formal qualifi ...
at the University of Heidelberg
Heidelberg University, officially the Ruprecht Karl University of Heidelberg (; ), is a public university, public research university in Heidelberg, Baden-Württemberg, Germany. Founded in 1386 on instruction of Pope Urban VI, Heidelberg is List ...
. In 1858, he was hired as full professor at the University of Ghent
Ghent University (, abbreviated as UGent) is a Public university, public research university located in Ghent, in the East Flanders province of Belgium.
Located in Flanders, Ghent University is the second largest Belgian university, consisting o ...
, then in 1867 he was called to Bonn
Bonn () is a federal city in the German state of North Rhine-Westphalia, located on the banks of the Rhine. With a population exceeding 300,000, it lies about south-southeast of Cologne, in the southernmost part of the Rhine-Ruhr region. This ...
, where he remained for the rest of his career. Basing his ideas on those of predecessors such as Williamson, Charles Gerhardt, Edward Frankland, William Odling, Auguste Laurent, Charles-Adolphe Wurtz and others, Kekulé was the principal formulator of the theory of chemical structure (1857–58). This theory proceeds from the idea of atomic valence, especially the tetravalence of carbon (which Kekulé announced late in 1857) and the ability of carbon atoms to link to each other (announced in a paper published in May 1858), to the determination of the bonding order of all of the atoms in a molecule. Archibald Scott Couper independently arrived at the idea of self-linking of carbon atoms (his paper appeared in June 1858), and provided the first molecular formulas where lines symbolize bonds connecting the atoms. For organic chemists, the theory of structure provided dramatic new clarity of understanding, and a reliable guide to both analytic and especially synthetic work. As a consequence, the field of organic chemistry developed explosively from this point. Among those who were most active in pursuing early structural investigations were, in addition to Kekulé and Couper, Frankland, Wurtz, Alexander Crum Brown
Alexander Crum Brown Fellow of the Royal Society of Edinburgh, FRSE Fellow of the Royal Society, FRS (26 March 1838 – 28 October 1922) was a Scottish Organic chemistry, organic chemist. Alexander Crum Brown Road in Edinburgh's King's Buildi ...
, Emil Erlenmeyer
Richard August Carl Emil Erlenmeyer (28 June 1825 – 22 January 1909), known simply as Emil Erlenmeyer, was a German chemist known for contributing to the early development of the theory of chemical structure and formulating the Erlenmeyer rul ...
, and Alexander Butlerov.
Kekulé's idea of assigning certain atoms to certain positions within the molecule, and schematically connecting them using what he called their "Verwandtschaftseinheiten" ("affinity units", now called " valences" or "bonds"), was based largely on evidence from chemical reactions, rather than on instrumental methods that could peer directly into the molecule, such as X-ray crystallography
X-ray crystallography is the experimental science of determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract in specific directions. By measuring th ...
. Such physical methods of structural determination had not yet been developed, so chemists of Kekulé's day had to rely almost entirely on so-called "wet" chemistry. Some chemists, notably Hermann Kolbe
Adolph Wilhelm Hermann Kolbe (27 September 1818 – 25 November 1884) was a German chemist and academic, and a major contributor to the birth of modern organic chemistry. He was a professor at Marburg and Leipzig. Kolbe was the first to apply t ...
, heavily criticized the use of structural formulas that were offered, as he thought, without proof. However, most chemists followed Kekulé's lead in pursuing and developing what some have called "classical" structure theory, which was modified after the discovery of electrons (1897) and the development of quantum mechanics (in the 1920s).
The idea that the number of valences of a given element was invariant was a key component of Kekulé's version of structural chemistry. This generalization suffered from many exceptions, and was subsequently replaced by the suggestion that valences were fixed at certain oxidation state
In chemistry, the oxidation state, or oxidation number, is the hypothetical Electrical charge, charge of an atom if all of its Chemical bond, bonds to other atoms are fully Ionic bond, ionic. It describes the degree of oxidation (loss of electrons ...
s. For example, periodic acid
Periodic acid ( ) is an oxoacid of iodine. It can exist in two forms: orthoperiodic acid, with the chemical formula , and metaperiodic acid, which has the formula . Periodic acids are colourless crystals. Periodic acid features iodine in the hig ...
according to Kekuléan structure theory could be represented by the chain structure I-O-O-O-O-H. By contrast, the modern structure of (meta) periodic acid has all four oxygen atoms surrounding the iodine in a tetrahedral geometry.
Benzene
Kekulé's most famous work was on the structure of benzene
Benzene is an Organic compound, organic chemical compound with the Chemical formula#Molecular formula, molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar hexagonal Ring (chemistry), ring with one hyd ...
.[ In 1865 Kekulé published a paper in French (for he was then still in Belgium) suggesting that the structure contained a six-membered ring of carbon atoms with alternating single and double bonds. The following year he published a much longer paper in German on the same subject.
The empirical formula for benzene had been long known, but its highly unsaturated structure was a challenge to determine. Archibald Scott Couper in 1858 and Joseph Loschmidt in 1861 suggested possible structures that contained multiple double bonds or multiple rings, but the study of ]aromatic compound
Aromatic compounds or arenes are organic compounds "with a chemistry typified by benzene" and "cyclically conjugated."
The word "aromatic" originates from the past grouping of molecules based on odor, before their general chemical properties were ...
s was in its earliest years, and too little evidence was then available to help chemists decide on any particular structure.
More evidence was available by 1865, especially regarding the relationships of aromatic isomer
In chemistry, isomers are molecules or polyatomic ions with identical molecular formula – that is, the same number of atoms of each element (chemistry), element – but distinct arrangements of atoms in space. ''Isomerism'' refers to the exi ...
s. Kekulé argued for his proposed structure by considering the number of isomers observed for derivatives of benzene. For every monoderivative of benzene (C6H5X, where X = Cl, OH, CH3, NH2, etc.) only one isomer was ever found, implying that all six carbons are equivalent, so that substitution on any carbon gives only a single possible product. For diderivatives such as the toluidine
There are three isomers of toluidine, which are organic compounds discovered and named by James Sheridan Muspratt and August Wilhelm von Hofmann in 1845. These isomers are O-Toluidine, ''o''-toluidine, ''m''-toluidine, and ''p''-toluidine, with the ...
s, C6H4(NH2)(CH3), three isomers were observed, for which Kekulé proposed structures with the two substituted carbon atoms separated by one, two and three carbon-carbon bonds, later named ortho, meta, and para isomers respectively.
The counting of possible isomers for diderivatives was, however, criticized by Albert Ladenburg, a former student of Kekulé, who argued that Kekulé's 1865 structure implied two distinct "ortho" structures, depending on whether the substituted carbons are separated by a single or a double bond. Since ortho derivatives of benzene were never actually found in more than one isomeric form, Kekulé modified his proposal in 1872 and suggested that the benzene molecule oscillates between two equivalent structures, in such a way that the single and double bonds continually interchange positions. This implies that all six carbon-carbon bonds are equivalent, as each is single half the time and double half the time. A firmer theoretical basis for a similar idea was later proposed in 1928 by Linus Pauling
Linus Carl Pauling ( ; February 28, 1901August 19, 1994) was an American chemist and peace activist. He published more than 1,200 papers and books, of which about 850 dealt with scientific topics. ''New Scientist'' called him one of the 20 gre ...
, who replaced Kekulé's oscillation by the concept of resonance
Resonance is a phenomenon that occurs when an object or system is subjected to an external force or vibration whose frequency matches a resonant frequency (or resonance frequency) of the system, defined as a frequency that generates a maximu ...
between quantum-mechanical structures.
Kekulé's dream
The new understanding of benzene, and hence of all aromatic compounds, proved to be so important for both pure and applied chemistry after 1865 that in 1890 the German Chemical Society organized an elaborate appreciation in Kekulé's honor, celebrating the twenty-fifth anniversary of his first benzene paper. Here Kekulé spoke of the creation of the theory. He said that he had discovered the ring shape of the benzene molecule after having a reverie or day-dream of a snake seizing its own tail (this is an ancient symbol known as the ouroboros
The ouroboros or uroboros (; ) is an ancient symbol depicting a serpent symbolism, snake or European dragon, dragon Autocannibalism, eating its own tail. The ouroboros entered Western tradition via Egyptian mythology, ancient Egyptian iconogra ...
).
Another depiction of benzene had appeared in 1886 in the ''Berichte der Durstigen Chemischen Gesellschaft'' (Journal of the Thirsty Chemical Society), a parody of the ''Berichte der Deutschen Chemischen Gesellschaft'', only the parody had six monkeys seizing each other in a circle, rather than a single snake as in Kekulé's anecdote. Some historians have suggested that the parody was a lampoon of the snake anecdote, possibly already well-known through oral transmission even if it had not yet appeared in print. Others have speculated that Kekulé's story in 1890 was a re-parody of the monkey spoof, and was a mere invention rather than a recollection of an event in his life.
Kekulé's 1890 speech, in which these anecdotes appeared, has been translated into English. If one takes the anecdote as reflecting an accurate memory of a real event, circumstances mentioned in the story suggest that it must have happened early in 1862.
He told another autobiographical anecdote in the same 1890 speech, of an earlier vision of dancing atoms and molecules that led to his theory of structure, published in May 1858. This happened, he claimed, while he was riding on the upper deck of a horse-drawn omnibus in London. Once again, if one takes the anecdote as reflecting an accurate memory of a real event, circumstances related in the anecdote suggest that it must have occurred in the late summer of 1855.
Works
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Honors
After his ennoblement in 1895, Kekulé’s name became Friedrich August Kekule von Stradonitz, and this is the form that some libraries use. This title was inherited by his son, genealogist Stephan Kekule von Stradonitz. Of the first five Nobel Prizes in Chemistry, Kekulé's former students won three: van 't Hoff in 1901, Fischer in 1902 and Baeyer
Johann Friedrich Wilhelm Adolf von Baeyer (; 31 October 1835 – 20 August 1917) was a German chemist who synthesised indigo and developed a nomenclature for cyclic compounds (that was subsequently extended and adopted as part of the IUPAC org ...
in 1905.
A larger-than-life monument of Kekulé, unveiled in 1903, is situated in front of the former Chemical Institute (completed 1868) at the University of Bonn
The University of Bonn, officially the Rhenish Friedrich Wilhelm University of Bonn (), is a public research university in Bonn, North Rhine-Westphalia, Germany. It was founded in its present form as the () on 18 October 1818 by Frederick Willi ...
. His statue is often humorously decorated by students, e.g. for Valentine's Day
Valentine's Day, also called Saint Valentine's Day or the Feast of Saint Valentine, is celebrated annually on February 14. It originated as a Christian feast day honoring a Christian martyrs, martyr named Saint Valentine, Valentine, and ...
or Halloween
Halloween, or Hallowe'en (less commonly known as Allhalloween, All Hallows' Eve, or All Saints' Eve), is a celebration geography of Halloween, observed in many countries on 31 October, the eve of the Western Christianity, Western Christian f ...
.
See also
* Non-Kekulé molecule
*Skeletal formula
The skeletal formula, line-angle formula, bond-line formula or shorthand formula of an organic compound is a type of minimalist structural formula representing a molecule's Atom, atoms, structural isomer, bonds and some details of its molecular ...
* Kekulé Program
* Auguste Laurent
References
Further reading
* Benfey, O. Theodor. "August Kekule and the Birth of the Structural Theory of Organic Chemistry in 1858." ''Journal of Chemical Education
The ''Journal of Chemical Education'' is a monthly peer-reviewed academic journal available in both print and electronic versions. It is published by the Division of Chemical Education of the American Chemical Society
The American Chemical S ...
''. Volume 35, No. 1, January 1958. p. 21–23. – Includes an English translation of Kekule's 1890 speech in which he spoke about his development of structure theory and benzene theory.
* Rocke, A. J., ''Image and Reality: Kekule, Kopp, and the Scientific Imagination'' (University of Chicago Press, 2010).
External links
Kekulés Traum
(Kekulé's dream, in German)
*
{{DEFAULTSORT:Kekule, August
1829 births
1896 deaths
19th-century German chemists
Scientists from Darmstadt
People from the Grand Duchy of Hesse
German organic chemists
German untitled nobility
University of Giessen alumni
Academic staff of the University of Bonn
Recipients of the Copley Medal
Foreign members of the Royal Society
Foreign associates of the National Academy of Sciences
Corresponding members of the Saint Petersburg Academy of Sciences
Recipients of the Pour le Mérite (civil class)