Editing August Kekulé (section)

==Benzene==
[[File:Historic Benzene Formulae Kekulé (original).png|thumb|Kekulé structure of benzene with alternating double bonds]]
Kekulé's most famous work was on the structure of [[benzene]].<ref name=EB1911/> 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.<ref>{{cite journal|title=Sur la constitution des substances aromatiques|author=Aug. Kekulé|journal=Bulletin de la Société Chimique de Paris|year=1865|volume=3|issue=2|pages=98–110}}</ref> The following year he published a much longer paper in German on the same subject.<ref>{{cite journal|title=Untersuchungen uber aromatische Verbindungen|author=Aug. Kekulé|journal=[[Annalen der Chemie und Pharmacie]]|year=1866|volume=137|issue=2|pages=129–196|doi=10.1002/jlac.18661370202|url=https://zenodo.org/record/1427249}}</ref>

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]]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]]s. Kekulé argued for his proposed structure by considering the number of isomers observed for derivatives of benzene. For every monoderivative of benzene (C<sub>6</sub>H<sub>5</sub>X, where X = Cl, OH, CH<sub>3</sub>, NH<sub>2</sub>, 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]]s, C<sub>6</sub>H<sub>4</sub>(NH<sub>2</sub>)(CH<sub>3</sub>), 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 [[Arene substitution pattern|ortho, meta, and para]] isomers respectively.<ref>{{Cite web|url=http://www.worldofchemicals.com/488/chemistry-articles/friedrich-august-kekule-von-stradonitz-principal-founder-of-chemical-structure.html|title=Friedrich August Kekule von Stradonitz –inventor of benzene structure – World Of Chemicals|website=worldofchemicals.com|date=28 May 2015|access-date=2 March 2018|archive-date=10 November 2022|archive-url=https://web.archive.org/web/20221110172704/https://www.worldofchemicals.com/488/chemistry-articles/friedrich-august-kekule-von-stradonitz-principal-founder-of-chemical-structure.html|url-status=dead}}</ref>

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.<ref>Ladenburg, Albert (1869) [https://books.google.com/books?id=Epg8AAAAIAAJ&pg=PA140 "Bemerkungen zur aromatischen Theorie"] (Observations on the aromatic theory), ''Berichte der Deutschen Chemischen Gesellschaft'', '''2''' : 140–142.</ref> 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.<ref>{{cite journal | last=Kekulé | first=August | title=Ueber einige Condensationsproducte des Aldehyds (On some condensation products of aldehydes)| journal=Annalen der Chemie und Pharmacie | publisher=Wiley | volume=162 | issue=2–3 | year=1872 | issn=0075-4617 | doi=10.1002/jlac.18721620211 | pages=309–320 | url=https://zenodo.org/record/1427307 | language=de}}</ref><ref>{{cite journal |author=Pierre Laszlo |url=http://www.hyle.org/journal/issues/10-1/rev_laszlo.htm |publisher=Hyle |journal=International Journal for Philosophy of Chemistry |volume=10 |issue=1 |date=April 2004 |issn=1433-5158 |title=Book Review: Jerome A. Berson: Chemical Discovery and the Logicians' Program. A Problematic Pairing, Wiley-VCH, Weinheim, 2003|access-date=6 July 2013}}</ref> 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]], who replaced Kekulé's oscillation by the concept of [[Resonance (chemistry)|resonance]] between quantum-mechanical structures.<ref>{{cite journal | last=Pauling| first=Linus | author-link=Linus Pauling| title=The Shared-Electron Chemical Bond | journal=Proceedings of the National Academy of Sciences | date=1 April 1928 | url=https://www.pnas.org/content/pnas/14/4/359.full.pdf |archive-url=https://web.archive.org/web/20200311040447/https://www.pnas.org/content/pnas/14/4/359.full.pdf |archive-date=2020-03-11 |url-status=live | access-date=29 November 2020 |volume=14 | issue=4 | pages=359–362| doi=10.1073/pnas.14.4.359| pmid=16587350 | pmc=1085493 | bibcode=1928PNAS...14..359P | doi-access=free }}</ref>

=== Kekulé's dream ===
[[File:Ouroboros-benzene.svg|thumb|200px|Kekulé's benzene ring in modern form, and the alchemical ouroboros symbol of a snake eating its tail]]
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]]).<ref>{{cite book | isbn = 978-0-486-28690-7 | title = From Alchemy to Chemistry | last1 = Read | first1 = John | year = 1957 | url = https://books.google.com/books?id=F6J-AUOWzpMC | pages = 179–180| publisher = Courier Corporation }}</ref>

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.<ref>Translated into English by D. Wilcox and F. Greenbaum, ''Journal of Chemical Education'', 42 (1965), 266–67.</ref> 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.<ref>{{cite journal|title=Hypothesis and Experiment in Kekulé's Benzene Theory|author=A.J. Rocke|journal=Annals of Science|year=1985|volume=42|issue=4|pages=355–81|doi=10.1080/00033798500200411}}</ref> 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,<ref>{{cite journal|title=Benzolfest: Rede|author=Aug. Kekulé|journal=[[Berichte der Deutschen Chemischen Gesellschaft]]|year=1890|volume=23|issue=1|pages=1302–11|doi=10.1002/cber.189002301204|url=https://zenodo.org/record/1425576}}</ref> in which these anecdotes appeared, has been translated into English.<ref>{{cite journal |author=O. T. Benfey|title=August Kekulé and the Birth of the Structural Theory of Organic Chemistry in 1858|journal=Journal of Chemical Education|volume=35|issue=1|year=1958|pages=21–23|doi=10.1021/ed035p21|bibcode=1958JChEd..35...21B}}</ref> 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.<ref>{{cite journal|author=Jean Gillis|title=Auguste Kekulé et son oeuvre, realisee a Gand de 1858 a 1867|journal=Mémoires de l'Académie Royale de Belgique|volume=37|issue=1|year=1966|pages=1–40}}</ref>

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.<ref>{{cite book |author=Alan J. Rocke|title=Image and Reality: Kekulé, Kopp, and the Scientific Imagination|publisher=University of Chicago Press|year=2010|pages=60–66|isbn=978-0-226-72332-7}}</ref>