Editing August Kekulé (section)

==Theory of chemical structure==
In 1856, Kekulé became [[Privatdozent]] at the [[University of Heidelberg]]. In 1858, he was hired as full professor at the [[University of Ghent]], then in 1867 he was called to [[University of Bonn|Bonn]], where he remained for the rest of his career. Basing his ideas on those of predecessors such as Williamson, [[Charles Frédéric Gerhardt|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)<ref>{{cite journal|title=Über die s. g. gepaarten Verbindungen und die Theorie der mehratomigen Radicale|author=Aug. Kekulé|journal=[[Annalen der Chemie und Pharmacie]]|year=1857|volume=104|issue=2|pages=129–150|doi= 10.1002/jlac.18571040202|url=https://zenodo.org/record/1427096}}</ref> and the ability of carbon atoms to link to each other (announced in a paper published in May 1858),<ref>{{cite journal|title=Ueber die Constitution und die Metamorphosen der chemischen Verbindungen und über die chemische Natur des Kohlenstoffs|author=Aug. Kekulé|journal=[[Annalen der Chemie und Pharmacie]]|year=1858|volume=106|issue=2|pages=129–159|doi= 10.1002/jlac.18581060202|url=https://zenodo.org/record/1427102}}</ref> 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),<ref>{{cite journal|title=Sur une nouvelle théorie chimique|author=A.S. Couper|journal=[[Annales de chimie et de physique]]|volume=53|year=1858|pages=488–489|url=http://gallica.bnf.fr/ark:/12148/bpt6k34794n/f468.table}}</ref> 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, [[Edward Frankland|Frankland]], [[Charles Adolphe Wurtz|Wurtz]], [[Alexander Crum Brown]], [[Emil Erlenmeyer]], and [[Alexander Butlerov]].<ref>{{cite book |author=Alan J. Rocke|title=Image and Reality: Kekulé, Kopp, and the Scientific Imagination|publisher=University of Chicago Press|year=2010|isbn=978-0-226-72332-7}}</ref>

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 "[[valence (chemistry)|valence]]s" 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]]. 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]], 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.<ref>{{Cite journal |date=1878 |title=Historical Data Concerning the Discovery of the Law of Valence |url=https://www.jstor.org/stable/2369316 |journal=American Journal of Mathematics |volume=1 |issue=3 |pages=282 |doi=10.2307/2369316 |jstor=2369316 |issn=0002-9327}}</ref> This generalization suffered from many exceptions, and was subsequently replaced by the suggestion that valences were fixed at certain [[oxidation state]]s.<ref>{{Cite journal |last=Constable |first=Edwin C. |date=2019-08-22 |title=What's in a Name?—A Short History of Coordination Chemistry from Then to Now |journal=[[Chemistry (MDPI journal)|Chemistry]] |language=en |volume=1 |issue=1 |pages=126–163 |doi=10.3390/chemistry1010010 |doi-access=free |issn=2624-8549 }}</ref> For example, [[periodic acid]] 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.<ref>{{Cite web |last=PubChem |title=Periodic acid |url=https://pubchem.ncbi.nlm.nih.gov/compound/Periodic-acid |access-date=2024-10-16 |website=pubchem.ncbi.nlm.nih.gov |language=en}}</ref>