The Structural Theory of Organic Chemistry, 1858-1874

Tuesday, October 5, 2010 August Kekulé (1829-1896)

Tuesday, October 5, 2010 Tuesday, October 5, 2010 Tuesday, October 5, 2010 I regard it as necessary and in the present state of chemical knowledge as in many cases possible to explain the properties of chemical compounds by going back to the elements themselves which compose these compounds. I no longer regard it as the chief problem of the time, to prove the presence of atomic groups which, on the strength of certain properties, may be regarded as radicals, and in this way to refer compounds to a few types which can hardly have any signiﬁcance beyond that of mere pattern formulas. On the contrary I hold that we must extend our investigation to the constitution of the radicals themselves; that we must ascertain the relation of the radicals to one another and, from the nature of the elements, deduce both the nature of the radicals and that of the compounds.

“On the Constitution and Metamorphosis of Chemical Compounds and on the Chemical Nature of Carbon,” 1858

Tuesday, October 5, 2010 Principles of Kekulé’s Carbon Theory

• Carbon is tetravalent. • Carbon atoms can form bonds to other carbon atoms. • Valence is constant for atoms of each element. • Carbon atoms can form “multiple” bonds with other atoms, when the number of atoms is insufﬁcient to “saturate” all the valences of carbon.

• Each atom in a molecule is considered independently (not hierarchical) • Putting together the atoms in a molecular formula following the rules of valence can explain chemical properties (arrangement) and isomerism (different connections between atoms) of known compounds. • Conversely, any connection between atoms that follows the rules of valence indicates the existence of a potential compound, whether chemists have found it or not.

Tuesday, October 5, 2010 Aleksandr Butlerov (1828-1886)

Tuesday, October 5, 2010 Starting from the assumption that each chemical atom possesses only a deﬁnite and limited amount of chemical force (afﬁnity) with which it takes part in forming a compound, I might call this chemical arrangement, or the type and manner of the mutual binding of the atoms in a compound substance, by the name of “chemical structure.” (chemische Struktur)

“Some Notes on the Chemical Structure of Bodies,” 1861

Tuesday, October 5, 2010 Kekulé’s “Sausage Formulas” from his Lehrbuch der Chemie (1860)

Tuesday, October 5, 2010 A “Sausage Formula” and its corresponding model

Tuesday, October 5, 2010 Joseph Loschmidt, 1861

Tuesday, October 5, 2010 Alexander Crum Brown (1838-1922)

Tuesday, October 5, 2010 Alexander Crum Brown’s Formulas, 1861 (left), and 1864

Tuesday, October 5, 2010 Possible Formulas for Maleic and Fumaric Acid

Tuesday, October 5, 2010 Frankland and Duppa, Journal of the Chemical Society,1867

Tuesday, October 5, 2010 H OH O H H O

H C C C OH HO C C C OH

H H H H

Ethylidene lactic acid Ethylene lactic acid (α-hydroxypropionic acid) (β-hydroxypropionic acid)

Tuesday, October 5, 2010 During my residence in Ghent in Belgium, I lived in an elegant bachelor apartment on the main street. However, my study was situated along a narrow alley and had no light during the day. for a chemist who spends the day in the laboratory this was not a disadvantage. There I was sitting, working on my textbook, but it was not going well; my mind was on other matters. I turned my chair toward the fireplace and sank into half sleep [Halbschlaf] Again the atoms fluttered before my eyes, This time smaller groups remained modestly in the background. My mind’s eye, sharpened by repeated visions of a similar kind, now distinguished larger forms in a variety of combinations. Long lines, often fitted together more densely ; everything in motion, twisting and turning like snakes. But look, what was that? One of the snakes had seized its own tail, and the figure whirled mockingly before my eyes. I awoke as by a stroke of lightning, and this time, too, I spent the rest of the night working out the consequences of the hypothesis.

August Kekulé, Benzolfest, 1890

Tuesday, October 5, 2010 Kekulé, Bulletin da la Societé Chimique (1865)

Tuesday, October 5, 2010 Tuesday, October 5, 2010 Wilhelm Koerner’s Assignment of ortho, meta, and para positions (1869)

Tuesday, October 5, 2010 Emil Erlenmeyer’s oxidation of naphthalene to phthalic acid

Tuesday, October 5, 2010 Synthesis of all possible chlorobenzenes (Beilstein, 1878)

Tuesday, October 5, 2010 [I]t seems quite obvious that chemistry, which only deals with bodies in a state of transformation, is powerless to judge this mechanical structure, as long as physical investigations are not brought to bear on this question...To be sure, we do not know what connection exists between the relative chemical effect of the atoms inside a compound molecule and their relative mechanical positions; we do not even know whether, in such a molecule, two atoms which directly affect each other chemically are in fact situated next to one another...

Aleksandr Butlerov, “Some Notes on the Chemical Structure of Bodies,” 1861

Tuesday, October 5, 2010 [These formulas are] used to express constitutional formulae, and by which, it is scarcely necessary to remark, I do not mean to indicate the physical, but merely the chemical position of the atoms. … and while it is no doubt liable, when not explained, to be mistaken for a representation of the physical position of the atoms, this misunderstanding can easily be prevented.

Alexander Crum Brown, “On the Theory of Isomeric Compounds,” 1864

Tuesday, October 5, 2010 Tuesday, October 5, 2010 Further Reading

Alan J. Rocke, Image and Reality: Kekulé, Kopp, and the Scientiﬁc Imagination, Chicago: The University of Chicago Press, 2010.

Tuesday, October 5, 2010