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by Dr. J. H. van't Hoff
translated into German by Dr. F Herrmann (1877)
Jacobus Henricus van't Hoff published the first 12-page version of this epochal pamphlet in Dutch when he was barely 22. The original title was "Voorstel tot Uitbreiding der Tegenwoordige in de Scheikunde gebruikte Structuurformules in de Ruimte, benevens een daarmee samenhangende Opmerking omtrent het Verband tusschen Optisch Actief Vermogen en chemische Constitutie van Organische Verbindingen" (Proposal for the Extension of Current Chemical Structural Formulas into Space, together with Related Observation on the Connection between Optically Active Power and the Chemical Constitution of Organic Compounds), a descriptive but lengthy title for such a short pamphlet. It was translated into French in 1875 as "La chimie dans l'espace" (Chemistry in Space), but this 1877 German translation "Die Lagerung der Atome im Raume" was the most influential and controversial.
Van't Hoff had studied with Kekul� in Bonn for several terms in 1872-73, so there is no doubt that he was familiar with Kekul�'s ball-and-stick lecture models of tetrahedral carbon.
He went on to found the discipline of physical chemistry and received the first Nobel Prize in Chemistry (1901). It recognized his contributions in physical chemistry, not in organic stereochemistry. To see the romantic self-image that he tried to project, click for his portrait(and a biography from the Nobel Institute).
Most of van't Hoff's contributions were theoretical, rather than experimental. In his obituary of van't Hoff, Bancroft wrote in 1911:In his whole life he never made what would be called a very accurate measurement, and he never cared to. I remember his saying to me eighteen years ago, "How fortunate it is that there are people who will do that sort of work for us!"
As befits a publication with this title, the 53-page German booklet was richly illustrated with 63 figures and many other formulae. This radical departure from precedent attracted lots of criticism. Do click here to samplecriticism from Kolbe and others.
I. For drawings one could consider the carbon atom in the center of the tetrahedron with labeled or colored vertices denoting the bonded atoms or groups. Thus an ethane molecule with 6 substituents and free rotation about the central C-C bond could be drawn as follows:
Note that van't Hoff, like Patern�, draws eclipsed rather than staggered conformations. You can't get everything right the first time.
Double bonds could be shown by tetrahedra sharing two vertices. Here are his drawings of the two diacids (fumaric and maleic acids, known at the time) from replacing one H on each carbon of ethylene with COOH. Now we call these two isomers trans and cis, or E and Z. "r" in the figures means the vertex should be colored red.
Triple bonds would share three vertices:
Perhaps the most amazing prediction, which would not be confirmed experimentally for another 60 years, was the following pair of stereoisomers. Think about these structures and whether they are diastereomers or enantiomers.
II. Alternatively one could make physical models by cutting and folding pieces of cardboard into tetrahedra with the faces labeled and/or colored to denote the attached groups. At the end of his pamphlet he gave the following pattern for a model that would represent substituted ethane as in the first drawing above. The letters show what color to use on the faces: w, r, b, s for weiss (white), rot (red), blau (blue), and schwartz (black).
Substitued ethane models of this type appear in the middle of a set made by van't Hoff himself in 1875 for his fellow student G. J. W. Bremer. (The set is preserved in the Leiden history of science museum and used with permission.)
While van't Hoff did no experimental work himself to support his stereochemical theory, he cited many examples of previously confusing observations that were in agreement with predictions of his theory. For example on page 37-38, he invokes tartaric acid as follows:
In the case of a symmetrical formula with two asymmetrical carbon atoms there are only three isomers, for which the rotatory power would be:
1) A + A = 2A, 2) A - A = 0, 3) -A -A = -2A
In the second case optical rotation disappears completely, although it involves asymmetric carbon atoms. We would like to name as theneutral case the situation where rotatory power due to one or several asymmetrical carbon atoms is cancelled by the equal and opposite rotatory power of another one or several asymmetrical carbon atoms, a case that would only occur when the formula is symmetrical.
Tartaric acid offers a perfect example for this statement. The formula for this substance is symmetrical and includes two asymmetric carbon atoms:
(CO2H) CH(OH) . CH(OH) (CO2H).
In fact three isomers are known for this formula:right- and left-handed tartaric acid with equal and opposite rotatory power, and optically inactive tartaric acid.
The optically inactive tartaric acid had already been named meso tartaric acid, and we have now generalized by using the name meso to designate all compounds in van't Hoff's neutralcase. The name "meso" means "between" (for example Mesopotamia is the land between two rivers, Tigris and Euphrates). The optical rotation of meso tartaric acid is certainly between those of the d- and l-tartaric acids. Note that rotatory power refers to the ability to rotate the plane of polarized light, and that van't Hoff indicates asymmetric carbons with italics.