Sachse's Strainless Rings (1890-93)

Conformational Isomers

Baeyer suggested that both large and small polymethylene rings should be strained, but Hermann Sachse, an obscure 28-year-old assistant in Berlin, soon pointed out that large rings need not be strained, because the carbons need not be coplanar. Unfortunately he was not good at expressing this simple idea in terms that organic chemists could easily understand, and he was far from having Baeyer's clout.

In Berichte, 23, 1363-1370 (1890) he gave the following patterns, and directed the reader to cut them out, fold them up, and paste a van't Hoff tetrahedron on each of the darkened triangles. If one does this, the first gives a clear model of the cyclohexane chair and the second, the boat. (Note: Sachse called these isomers "symmetrical" and "unsymmetrical", respectively. The silly but convenient names "chair" and "boat" came later.)

Sachse 1890 Chair Template
(Fold back along diagonal lines and tape A-B to G-H to make six faces of an open ended octahedron. Realized in blue below.)

Sachse's Boat Template
(Fold each back along diagonals and tape edges to make six faces of an open ended octahedron, then join the octahedra
through their open faces with a against a, b against b, and c against c to make an hourglass. Realized in blue below.)

Sachse had it all worked out. He saw the axial and equatorial positions for substituents. He saw how two chairs could interconvert with a small barrier. He saw how certain substituents might favor one of the chairs

Apparently Sachse didn't have enough clout to make anyone follow his instructions. Thus Julius Wagner, who was supposed to prepare a summary of Sachse's paper for the Chemisches Central-Blatt (1890), wrote dismissively:

The author thinks he can clarify the easy rearrangement of maleinoid hexahydromellitic acid into the fumaroid without formation of the other possible forms by hypotheses about the configuration [we would now say conformation] of the hexamethylene ring. It is not possible to write an abstract of this paper, especially since the author's explanations are hardly understandable without models.

Sachse was apparently beneath Baeyer's notice, although Baeyer "answered" his criticism indirectly and perhaps condescendingly in Liebig's Annalen, 258, 145 (1890):

4) The Configuration of the Hexamethylene Ring

A further proposal is that the atoms in hexamethylene are arranged as in Kekulé's model, that is that the arrangement of the atoms in space is the one with a minimum distortion of the valence directions. Thus the 6 carbon atoms must lie in one plane and 6 hydrogen atoms lie in each of two equidistant parallel planes. Further each of the 12 hydrogen atoms must have the same position relative to the other 17 atoms. The experimental test of the correctness of this assumption is relatively easy, for example sufficient evidence is that there is a single isomer of hexahydrobenzoic acid [i.e. cyclohexane carboxylic acid gives no isomers]. Meanwhile, as long as our knowledge in this field is so incomplete, we must be satisfied that the above assumption is the most likely, and no known fact contradicts it.

Sachse knew he was right and would not give up. In 1892 he supported his point of view with a 41-page trigonometric paper [Zeitschrift für physicalische chemie, 10, 201-241 (1892)]. Page 216 is not atypical.

You can guess how far this got with an audience of organic chemists!

He tried again in 1893 using geometry as well as trigonometry [Z. phys. chem. 11, 185-219 (1893)]. The following is representative of its 34 pages:

Again, good luck with an audience of organic chemists!

Sachse died at the age of 31 in March 1893, before this last paper was printed. His efforts had failed not because he was wrong, but at least in part because he did not express himself clearly to the audience who would have been interested in this perceptive contribution.

In his "Reminiscences" in 1905 Baeyer wrote:

Sachse... disagreed with my opinion that larger rings are planar. He is certainly right from a mathematical point of view; yet in reality, strangely enough, my theory appears to be correct. The reason is not clear...
(translated by R. Huisgen)

Only in 1918, 25 years too late for Sachse, would his theory be revived and proven correct by Ernst Mohr [J. Prakt. Chem.[2] 98, 315 (1918)] on the basis of the x-ray structure of diamond, which had been determined by Bragg and Bragg in 1913 using X-ray diffraction.

Using the figures below, drawn so clearly that no one could misunderstand them, Mohr showed cyclohexane within the structure of diamond.

In Figure 1, Mohr showed Sachse's chair form cyclohexne abstracted from the diamond lattice (below) and by extension imagined Sachse's boat form (above):

In diamond Mohr also saw the fused chair cyclohexane structure of decalin:

Surprisingly, full recognition of Sachse's premonitions would come only after half a century, with a 1950 publication by Britain's D. H. R. Barton, whose conformational diagrams were much less accurate than Mohr's from 32 years earlier, or Sachse's of 1890. Since 1948 Conformational Analysis has played a central role in the theory of organic chemistry.

Know your audience, and express yourself in terms they can understand.


For a fuller discussion of this topic see Colin A. Russell "The Origins of Conformational Analysis" in O. B. Ramsay, ed., "van't Hoff-Le Bel Centennial", ACS Symposium Series 12, Am. Chem. Soc., Washington, D.C., 1975, pp. 159-178.

Thanks to Matt Weinschenk for constructing the Sachse models.
copyright 2001 J.M.McBride