Sachse-Mohr theory

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The Sachse-Mohr theory transfers the tetrahedron model of the carbon atom by Jacobus Henricus van 't Hoff to cyclic systems.

Chair conformation (left) and boat conformation ( right) of cyclohexane [empirical formula: C 6 H 12 ]. At the ends of the exocyclic bonds shown are a total of twelve hydrogen atoms, which are not shown for the sake of clarity.

1890 presented Hermann axis of the hypothesis to that in cyclohexane , which was previously regarded as a planar hexagon, all having six carbon atoms in tetrahedral symmetry and are arranged without tension to each other. While maintaining the normal valence angle (109.5 °), Sachse was able to construct two space models for cyclohexane, the chair and the boat shape (= chair and boat conformation). Sachse's assumption contradicted Baeyer's stress theory and was initially rejected in specialist circles because it was not possible at that time to separate the two conformers, i.e. the theory could not be verified experimentally.

Both conformers of the decalin .

Ernst Mohr (1873–1926) took up the Sachse hypothesis in 1918 and applied it to decalin . In doing so, he assumed the existence of two isomers - which are not possible with a flat structure - the cis - and the trans -decalin, both of which could be isolated in 1925 by the German chemist Walter Hückel (1895–1973).

It is now generally accepted that the Sachse-Mohr theory is correct. The six equatorial and six axial hydrogen atoms (ratio 1: 1) of the chair conformation of cyclohexane can be detected experimentally by 1 H-NMR spectroscopy by recording the spectrum at low temperature. This detection does not work at a higher temperature (e.g. 20 ° C ), as the chair conformations then transform into one another faster than the NMR spectrometer measures.

Individual evidence

  1. ↑ Wolfgang Bähr, Hans Theobald: Organic Stereochemistry , Springer Verlag, 1973, pp. 99-100, ISBN 3-540-06339-0 .
  2. ↑ H. Sachse: About the geometric isomers of hexamethylene derivatives , Ber. German chem. Ges. 23 , 1363 ( 1890 ).
  3. ↑ Ernst Mohr, J. Prakt. Chem. 98 , 315 ( 1918 ).
  4. ↑ Walter Hückel, Liebigs Ann. Chem. 441 , 1 ( 1925 ).