Sache mohr theory

sahemohr theory-strainless theory

1. Sache –Mohr Theory

2. Sachse -Mohr theory The  Sachse -Mohr theory  transfers the  tetrahedron model of the carbon atom of Jacobus Henricus van 't Hoff to cyclic systems.  In 1890, Hermann Sachse hypothesized that in the  cyclohexane , which was previously considered a planar hexagon, all six carbon atoms have tetrahedral symmetry and are arranged to each other without tension.

3. Sachse -Mohr theory Maintaining the normal valence angles (109.5°), Sachse was able to construct two room models for Cyclohexane, the armchair shape and the boat shape (= armchair and boat conformation ). The assumption of Sachse was contrary to Baeyer's theory of tension and was initially rejected in professional circles, because at that time it was not possible to separate the two more conformists, so the theory could not be verified experimentally.

4. 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 .

5. Decalin isomer

6. Sachse -Mohr theory Ernst Mohr (1873-1926) took up the hypothesis of Sachse in 1918 and applied it to  Decalin .   He assumed the existence of two isomers, the  cis - and the  trans - decalin , both of which could be isolated by the German chemist Walter Hückel  (1895-1973) in 1925.  It is now generally accepted that the Sachse -Mohr theory is correct.

7. Heat Combustion The quantity of heat evolved when one mole of compound is burned to CO2 and water. Heat of combustion support of Baeyer’s proposal that rings smaller or larger than cyclopentane and cyclohexane are unstable.

8. Chemical stability is measured in various ways The heat of formation Heat of combustion Dipole moment Absorption spectra One of the most convenient to work with in respect to hydrocarbons is the heat of combustion. If the strain in a ring changes with size of the ring, then this should be observed by changes in the heat of combustion

9. From combustion table stability increases up to the five and six membered rings and then remains effectively constant. This can be explained by rejecting the postulate that all rings are planar and by assuming that rings with six or more carbon are puckered. The normal valency angle being retained and thereby producing strainless rings.

10. The two forms of cyclohexane are Chair and boat form. The arrangement is being produced by twisting or rotation of bonds. The terms rotational isomer and constellation have also been used in the same sense as conformation.

11. Cyclohexane

12. Conformation of cyclohexane Cyclohexane ring can assume many shapes. Chair form has lowest energy while the half chair has the highest energy.

13. Cyclohexane

14. Cyclohexane

15. In order to account for the stability of cyclo hexane and higher members. Sache and Mohr 1918 proposed that such rings can become free from strain if all the ring carbons are not forced into one plane assumed by B aeyer. If the ring assumed a folder or puckered condition, the normal tetrahedral angles of 109 28 are retained and as a result the strain within the ring is relieved.

16. It exist in two non planar puckered conformations both of which are completley free from strain [ Chair form and Boat form]. Non planar strain free rings in which carbon have normal tetra hedral angle. Chair form of cyclohexane is more stable than boat form . In ordinary condition cyclohexane exist in the chair form.

17. Examination of the chair form of cyclohexane reveals that the hydrogen atoms can be divided into two categories. Six bonds to hydrogen atoms point straight up or down almost perpendicular to the plane of molecule is axial hydrogen. Six hydrogens lie slightly above or slightly below the plane of cyclohexane ring i.e equatorial hydrogen.

18. Boat form Chair form

19. Equatorial Axial

20. CycloHexane

21. The conformation arise due to rotation of C-C bond. The chair and boat form is two extreme case

22. Hydrogen lies in the rough plane of ring is equatorial

23. Hydrogen lies parallel to the axis is axial hydrogen

24. Six axial and six equatorial in cyclohexane. In the flipping and reflipping between conformation axial becomes equatorial while equatorial become axial. Bulkier the group the greater is the energy difference between axial and equatorial conformers. Cyclohexane ring with bulky substituent is more likely to have that group in the equatorial position. When the size of the substituent reaches tertiary butyl, the difference in energies between the conformers become quite large.

25. Cis -Trans Isomerism

26. Orbital picture of angle strain A bond to form two atoms must be located so that an orbital of one overlaps an orbital of the other. For a given pair of atoms , the greater the overlap of atomic orbitals, the stronger the bond. When carbon is bonded to four other atoms, its bonding orbitals ( sp3 orbitals) are directed to the corners of a tetrahedron; the angle between any pair of orbitals is thus 109.5. Formation of a bond with another carbon atom involves overlap of one of these sp3 orbitals with a similar sp3 orbital of the other carbon atom.

27. In cyclopropane the CCC bond angle cannot be 109.5, but instead must be 60. As a result, the carbon atoms cannot be located to permit their sp3 orbitals to point toward each other. There is less overlap and the bond is weaker than the usual carbon-carbon bond.

28. The decrease in stability of a cyclic compound attributed to angle strain is due to poor overlap of atomic orbitals in the formation of the carbon-carbon bonds .

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