Auwers-Skita rule

What is Auwers-Skita rule?

Auwers-Skita rule, also called conformational rule, is an empirical rule related to the physical properties of geometrical isomers and their configurations. This rule was initially proposed, in 1920, simultaneously by Karl von Auwers and by A. Skita, and was restated by Norman Allinger in 1956. This rule was extensively applied during the assignment of the structures of isomers before the NMR techniques were commonly available.

Auwers-Skita rule
Auwers-Skita rule
  • R = OH      54 ºC
  • R = NH2  -25 ºC
Auwers-Skita rule
Auwers-Skita rule

R = OH      65 ºC
R = NH2  -13 ºC

Auwers-Skita rule
Auwers-Skita rule

cisN-methyl-N-neopentyl-4-methylcyclohexylamine, nD20 = 1.457, b.p. 90–91 ºC (4.5 mmHg).

Auwers-Skita rule
Auwers-Skita rule

transN-methyl-N-neopentyl-4-methylcyclohexylamine, nD20 = 1.452, b.p. 88–89 ºC (4.5 mmHg).

The Auwers-Skita rule, applying to alicyclic epimers not differing in dipole moment, states that the isomer of smaller molecular volume and therefore higher physical constants (e.g., density, index of refraction, and boiling point) has the higher heat content (enthalpy), however, molecules of higher enthalpy have a lower melting point. Between the pair of cis– and trans– isomers, cis-compounds are higher in specific gravity and refractive index but smaller in molecular refraction. Between stereoisomers, the transform of a pair of stereoisomers has the lower density, boiling point, and refractive index.

The Auwers-Skita rule can even be used to assign the conformers, in which the conformer of higher enthalpy has a lower molecular volume. In cyclic stereoisomers, when the substituents are bound to configurationally identical ring systems, the isomer with the higher density and the higher refractive index is that which has the higher heat content. Usually, the (higher) boiling point and (lower) molar refraction can be related in a similar manner. In other words, isomers having the greater number of axial substituents within a series will have the higher boiling point and refractive index and the greater density.

Although the Auwers-Skita rule has been proved successful with a variety of disubstituted cyclohexanes, it is recognized that such an empirical rule is of limited reliability, especially in complex systems, such as the configuration for isopinocampheol, 3-methylcyclohexanol, thiabicyclo[3.3.0]-octanes, and other 1,3-disubstituted cyclohexenes.

References

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