Anomeric Effect

Objective

The objective of this exercise is to determine the origin of the anomeric effect.

Procedure

Substituents on a six-membered ring typically adopt the equatorial position to minimize steric interactions with the ring. In some instances, however, electronic effects result in a preference for the axial position, which is known as the anomeric effect. Polysaccharide provide common examples of anomeric linkages, in which axial oxygens frequently form the glycosidic bonds which join sugar rings.

In this exercise, you will search for an anomeric effect in 2-chlorotetrahydropyran.

A. Use the WebMO editor to build 2-chlorotetrahydropyran, with the chlorine atom in the axial position. Use PM3 to optimize the geometry of the resulting conformation, and measure its final energy. Using the optimized geometry, construct the equatorial conformation by simply changing two dihedral angles by 180°. Use PM3 to measure the final energy of the second conformation, and report the heats of formation of each conformation in the table below.

B. Repeat the calculations for chlorocyclohexane.

C. Calculate D_fG_eq-D_fG_ax for each compound and compare your results in a table with the experimental results.

Compound D_fG_eq
(kJ/mol) D_fG_ax
(kJ/mol) Calculated
D_fG_eq-D_fG_eq
(kJ/mol) Experimental
D_fG_eq-D_fG_eq
(kJ/mol)

2-chlorotetrahydropyran 7.5

2-chlorotetrahydropyran -2.1

For which compound does an anomeric effect exist? By comparing the structural differences between the two compounds, what factors likely cause the anomeric effect?

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