stability of the chair conformations of d
TRANSCRIPT
Stability of the chair conformations of D-glucose and D-mannose:
For D-glucopyranose, the β-anomer is more stable. The main effect in this case is the absence of 1,3-diaxial interactions between the anomeric hydroxyl group (shown in blue) and C3- and C5-hydrogen atoms (shown in green).
But in case of D-mannopyranose, the α-anomer is more stable because this form avoids dipolar repulsion between the anomeric hydroxyl group (shown in blue) and the hydroxyl group on the next carbon (C2, shown in green) in the ring. Here, the dipolar repulsion is more prominent than the Me-H 1,3-diaxial interactions. So, the equilibrium shifts towards the α-anomer.
For these reasons, the aqueous solution of D-glucose contains the α- and β-form in a ratio of 36:64, whereas the aqueous solution of D-mannose contains the α- and β-form in a ratio of 69:31. This anomaly is due to the anomeric effect (sometimes known as Edward-Lemieux effect).
If we replace the –OH group with other groups, then the effect will be prominent in the equilibrium mixture.
The substituent on C2 can influence the anomeric effect. When this is equatorial, as in glucose and galactose, the anomeric effect is weakened, and is enhanced in the case of an axial C–2–substituent as in mannose.
The most widely accepted reason behind the anomeric effect can be described by the MO diagram. It is a stereoelectronic effect, in which a lone pair of electrons located in a non-bonded molecular orbital of the oxygen atom overlaps with the antibonding orbital of the C-O bond. This favorable nX* delocalization of non-bonding electrons (sometimes called as double-no bond resonance or negative hyperconjugation) is only possible with an anti-periplanar arrangement of the involved orbitals as found in the axial anomer.
It is to be kept in mind that anomeric effect occurs due to a combination of factors like stereoelectronic effects, dipole stabilization effect, electrostatic repulsion, ALPH model etc. So, one cannot quantify it in each and every case. It is overall a qualitative effect.