Question

How do you identify `alpha` and `beta` forms in sugars?

Answer 1

You look at the position of the `"OH"` (or `"OR"`) group at the anomeric carbon.

Explanation:

The cyclic forms of carbohydrates can exist in α and β forms, based on the position of the substituent at the anomeric carbon.

To assign the α or β configuration to the cyclic form of a carbohydrate, look at the relative positions of the `"CH"_2"OH"` group and the `"OH"` (or `"OR"`) group at the anomeric carbon.

(from chemistry2.csudh.edu)

If you consider the 6-membered ring to be flat, the exocyclic `"O"` atom at the anomeric carbon is on the "bottom" side of the ring, and the `"CH"_2"OH"` group is on the "top" side of the ring.

The `"CH"_2"OH"` group and the anomeric #"OH" group are on opposite sides of the ring.

In the β- form, the exocyclic `"O"` atom at the anomeric carbon is on the same side of the ring as the `"CH"_2"OH"` group.

Lactose is a disaccharide that consists of galactose and glucose units.

(from www.studydroid.com)

Its systematic name is β-D-galactopyranosyl-(1→4)-D-glucose.

That tells you that the `"O"` atom on `"C-1"` of the galactose is β (on the same side as the `"CH"_2"OH"` group) and joined in a glycoside linkage to `"C-4 "`of the glucose unit..

If the glucose unit has the anomeric `"OH"` group in the α position, we have α-lactose.

If the glucose unit has the anomeric `"OH"` group in the β position, we have β-lactose.