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

1 Answer
Jul 22, 2015

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.

Alpha Beta Glucose
(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.

studydroid.com
(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.