Draw the structure that completes the mutarotation reaction between the two cyclic forms of (a) galactose and (b) fructose.
b.

Mutarotation is the change in optical rotation that occurs when an anomeric carbon in a sugar ring opens and closes, interconverting between its alpha and beta forms. This process is significant in carbohydrates, as it affects the sugar's reactivity and properties. For example, in the case of galactose and fructose, the mutarotation involves the conversion of their cyclic forms into their respective open-chain forms and back to cyclic forms.

Cyclic forms of sugars, also known as hemiacetals or hemiketals, are the structures that sugars adopt when they cyclize, typically involving the reaction of a carbonyl group with a hydroxyl group. For instance, galactose can exist in both alpha and beta forms, depending on the orientation of the hydroxyl group at the anomeric carbon. Understanding these structures is crucial for predicting the outcomes of reactions like mutarotation.

The anomeric carbon is the carbon atom in a sugar that was originally part of the carbonyl group (aldehyde or ketone) and becomes a new chiral center upon cyclization. It plays a pivotal role in determining the alpha or beta configuration of the sugar. In the context of mutarotation, the anomeric carbon's configuration influences the sugar's reactivity and interactions with other molecules, making it essential for understanding the reaction mechanisms.