Chapter 20, Problem 20.75CP

Interpretation Introduction

Interpretation:

Whether the $\text{α}\text{and}\text{β}$ forms of disaccharide lactose are enantiomers of each other has to be explained.

Concept introduction:

Enantiomers are the compounds that are non-superimposable mirror images of each other.

Haworth projection is used to represent sugars in their cyclic forms.

In Haworth projection,

$\text{α}$-indicates the $–OH$ group of anomeric carbon $C1$ and ${\text{CH}}_{\text{2}}\text{OH}$ substituent on the $C5$ carbon is trans to each other. $\beta$-indicates the $–OH$ group of anomeric carbon $C2$ and ${\text{CH}}_{\text{2}}\text{OH}$ substituent on the $C5$ carbon is cis to each other.

Lactose is a disaccharide made up of a galactose molecule attached to glucose by $\text{β-1,4 glycosidic linkage}$. Therefore, the anomeric carbon of glucose molecule is attached by a $–OH$ group. The $\text{α}\text{and}\text{β}$ forms of lactose are on the basis of $\text{α}\text{and}\text{β}$ form of glucose present in it.