What does alpha acetal linkage mean?

An alpha acetal linkage is a glycoside linkage that points in the opposite direction to the ##"CH"_2"OH"## group.

Let's begin by defining terms.

An acetal is a functional group formed when an aldehyde reacts with an alcohol to form a functional group with two ether groups on the same carbon.

The term "alpha acetal" usually refers to carbohydrates and their derivatives.

In the cyclic form of a carbohydrate, the anomeric carbon is the carbon that was the carbonyl carbon in the acyclic form.

Notice that the ##"OH"## on the anomeric carbon can be "up" or "down" (more on this later).

A glycosidic linkage is the bond that forms when the hemiacetal ##"OH"## of the anomeric carbon reacts with another alcohol group to form an acetal.

For example,

The ##"OH"## group could come from another carbohydrate, For example,

If the glycoside linkage points "down" (i.e., in a direction opposite to the ##"CH"_2"OH"## group), it is called an alpha acetal or an alpha glycoside linkage.