Q. 13.41 - An Introduction to General, Organic, and Biological Chemistry

Question

For each of the following, give the monosaccharide units produced by hydrolysis, the type of glycosidic bond, and the name of the disaccharide including $\alpha$or $\beta$ :

Step-by-Step Solution

Answer

(Part a) The name of the given disaccharide is $\beta -$ lactose

(Part b) The name of the given disaccharide is $\alpha -$ maltose

Step 1: Given information (Part a)

The given disaccharide is 

Step 2: Explanation for (Part a)

Monosaccharide is a polyhydroxy compound that contains an aldehyde or a ketone group. It is composed of only one unit.

When the $-\mathrm{OH}$ group on the carbon atom 1 is down, then that isomer is called a $\alpha$ isomer, and when the $-\mathrm{OH}$ group on the carbon atom 1 is up, then that isomer is called a $\beta$isomer.

The structure of disaccharide is given below 

Disaccharide on hydrolysis gives two monosaccharide units as follows:

In the given disaccharide structure, the glycosidic bond is formed between the $\mathrm{C}1$ carbon atom of the $\beta$ - galactose molecule and the C4 carbon atom of the glucose molecule. Therefore, the glycosidic link in the given structure is $\beta -1,4$.

The name of given disaccharide is $\beta$-lactose .

Step 3 : Given information (b)

The given disaccharide is 

Step 4 : Explanation for (b)

The structure of disaccharide is given below 

Disaccharide on hydrolysis gives two monosaccharide units as follows:

In the given disaccharide structure, a glycosidic bond is formed between the C1 carbon atom of a $\alpha$-glucose molecule and the C4 carbon atom of another $\alpha$-glucose molecule. Therefore, the glycosidic link in the given structure is $\alpha -1,4$.

The name of the given disaccharide is $\alpha$-maltose.