Question

a. How many stereogenic centers are present in -D-galactose?

b. Label the hemiacetal carbon in -D-galactose.

c. Draw the structure of -D-galactose.

d. Draw the structure of the polyhydroxy aldehyde that cyclizes to - and

-D-galactose.

e. From what you learned in Section 21.16B, what product (s) is (are) formed

when -D-galactose is treated with CH₃OH and an acid catalyst?

Short answer

There are five stereogenic centers in -D-galactose.

a.

b.

c.

d.

e.

Step-by-step solution

Carbohydrates

Carbohydrates fall under the category of biomolecules and have a significant role in controlling the function and structure of living cells. Cyclic acetals and hemiacetals are a part of several carbohydrates.

D-galactose

Galactose belongs to the category of aldohexoses. Galactose molecules can be connected with glucose leading to the formation of lactose.

Stereogenic centres in -D-galactose and the polyhydroxy aldehyde that cyclizes to  - and -D-galactose

a. There are five stereogenic centres in -D-galactose. The stereogenic centers are represented by asterisks in the figure as:

Stereogenic centres in-D-galactose

b. The carbon containing the alkoxy group and the hydroxyl group is referred to as the hemiacetal carbon. The hemiacetal carbon of -D-galactose is represented by an asterisk and can be given as:

Hemiacetal carbon of -D-galactose

c. The position of the OH group is in the equatorial position for -D-galactose. The structure of -D-galactose can be given as:

Structure of -D-galactose

d. A hemiacetal is formed on the cyclization of poly hydroxyl aldehyde. The hydroxyl group on the fifth carbon attacks the aldehyde group leading to the formation of

hemiacetal. The structure of polyhydroxy aldehyde that cyclizes to -D-galactose and -D-galactose can be given as:

Structure of polyhydroxy aldehyde

e. Cyclic hemiacetals are converted to acetals when it is treated with alcohol in the presence of acid. The reaction can be given as:

Treatment of -D-galactose with CH₃OH