Chapter 21, Problem 21.39P

Interpretation Introduction

(a)

Interpretation: The number of stereogenic centers present in $\alpha \text{-D-galactose}$ are to be predicted.

Concept introduction: Carbohydrates are naturally occurring compounds. Carbohydrates are polyhydroxy aldehydes and ketones. Galactose is a aldohexose as it contains six carbon atoms as well as an aldehyde functional group. The molecular formula of galactose ${\text{C}}_{\text{6}}{\text{H}}_{\text{12}}{\text{O}}_{\text{6}}$.

Answer to Problem 21.39P

There are five stereogenic centers present in $\alpha \text{-D-galactose}$.

Explanation of Solution

The stereogenic centers in $\alpha \text{-D-galactose}$ are shown below.

Figure 1

The stereogenic centers are marked by star. There are five stereogenic centers present in $\alpha \text{-D-galactose}$.

Conclusion

There are five stereogenic centers present in $\alpha \text{-D-galactose}$.

Interpretation Introduction

(b)

Interpretation: The hemiacetal carbon in $\alpha \text{-D-galactose}$ is to be labeled.

Concept introduction: Carbohydrates are naturally occurring compounds. Carbohydrates are polyhydroxy aldehydes and ketones. Galactose is a aldohexose as it contains six carbon atoms as well as an aldehyde functional group. The molecular formula of galactose ${\text{C}}_{\text{6}}{\text{H}}_{\text{12}}{\text{O}}_{\text{6}}$.

Aldehydes or ketones on reaction with one equivalent of alcohol form hemiacetal and on reaction with two equivalents of alcohol it forms acetals. This is nucleophilic addition reaction. These reactions takes place in presence of acids, commonly $p\text{-toluenesulfonicacid}$.

Ethers contain only one alkoxy group on a carbon atom while acetals contain two alkoxy groups on a single carbon atom.

Hemiacetals contains one alkoxy group and one hydroxyl group attached to same carbon atom.

Answer to Problem 21.39P

The hemiacetal carbon in $\alpha \text{-D-galactose}$ is,

Explanation of Solution

The hemiacetal carbon in $\alpha \text{-D-galactose}$ is shown below.

Figure 2

The highlighted carbon contains alkoxy group and hydroxyl group. Hence, this carbon is labeled as hemiacetal carbon.

Conclusion

The hemiacetal carbon in $\alpha \text{-D-galactose}$ is shown in Figure 2.

Interpretation Introduction

(c)

Interpretation: The structure of $\beta \text{-D-galactose}$ is to be drawn.

Concept introduction: Carbohydrates are naturally occurring compounds. Carbohydrates are polyhydroxy aldehydes and ketones. Galactose is a aldohexose as it contains six carbon atoms as well as an aldehyde functional group. The molecular formula of galactose ${\text{C}}_{\text{6}}{\text{H}}_{\text{12}}{\text{O}}_{\text{6}}$.

Answer to Problem 21.39P

The structure of $\beta \text{-D-galactose}$ is shown in Figure 3.

Explanation of Solution

In $\alpha \text{-D-galactose}$, the $\text{OH}$ group is in axial position while in $\beta \text{-D-galactose}$ the $\text{OH}$ group is in equitorial position. The structure of $\beta \text{-D-galactose}$ is shown below.

Figure 3

Conclusion

The structure of $\beta \text{-D-galactose}$ is shown in Figure 3.

Interpretation Introduction

(d)

Interpretation: The structure of poly hydroxy aldehyde that cyclizes to $\alpha$ and $\beta \text{-D-galactose}$ is to be drawn.

Concept introduction: Carbohydrates are naturally occurring compounds. Carbohydrates are polyhydroxy aldehydes and ketones. Galactose is a aldohexose as it contains six carbon atoms as well as an aldehyde functional group. The molecular formula of galactose ${\text{C}}_{\text{6}}{\text{H}}_{\text{12}}{\text{O}}_{\text{6}}$.

Aldehydes or ketones on reaction with one equivalent of alcohol form hemiacetal and on reaction with two equivalents of alcohol it forms acetals. This is nucleophilic addition reaction. These reactions takes place in presence of acids, commonly $p\text{-toluenesulfonicacid}$.

In $\alpha \text{-D-galactose}$, the $\text{OH}$ group is in axial position while in $\beta \text{-D-galactose}$ the $\text{OH}$ group is in equitorial position.

Answer to Problem 21.39P

The structure of poly hydroxy aldehyde that cyclizes to $\alpha$ and $\beta \text{-D-galactose}$ is,

Explanation of Solution

The cyclization of poly hydroxy aldehyde results in the formation of hemiacetal. The hydroxyl group on $\text{C}5$ attacks on the aldehyde group to form hemiacetal. The structure of poly hydroxy aldehyde that cyclizes to $\alpha$ and $\beta \text{-D-galactose}$ is shown below.

Figure 4

Conclusion

The structure of poly hydroxy aldehyde that cyclizes to $\alpha$ and $\beta \text{-D-galactose}$ is shown in Figure 4.

Interpretation Introduction

(e)

Interpretation: The products formed when $\alpha \text{-D-galactose}$ is treated with ${\text{CH}}_{\text{3}}\text{OH}$ and an acid are to be predicted.

Concept introduction: Carbohydrates are naturally occurring compounds. Carbohydrates are polyhydroxy aldehydes and ketones. Galactose is a aldohexose as it contains six carbon atoms as well as an aldehyde functional group. The molecular formula of galactose ${\text{C}}_{\text{6}}{\text{H}}_{\text{12}}{\text{O}}_{\text{6}}$.

Aldehydes or ketones on reaction with one equivalent of alcohol form hemiacetal and on reaction with two equivalents of alcohol it forms acetals. This is nucleophilic addition reaction. These reactions takes place in presence of acids, commonly $p\text{-toluenesulfonicacid}$.

Answer to Problem 21.39P

The products formed when $\alpha \text{-D-galactose}$ is treated with ${\text{CH}}_{\text{3}}\text{OH}$ and an acid are,

Explanation of Solution

Cyclic hemiacetals can be converted to acetals by treatment with alcohol in presence of acid. The hydroxyl group of hemiacetal is converted to alkoxy group. The $\alpha \text{-D-galactose}$ on treatment with ${\text{CH}}_{\text{3}}\text{OH}$ and an acid, results in the formation of acetal. The corresponding reaction is as follows,

Figure 5

Conclusion

The products formed when $\alpha \text{-D-galactose}$ is treated with ${\text{CH}}_{\text{3}}\text{OH}$ and an acid are shown in Figure 5.