Chapter 21, Problem 21.38P

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 ${\text{C}}_{\text{6}}{\text{H}}_{\text{12}}{\text{O}}_{\text{6}}$.

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.

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 ${\text{C}}_{\text{6}}{\text{H}}_{\text{12}}{\text{O}}_{\text{6}}$.

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.

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 $p\text{-toluenesulfonicacid}$.