Chapter 25, Problem 25.24P

(a)

Interpretation Introduction

Interpretation:

The product formed by the reaction of D-galactose with ${\text{NaBH}}_{\text{4}}$ and ${\text{H}}_{\text{2}}\text{O}$ has to be drawn in Fischer projection and the product is optically active or inactive has to be stated.

Concept Introduction:

The reaction of ${\text{NaBH}}_{\text{4}}$ and ${\text{H}}_{\text{2}}\text{O}$ with carbohydrates is a reduction reaction.  The carbonyl group in the carbohydrate is reduced to alcoholic group and forms alditols.

Explanation of Solution

The D-galactose is an aldohexose molecule.  The carbonyl group present in D-galactose is aldehyde ($\text{-CHO}$).  When the D-galactose molecule is reacted with ${\text{NaBH}}_{\text{4}}$ and ${\text{H}}_{\text{2}}\text{O}$, the aldehyde group is reduced to alcohol group (${\text{-CH}}_{\text{2}}\text{OH}$) forming the alditol product.

The formed alditol is optically inactive as it is a meso compound and shows plane of symmetry.

(b)

Interpretation Introduction

Interpretation:

The product formed by the reaction of D-galactose with ${\text{H}}_{\text{2}}\text{/Pt}$ has to be drawn in Fischer projection and the product is optically active or inactive has to be stated.

Concept Introduction:

The reagent ${\text{H}}_{\text{2}}\text{/Pt}$ is a reducing agent which reduces the carbonyl group into alcoholic group.  The carbonyl group in the carbohydrate is reduced to alcoholic group and forms alditols.

Explanation of Solution

When the D-galactose molecule is made to react with ${\text{H}}_{\text{2}}\text{/Pt}$, the aldehyde group is reduced to alcoholic group forming the alditol.

The formed alditol is optically inactive as it is a meso compound and it shows plane of symmetry.

(c)

Interpretation Introduction

Interpretation:

The product formed by the reaction of D-galactose with warm ${\text{HNO}}_{\text{3}}$ has to be drawn in Fischer projection and the product is optically active or inactive has to be stated.

Concept Introduction:

The reaction of warm ${\text{HNO}}_{\text{3}}$ with carbohydrates is a oxidation reaction.  The carbonyl groups and alcoholic groups in the carbohydrate is oxidised to acid groups.

Explanation of Solution

The nitric acid (${\text{HNO}}_{\text{3}}$) is an oxidising agent.  It oxidises the alcoholic groups and carbonyl groups into acidic groups.  In D-galactose, the aldehyde group and the alcoholic group (C6) are oxidised to acid groups forming D-galactaric acid.

The product formed is a meso compound and shows plane of symmetry.  So, it is optically inactive.

(d)

Interpretation Introduction

Interpretation:

The product formed by the reaction of D-galactose with ${\text{Br}}_{\text{2}}{\text{/H}}_{\text{2}}{\text{O/CaCO}}_{\text{3}}$ has to be drawn in Fischer projection and the product is optically active or inactive has to be stated.

Concept Introduction:

The reaction of ${\text{Br}}_{\text{2}}{\text{/H}}_{\text{2}}{\text{O/CaCO}}_{\text{3}}$ with carbohydrates is a oxidation reaction.  The carbonyl group in the carbohydrate is oxidised to acid group.  It is a mild oxidising agent compared to ${\text{HNO}}_{\text{3}}$.

Explanation of Solution

When the D-galactose reacts with ${\text{Br}}_{\text{2}}{\text{/H}}_{\text{2}}{\text{O/CaCO}}_{\text{3}}$, the aldehyde group is oxidised to acid group forming the acidic product, D-galactonic acid.

The formed product is optically active.

(e)

Interpretation Introduction

Interpretation:

The product formed by the reaction of D-galactose with ${\text{H}}_{\text{5}}{\text{IO}}_{\text{6}}$ has to be drawn in Fischer projection and the product is optically active or inactive has to be stated.

Concept Introduction:

The reaction of ${\text{H}}_{\text{5}}{\text{IO}}_{\text{6}}$ oxidises the carbohydrate molecule by cleaving the carbon-carbon bonds.  It forms many cleaved products of the carbohydrate.

Explanation of Solution

The D-galactose molecule consumes ${\text{5H}}_{\text{5}}{\text{IO}}_{\text{6}}$ molecules to cleave the six carbon-carbon bonds.  The aldehyde group (C1) and hydroxyl groups (C2-C5) are reduced to acid groups and the C6 alcohol group is converted to aldehyde group.  It forms six products among which five are formic acid molecules and one is formaldehyde.

The products formed are achiral and are optically inactive.

(f)

Interpretation Introduction

Interpretation:

The product formed by the reaction of D-galactose with aniline (${\text{C}}_{\text{6}}{\text{H}}_{\text{5}}{\text{NH}}_{\text{2}}$) has to be drawn in Fischer projection and the product is optically active or inactive has to be stated.

Explanation of Solution

The D-galactose molecule reacts with aniline molecule and the aldehyde group of carbohydrate is reacted with the amine group of aniline.  By the loss of water molecule, it forms Schiff’s base of D-galactose.

The formed product is a chiral compound and optically active.