Chapter 14, Problem 14.23UKC

a.

Interpretation Introduction

Interpretation:

Structure of an L-aldopentose has to be given.

Concept Introduction:

Simplest carbohydrates are known as monosaccharides.  They contain three to six carbons generally in a chain form with a carbonyl group present in the terminal or the adjacent carbon atom from the terminal.  Monosaccharides that have the carbonyl group at the terminal carbon atom $\text{C1}$ are known as aldoses and the monosaccharides that have the carbonyl group on the adjacent carbon atom $\text{C2}$ are known as ketoses.

The number of carbon atoms present in the chain characterize the monosaccharide.  They are given below.

• Carbon chain with three carbon atoms is triose.
• Carbon chain with four carbon atoms is tetrose.
• Carbon chain with five carbon atoms is as pentose.
• Carbon chain with six carbon atoms is as hexose.

A carbon atom that is bonded to four different groups is known as a chiral carbon atom.  This can rotate the plane polarized light.  D- and L- isomers of monosaccharide can be identified by looking into the chiral center that is farther from the carbonyl group.  In a Fischer projection, if the $\text{OH}$ group is present on the right side in the chiral center that is farthest from the carbonyl group means it is a D monosaccharide and if the $\text{OH}$ group is present on the left side means it is a L monosaccharide.

b.

Interpretation Introduction

Interpretation:

Structure of a D-aldotetrose has to be given.

Concept Introduction:

Refer part “a.”.

c.

Interpretation Introduction

Interpretation:

Structure of a five-carbon alditol has to be given.

Concept Introduction:

Carbohydrates undergo reduction similar to an alkene.  When an aldose is treated with hydrogen in presence of palladium as catalyst the carbonyl group present in the aldose is reduced resulting in formation of alditol.  This is also known as “sugar alcohol”.