Chapter 22, Problem 1PP

Interpretation Introduction

Interpretation:

The chirality centers in aldotetrose and ketopentose are to be calculated and the stereoisomers for each general case are to be determined.

Concept Introduction:

Carbohydrates are categorized mainly as monosaccharides, disaccharides, and polysaccharides. Monosaccharides are single sugar units, mainly glucose and fructose, disaccharides are two sugar units, such as sucrose, and polysaccharides are more than two sugar units, such as starch and cellulose.

Monosaccharides containing 3-carbon atoms are called triose, 4-carbon atoms called tetrose, 5-carbon atoms called pentose, and so on.

In chiral molecules, carbon atom having four nonidentical substituent groups is called the chirality center of that molecule. Chirality center may also be called stereocenter, which signifies any point in the molecule where the interchanging of any two groups may lead to stereoisomers. The carbon of a carbohydrate can be considered as chiral when the carbon has all four different substituents attached to it.

The stereoisomers are calculated as follows:

$\text{Number of stereoisomers}=2^n$

Here, $n$ is the number of chiral centers.

Answer to Problem 1PP

Solution:

a) Two

b) Two

c) Four

Explanation of Solution

a) The aldotetrose

A monosaccharide containing four carbon atoms is called a tetrose. An aldotetrose is a monosaccharide that contains aldehyde group.

The structure of aldotetrose is as follows:

The carbon atom attached to four different groups is chiral carbon. The chiral center in ketopentose is marked by * as follows:

Hence, an aldotetrose has two chirality centers.

b) The ketopentose

A monosaccharide containing five carbon atoms is called a pentose. A pentose containing a keto group is called a ketopentose.

The structure of ketopentose is as follows:

The carbon atom attached to four different groups is chiral carbon. The chiral center in ketopentose is marked by * as follows:

Hence, a ketopentose has two chirality centers.

c) The number of stereoisomers that will be expected from each general structure

Stereoisomers of a molecule have the same molecular formula, but different arrangement of atoms in space. Stereoisomers are different from enantiomers as they are not mirror images of each other, while enantiomers are mirror images of one another.

The compounds aldotetrose and ketopentose have two sets of enantiomers. The number of stereoisomers is calculated as:

$\text{Number of stereoisomers}=2^n$

Substitute 2 for $n$ in the above expression as follows:

$\begin{array}{c}\text{Number of stereoisomers }=2^2\\ =4\end{array}$

Hence, they will have four stereoisomers for each general structure.