Chapter 16, Problem 19P

(a)

Interpretation Introduction

Interpretation:

The enantiomers among the eight aldopentoses have to be found.

Concept Introduction:

Enantiomers are the pairs which are mirror images of each other.  These mirror images are non-superimposable to each other.  This means the forms of a compound which exists in two non-superimposable forms each of which is a mirror image of other form is known as enantiomers.

(b)

Interpretation Introduction

Interpretation:

The $\text{C-2}$ epimers among the eight aldopentoses have to be found.

Concept Introduction:

Epimer: Epimer is one of a pair of stereoisomers. The two isomers differ in configuration at only one stereogenic center.

$C-2$ Epimers are the compounds which has different configuration at second carbon atom and have same configuration at all other carbon atoms.

The stereogenic center present in the compound which makes the orientation of molecules different is known as epimers.