Chapter 22, Problem 15PP

Interpretation Introduction

Interpretation:

The structures of $\text{D}–(+)–\text{xylose}$, which oxidized to optically inactive aldaric acid and $\text{ D}–(-)–\text{lyxose}$, which oxidized to optically active product, are to be determined.

Concept introduction:

舧 A carbohydrate is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen-oxygen atom ratio of 2:1

舧 ${\text{HNO}}_{\text{3}}$ is an oxidizing agent.

舧 Carbohydrates are oxidized by ${\text{HNO}}_{\text{3}}$ to give aldaric acid as a product.

舧 Aldaric acids are carbohydrates having two carboxylic acids. They are formed due to oxidation reaction of aldoses with dilute ${\text{HNO}}_{\text{3}}$ and their general reaction can be represented as:

舧 The molecules that are nonsuperimposable or not identical with their mirror images are known as chiral molecules.

舧 A pair of two mirror images that are nonidentical is known as enantiomers, which are optically active.

舧 The stereoisomers that are nonsuperimposable on each other and not mirror images of each other are known as diastereomers.

舧 The achiral compounds in which plane of symmetry is present internally and consists of chiral centres are known as meso compounds, but they are optically inactive.

舧 Compounds that have a plane of symmetry tend to exist in meso forms. A meso form arises when the two stereoisomers produce superimposable images, and hence, compounds having meso forms are optically inactive.

舧 The Kiliani-Fischer reaction is used for increasing the number of carbon atoms of respective aldoses, thereby, lengthening the carbon chain of the compound. It is used to produce epimers of higher aldoses from a lower aldose, i.e., an aldose with a lesser number of carbon atoms produces a mixture having two epimeric aldoses with a higher number of carbon atoms.

舧 Oxidation of aldoses by nitric acid produces optically inactive aldaric acids. This is due to the formation of meso compounds. Compounds that do not form meso structures are optically active.