Molecules and Optical Isomerism

CHAPTER 15 Organic Compounds and the Atomic Properties of Carbon 

Unlike constitutional isomers, optical isomers are identical in all but two respects  

Optical isomerism plays a vital role in living cells. Nearly all carbohydrates and amino acids are optically active, but only one of the isomers is biologically usable. For example, r/-glucose is metabolized for energy, but /-glucose is excreted unused. Similarly, /-alanine is incorporated naturally into proteins, but r/-alanine is not. Many drugs are chiral molecules of which one optical isomer is biologically active and the other has either a different type of activity or none at all. 

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Constitutional Isomerism and the Physical Properties of Alkanes Chiral Molecules and Optical Isomerism Alkenes Alkynes... 

Cyclic selenium sulfides constitute a system of considerable structural complexity. In addition to the various stoichiometric compositions of the molecules there are many possibilities for positional and optical isomerism. In the case of the eight-membered ring molecules the following 28 molecules (excluding optical isomers) can exist ... 

Configurational centres impose a rigid shape on sections of the molecule in which they occur. However, their presence gives rise to geometric and optical isomerism. Since these stereoisomers have different shapes, biologically active stereoisomers will often exhibit differences in their potencies and/or activities (Table 2.1). These pharmacological variations are particularly likely when a chiral centre is located in a critical position in the structure of the molecule. The consequence of these differences is that it is now necessary to make and test separately all the individual stereoisomers of a drug. 

Stereoisomerism is the arrangement of atoms in molecules whose connectivity remains the same but their arrangement in space is different in each isomer (atoms are connected in the same sequence). The two main types of stereoisomerism are cis-trans or Z-E isomerism and optical isomerism. 

Here we introduce a further category of isomerism known as stereoisomerism, in which the molecules concerned have the same molecular formula and structural formula, but their atoms are arranged differently in space. There are three types of stereoisomerism — conformational isomerism, cis— trans isomerism and optical isomerism (Figure 20.46). Configurational isomers have permanent Figure 20.46 The different types of isomerism differences in their structural geometry and cannot... 

Many substances exhibit the property of isomerism they occur in two or more molecular forms that have the same composition but differ from each other in structure and in their properties. One type of isomerism, known as optical isomerism, is exhibited by molecules that have the same constituent atoms but are arranged in different spatial distributions, where one of the optical isomers is a mirror image of the other (see Textbox 63). 

Optical isomerism is exhibited by compounds that are chiral, i.e., are not superimposable on their mirror images. Such a compound and its mirror image are called optical isomers or enantiomers. They have identical physical and chemical properties except when they interact with other chiral molecules. 

Optical Isomerism of (D-L-Isomerism) and Tacticity of Polymers Optical isomerism has its origin in the way different substituents occupy positions on an asymmetric carbon atom in a polymer molecule. For example, polyethylene molecule has fully saturated carbon atoms as shown in the following chemical formula ... 

Another type of isomerism is optical isomerism. These molecules are capable of rotating light to either the left or right and are said to be optically active. The presence of an asymmetric or chiral carbon (a carbon atom with four different groups attached to it) will make a compound optically active. 

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