As it has already been discussed in Chapter 1, constitutional (or structural) isomers are compounds that have the same molecular formula but different structural formula. There is another type of isomerism, namely stereoisomerism, which is important not only in organic chemistry but, especially, in bioorganic and biological chemistry. The branch of chemistry that concerns with the spatial aspects of molecules is called stereochemistry.
ДХД Stereoisomers are compounds that have the same order of atoms attachment, but differ only in the arrangement of their atoms or groups in space.
In other words, stereoisomers are spatial isomers. They can be subdivided into two categories according to the principles by which they are interconverted:
• configurations! isomers that can be interconverted only by breaking a cova-lent bond and creating a new covalent bond;
• conformationa! isomers that can be interconverted by rotation about single bonds.
The principal types of isomerism occurring in organic chemistry are summarized below:
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Conformational isomerism has been discussed in Chapter 6; in this chapter configurational isomerism will be considered.
Molecules differing only in configuration are termed configurational isomers.
The configuration of a molecule of definite structure is the arrangement of its atoms or groups in space without regard to arrangements that differ only due to rotation about one or more single bonds.
10.1. CHIRAL AND ACHIRAL OBJECTS
A chiral object is one that exhibits the property of «handedness». The word chiral (pronounced «käi-ral») comes from the Greek word cheir, meaning hand. Many familiar objects are chiral, for example, hands and gloves, feet and shoes, helices (including the double helix of DNA), screws and other threaded objects. It is well known that the reflection of a left hand is not another left hand, but a right hand (Fig. 10.1, a). So both hands cannot be superposed (placed one upon another). Thus a left and right hand are said to be enantiomers. As we shall soon see, a similar relationship is correct with molecules.