Compounds having the same molecular formula may differ from one another in the nature or sequence in which the individual atoms are bound. Such compounds are known as isomers and they may differ significantly in their chemical and physical properties, depending on the structures. For instance ethylene oxide and acetaldehyde both have the formula C2H4O but they differ in their constitution. When substances have the same constitution but differ from one another in the manner in which the individual atoms (or groups) are arranged in space, then they are termed stereoisomers. When two stereoisomers are so related to each other that one is the nonsuperimposable mirror image of the other, then the two are said to be enantiomeric and each enantiomer is chiral. They differ from one another in having an equal and opposite optical rotation. Stereoisomers which are not enantiomers are called diastereomers. Diastereomers may therefore be defined as substances which have the same constitution, which are not mirror images and which differ from one another in having a different configuration at one or more asymmetric centers in the molecule [1–3]. Substances may also exist as conformers; the conformational isomerism results from the existence of discrete isomers due to barriers in the rotation about single bonds.
KeywordsSequence Rule Asymmetric Center Attached Atom Chiral Reagent Stereoselective Reaction
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