Optical Isomerism
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Optical isomerism is a form of stereoisomerism. This kind of isomerism occurs as a result of chirality in molecules.
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Chirality refers to a molecular property where the molecule is not superimposable on its mirror image, much like how a left and right hand are mirror images but not superimposable.
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A chiral molecule often has a carbon atom that is attached to four different groups. This carbon atom is known as the chiral or asymmetric centre.
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The mirror image of a chiral molecule is called its enantiomer. Each pair of enantiomers is designated as either ‘R’ or ‘S’ according to a set of priority rules.
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Enantiomer pairs have identical physical properties (eg., melting points, boiling points), but they rotate plane-polarised light in opposite directions. One enantiomer will rotate light clockwise (dextrorotatory) while the other will rotate it anti-clockwise (laevorotatory).
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The measure of how much a substance can rotate plane-polarised light is called ‘optical activity’. The optical activity is dependent on the structure of the molecule and the concentration of the chiral compound.
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The mixture of equal amounts of two enantiomers is known as a racemic mixture. Racemic mixtures do not rotate light because the rotations of the two enantiomers cancel each other out.
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Optical isomerism is important in many biological systems as many bio-chemicals such as drugs, enzymes, and hormones are chiral. In these cases, only one of the enantiomers typically has the desired physiological effect.
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Reactions that produce chiral molecules often yield racemic mixtures unless a chiral catalyst is used. These reactions can be incredibly important in creating pharmaceuticals and other useful chemicals.
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To distinguish between enantiomers, substances such as plane-polarised light and chiral compounds can be used.
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As potential chemists, it’s crucial to understand optical isomerism because it’s a fundamental concept that plays a significant role in the synthesis and application of many organic compounds.