Optical isomerism

Optical isomerism

Mirror image isomers of asymmetrical shapes and solids are non-superimposable.

Chemistry

Keywords

optical isomerism, stereoisomerism, enantiomer, racemic mixture, chirality, chiral, chirality centre, mirror image, alanine, glucose, thalidomide, limonene, isobuthane, L-alanine, D-alanine, D-glucose, L-glucose, D-limonene, L-limonene, optically, ligand, configuration, dextrorotatory, levorotatory, isomer, polarimeter, methane, organic chemistry, chemistry

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Narration

There are symmetrical and asymmetrical molecules. The mirror image isomers of a symmetrical molecule are superimposable. Methane is such a symmetrical molecule.

Isobuthane molecules are also symmetrical. Symmetrical molecules and their mirror image isomers are identical. However, the mirror image isomers of asymmetrical shapes and solids are non-superimposable.

This can be demonstrated with our right and left hands or with screw threads. Asymmetrical molecules are called chiral molecules.

Bromofluorochloromethane is such a molecule: in its structure 4 different atoms are attached to the central carbon atom. Thus it is a chiral molecule: its mirror image is not superimposable. The two mirror image isomers of a chiral molecule are called enantiomers. When enantiomers are present in a solution in equal amounts, the solution is called racemate, or a racemic mixture.

Amino acids are also chiral molecules, as shown by the example of alanine. In this molecule, 4 different atomic groups are attached to the central carbon atom: a hydrogen atom, an amino group, a carboxyl group and a methyl group. L and D amino acids are determined by the order of attachment of atoms and atomic groups. L amino acids occur in living beings. Their mirror image isomers are called D amino acids. The designation D and L derives from a complex system, relative to the isomers of the glyceraldehyde molecule.

Asymmetrical molecules are optically active, that is, they rotate the plane of polarized light. Mirror image isomers rotate the plane in opposite directions. Thus there are levorotatory and dextrorotatory forms. Racemic mixtures do not rotate the plane of polarized light.

Enantiomers often have different effects on living beings. D-limonene, for example, smells like orange and occurs in citrus fruits such as oranges and lemons, while L-limonene has a piney smell and occurs in pine oil.

A well-known, tragic case related to optical isomerism was the Contergan or thalidomide scandal. This medicine was used as a sedative and to alleviate morning sickness in pregnant women. Its active ingredient, thalidomide is a chiral molecule, the drug contained it in a racemic mixture. However, as it later turned out, one of the enantiomers of the molecule caused a congenital disorder involving malformation of the limbs. Due to this, the drug was withdrawn from the market in 1961 and regulations for the process of approving medicines were tightened.

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