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Optical isomerism

Optical isomers are molecules that differ three-dimensionally by the placement of substituents around one or more atoms in a molecule. Optical isomers were given their name because they were first able to be distinguished by how they rotated plane-polarized light. Isomers are molecules that have the same molecular formula, but have a different arrangement of the atoms in space. That excludes any different arrangements which are simply due to the molecule rotating as a whole, or rotating about particular bonds.

Simple substances which show optical isomerism exist as two isomers known as enantiomers.

  • A solution of one enantiomer rotates the plane of polarisation in a clockwise direction. This enantiomer is known as the (+) form.

    • For example, one of the optical isomers (enantiomers) of the amino acid alanine is known as (+)alanine.

  • A solution of the other enantiomer rotates the plane of polarisation in an anti-clockwise direction. This enantiomer is known as the (-) form. So the other enantiomer of alanine is known as or (-)alanine.
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