Stereoisomers are species with the same structural formula but with a different arrangement of the atoms in space.
A chiral carbon is a carbon atom attached to four different atoms or group of atoms.
Optical isomers are stereoisomers that are non-superimposable mirror images of each other.
In stereoisomerism the atoms making up the isomers are joined up in the same order but have different arrangements in space.
Alkenes can show E/Z isomerism provided that each of the carbon atoms in the double bond is attached to two different groups.
There is a second type of stereiusomerism, known as optical isomerism - so called because the isomers have different effects on the plane-polarised light.
- Optical isomerism arises in organic molecules that contain carbon atoms attached to four different atoms or groups of atoms.
- A carbon atom with four different groups attached is said to be chiral.
- If a molecule has a chiral centre in its structure, then two mirror-image arrangements are possible in space. Neither arrangement can be placed over the other.
- Tese arrangements are non-superimposablem mirror images and are called potical isomers.
- Optical isomers are chemically identical. However, optical isomers rotate plane-polarised light in different directions.
- There are only ever two optical isomers formed for each chiral carbons.
Properties of optical isomers
- Optical isomers rotate plane-polarised light differently.
- One of the optical isomers rotates the light clockwise and the other rotates the light anti-clockwise.
- A mixture containing equal amounts of each isomer is known as a racemic mixture.
- A racemic mixture has no effect on plane-polarised light because the rotations cancel each other out.
Naturally occuring alpha-amino acids are optically active - the one expection is glycerine, which has two hydrogen atoms on the central carbon atom.
Optical activity is important…