Chemistry (isomers)

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  • Created by: Em
  • Created on: 05-10-10 10:00

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…

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