Chemistry A2 notes – Optical isomers

  • An optical isomer occurs when the carbon atom (chiral centre; this is marked by a *) is attached to 4 different groups.
  • this results in 2 isomers (pair of enantiomers) that are mirror images
  • they are NOT identical and can’t superimpose
  • the 2 isomers differ in the way that they rotate the plane of polarisation of polarised light

How to distinguish between the pair of enantiomers

  • the 2 isomers differ in the way that they rotate the plane of polarisation of polarised light
  • (plane-polarised light is light that has been filtered into 2D plane)
  • the light passes through a polaroid or liquid containing a chiral molecule which causes the plane of light to rotate. This is detected using a polarimeter
  • chiral molecules rotate plane-polarised light.
  • the 2 isomers rotate equally but in opposite directions.
  • The + isomer (dextrorotatory) rotates the plane of polarisation CLOCKWISE.
  • The – isomer (laevorotatory) rotates the plane of polarisation ANTI-CLOCKWISE.
  • a substance which can rotate plane of polarised light is optically active

The importance of optical isomers in biochemistry

Many drugs and enzymes are chiral so only 1 of the optical isomers can interact effectively with the target molecule.

Many drugs work by a molecule of the active ingredient fitting the receptor on the cell, similarly enzymes have a specific active site that only a specific substrate can fit into.

Receptors have a 3D structure so only 1 of the pair of optical isomers will fit. So one optical isomer will be the effective drug and the other optical isomer will be inactive.


The oppostie effect they have on the rotation of plane polarised light is no overall rotation, so an equimolar mix of 2 optical isomers will have no effect on the plane of polarised light. So they are NOT optically active.

Racemic mixtures – equimolar mixture of 2 optical isomers that aren’t optically active.

Separating the 2 isomers is difficult and expensive as they have similar properties.

Chiral molecules show optical activity if 1 isomer is in greater quantity than the other.


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