Racemates & Optical Activity (Edexcel International A Level Chemistry)

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Racemates & Optical Activity

Properties of optical isomers

  • The chemical properties of optical isomers are generally identical, with one exception
    • Optical isomers interact with biological sensors in different ways
      • For example, one enantiomer of carvone smells of spearmint, while the other smells of caraway

        20-3-3-carvone-optical-isomers

                         Carvone optical isomers have distinctive smells

  • Optical isomers have identical physical properties, with one exception
    • Isomers differ in their ability to rotate the plane of polarised light

Organic Chemistry - Unpolarised Light, downloadable AS & A Level Chemistry revision notes

When unpolarised light is passed through a polariser, the light becomes polarised as the waves will vibrate in one plane only

  • The major difference between the two enantiomers is:
    • One enantiomer rotates plane polarised light in a clockwise manner and the other in an anticlockwise fashion
    • A common way to differentiate the isomers is to use (+) and (-), but there are other systems using d and l, D and L, or R and S

  • The rotation of plane polarised light can be used to determine the identity of an optical isomer of a single substance
    • For example, pass plane polarised light through a sample containing one of the two optical isomers of a single substance
    • Depending on which isomer the sample contains, the plane of polarised light will be rotated either clockwise or anti-clockwise by a fixed number of degrees

Organic Chemistry - Effect of Optical Isomers on Plane of Polarised LightEffect of Optical Isomers on Plane of Polarised Light, downloadable AS & A Level Chemistry revision notes

Each enantiomer rotates the plane of polarised light in a different direction

  • A racemic mixture (or racemate) is a mixture containing equal amounts of each enantiomer
    • One enantiomer rotates light clockwise, the other rotates light anticlockwise

  • A racemic mixture is optically inactive as the enantiomers will cancel out each others effect
    • This means that the plane of polarised light will not change

      20-3-3-racemic-mixture

           Racemic mixtures are optically inactive

Racemic mixtures and drugs

  • In the pharmaceutical industry, it is much easier to produce synthetic drugs that are racemic mixtures than producing one enantiomer of the drug
  • Around 56% of all drugs in use are chiral and of those 88% are sold as racemic mixtures
  • Separating the enantiomers gives a compound that is described as enantiopure, it contains only one enantiomer
  • This separation process is very expensive and time consuming, so for many drugs it is not worthwhile, even though only half the of the drug is pharmacologically active
  • For example, the pain reliever ibuprofen is sold as a racemic mixture

Ibuprofen, downloadable AS & A Level Chemistry revision notes

The structure of ibuprofen showing the chiral carbon that is responsible for the racemic mixture produced in the synthesis of the drug

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Richard

Author: Richard

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Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.