3.07 Optical Isomerism

• What property must a carbon atom have for the molecule to display optical isomerism about that carbon atom?

  4 different substituents attached to one carbon atom
• What are the similarities and differences between two optical isomers?

  • Same atoms and bonds, but they are non-superimposable mirror images of one another. NOT IDENTICAL in chemical properties necessary.
  • Differ in the way they rotate plane polarised light - rotate plane of polarisation by the same angle but in different directions.
• What word is used to describe optically active molecules?

  Chiral
• What are the pairs of isomers called?

  Enantiomers
• What is the chiral centre?

  The carbon that has four different substituents attached to it
• How is the chiral centre denoted?

  C*
• Give two examples of chiral molecules. Draw one of them.

  • All alpha amino acids, except glycine
  • Lactic acid
• How is light polarised?

  By passing it through a polaroid filter, so oscillations are only in one plane.
• What effect does the racemic mixture have on plane polarised light?

  None, as the rotation by each enantiomer cancels out to nothing
• What effect does the + isomer have on plane polarised light?

  Rotates plane of polarisation by x° clockwise
• What effect does the - isomer have on plane polarised light?

  Rotates plane of polarisation by x° anticlockwise (same angle, opposite direction)
• What is the structure of a polarimeter?

  Light source (unpolarised light) → polarising filter (polarised light) → polarised light passes through compartment containing sample → detector determines the angle of rotation of the plane polarised light
• What are polarimeters used for?

  To identify which enantiomer is present, the purity of the sample, the concentration of the sample, etc.
• What is the first stage of synthesis of lactic acid from ethanal?

  CH3CHO + HCN → CH3CH(OH)CN
• Why is the CH3CH(OH)CN formed chiral?

  H, CH3, OH and CN groups attached to the 4 central chiral atoms - 4 substituents
• What is the second stage of the synthesis of lactic acid from ethanal?

  Hydrolysis: CH3CH(OH)CN + HCl + 2H2O → CH3CH(OH)COOH + NH4Cl
• How does the second stage of the synthesis of lactic acid from ethanal affect the chirality?

  Does not affect it - still racemic as chirality not affected by this stage.
• Are racemic mixtures formed in nature? Why?

  Not often, as enzyme mechanisms are 3D so form only one enantiomer (+complementary to only one substrate)
• Why is optical isomerism a problem for the drug industry?

  Sometimes, only one enantiomer is effective due to enzyme's active site/cell receptors being 3D
• What are the options to resolve the issue of only one enantiomer being effective?

  1. Separate enantiomers - difficult and expensive as have very similar properties
  2. Sell racemate - wasteful as half of it is inactive
  3. Design alternative synthesis to only produce one enantiomer
• Examples of optically active drugs?

  Ibuprofen, Thalidomide
• Why is ibuprofen able to be sold as a racemate, even though the positive isomer is needed to treat inflammation?

  Sold as a 50% racemate, however the body converts 60% of R- isomer to S+ → end up with 80% S+ isomer