Optical isomers

• What property must a carbon atom have for the molecule to display OPTICAL ISOMERISM

  • 4 different substituents (groups) 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
  • THEY ARE NON IDENTICAL in chemical properties
  • 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 a pair of optical isomers called

  Enantiomers
• What is the chiral centre

  The CARBON that has FOUR different substituents attached to it
• How can a chiral centre be denoted

  C*

  Put a star on the carbon
• Give two examples of chiral molecules. Draw it

  • All alpha amino acids, EXCEPT glycine
  • Latic acid 2-hydroxypropanoic 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
  • 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^o clockwise
• What effect does the - isomer have on PLANE polarised light

  Rotates plane of polarisation by X^o ANTI-CLOCKWISE

  (same angle, opposite direction)
• What is the structure of a polarimeter

  Light source (unpolarised light) -> polarising filter (polarised light) -> polarised light passes through the compartment containing the 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
• What is the first stage of the synthesis of lactic acid from ethanal

  • Reagent = KCN and HCL
  • CH3CHO + HCN -> CH3CH(OH)CN
• Why is the CH3CH(OH)CN molecule formed chiral

  Four different groups are attached to the chiral carbon: H, CH3, OH and CN
• 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 hydrolysis of the lactic acid affect the chirality

  • DOES NOT AFFECT IT
  • Still racemic as chirality is not affected by this stage
• Are racemic mixtures formed in nature

  • Not very often
  • Enzymes mechanisms are 3D so only form one enantiomer
• 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

  • Separate enantiomers -> difficult and expensive as they have very similar properties
  • Sell racemate -> wasteful as half is inactive
  • Design alternative synthesis to only produce one enantiomer
• Examples of optically active drugs

  • Ibuprofen
  • Thalidomide