• Chiral molecules are molecules that contain a carbon atom that is attached to four different atoms or groups of atoms
  • An example of a compound with a chiral centre is the CH₃CHBrOH compound
• A molecule with a chiral centre has to enantiomers which are non-superimposable mirror images of each other

Examples of a molecule with a chiral centre

• These enantiomers have similar chemical properties but differ from each other in their ability to rotate the plane of plane polarised light

Chiral drugs

• Many drugs are derived from natural sources such as plants
• The chiral drugs extracted from these natural sources often contain a single optical isomer only
• An example of a drug derived from plants is the anti-cancer drug Taxol
• Taxol has many chiral centres so many optical isomers of this single compound could exist
• However, only one of these optical isomers is present in the bark of yew the tree which is extremely beneficial

• In biological systems (such as cells), molecules are made and broken down by reactions involving biological catalysts called enzymes
• Enzymes only bind molecules (substrates) that fit the shape of the enzyme’s active site by a lock-and-key mechanism
• If the substrate molecule doesn’t have the correct shape, it will not bind to the enzyme 
• Therefore, if there were many optical isomers (with each a different arrangement around the chiral centre) of the drug Taxol, they will no longer be able to fit the enzyme’s active site for a reaction to occur
• Higher doses of the drugs would have to be administered for it to be effective
• This is why it is so useful that natural sources (such as the yew tree) produce only one optical isomer