AQA A Level Chemistry复习笔记7.1.3 Racemic Mixtures

Net Optical activity

• Let’s suppose that in a solution, there is 20% of the enantiomer which rotates the plane polarised light clockwise and 80% of the enantiomer which rotates the plane of polarised light anticlockwise
• There is an uneven mixture of each enantiomer, so the reaction mixture is said to be optically active
  • The net effect is that the plane of polarised light will be rotated anticlockwise

   

• Similarly, if the percentages of the enantiomers are reversed, the reaction mixture is still optically active but now the plane of polarised light will be rotated clockwise
  • In this case, there is 20% of the enantiomer, which rotates the plane anticlockwise
  • And 80% of the enantiomer, which rotates the plane clockwise

   

• A racemic mixture (or racemate) is a mixture in which there are equal amounts of enantiomers present in the solution
• A racemic mixture is optically inactive as the enantiomers will cancel out each others effect and the plane of polarised light will not change

When one of the enantiomers is in excess, the mixture is optically active; when there are equal amounts of each enantiomer the mixture is 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

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

• However, there are two important reasons that sometimes only one enantiomer is produced:
  • The other enantiomer could be harmful
  • The drug is very expensive to synthesize so you don't want to waste materials and resources to produce a drug that is only 50% effective

   

• The tragic story of the drug thalidomide illustrates the dangers of marketing a racemic mixture
  • Thalidomide was a drug introduced in the late 1950s as a sedative and treatment for morning sickness in pregnant women
  • A few years later, children of mothers who had taken thalidomide were born with missing limbs and it was later linked to the presence of the other enantiomer that caused birth defects
  • Fortunately the testing and regulation of drugs these days means it is very unlikely that such a tragedy could be repeated
  • The irony is that even if the drug had been sold as one enantiomer, some birth defects may still have occurred as we now know that some drugs an invert from one optical isomer to the other during metabolic reactions in the human body

   

One enantiomer of thalidomide was a very effect treatment for morning sickness, skin cancers and leprosy, but the other enantiomer caused horrific birth defects