Synthesis of optically active compounds

Question 1

What is a recemic mixture?

Answer 1

• great many of the reactions that are used in organic synthesis produce a 50:50 mixture of two optical isomers
• this is called a racemic mixture or racement
• the mixture is not optically active because the effects of the two isomers cancel out

Question 2

Why are usually only one of the isomers formed in biological processes?

Answer 2

using enzyme catalysts, which hold the molecules in place so it only produced one of the possible isomers

Question 3

How are optical isomers used in industry?

Answer 3

• some drugs are optically active molecules
• for some purposes, a recemic mixture of the two optical isomers will do
• for other uses only one isomer is required
• e.g. ibuprofen

Question 4

Why do optical isomers cause problems in pharmaceuticals?

Answer 4

pharmaceutical companies are left with 3 options:

• separate the two isomers
  
  • difficult
  • expensive
  • this is because the optical isomers have similar properties
• sell the mixture as a drug
  
  • wasteful as half is inactive
• design an alternative synthesis of the drug
  
  • only makes the required isomer

Question 5

What was thalidomide used for?

Answer 5

• insomnia
• eventually used for morning sickness

Question 6

WHat was the problem with the testing on thalidomide?

Answer 6

• not tested on pregnant animals or humans

Question 7

What is the problem with thalidomide?

Answer 7

• forms a recemic mixture
• S-form causes birth defects, whereas the R-form is a safe sedative
  
  • evidence that the R-form is converted into S-form so even a pure form would not be safe

Question 8

How is thalidomide used now?

Answer 8

• leprosy and cancer treatments
• blood vessels are prevented from being grown to tumours
• chemists have also produced a number of related compounds that are upto 4000 times more effective and have fewer side effects

Question 9

Butanone is reduced in a two-step reaction using NaBH4 followed by dilute hydrochloric acid.

CH₃CH₂COCH₃ + 2[H] CH₃CH₂CH(OH)CH₃

By considering the mechanism of the reaction, explain why the product has no effect on plane polarised light.

Answer 9

• stage 1: Formation of product
  
  • nucleophillic attack
  • planar carbonyl group
  • H- attacks from either side
• stage 2: nature of product
  
  • exists in two chiral forms
  • equal amounts of each enantiomer / recemic mixture fromed
• stage 3: optical activity
  
  • enantiomers rotate the plane of polarised light equally
  • with a recemic / equal mxiture the effects cancel