Chirality & Drug Synthesis

Chemistry: Cambridge International A Level, topic 3.10.1

Optical Isomerism, Chirality, & Drug Synthesis

Optical isomerism is a form of stereoisomerism. Optical isomers (also called enantiomers) are non-superimposable mirror images of each other. They contain chiral centres.

Non-superimposable mirror images

Much like your left and right hands, some molecules have mirror images.
These mirror images are non-superimposable upon each other.
This means that they have a different spatial structure.
No matter how you twist your hands, you can’t make a left hand into a right hand.

Chiral centres

Molecules that have four different groups bonded to one carbon atom have two mirror images which are non-superimposable.
The carbon atom with four different attached groups is called a chiral center. Chiral centers lead to chirality in molecules.

Drawing chiral centres

Dashed bonds indicate an atom below the plane of the paper.
Thick wedge bonds indicate an atom above the plane of the paper.
The different optical isomers of bromohydroxyacetonitrile are above.
Try rotating them to make them the same - you can’t!

Chiral centres & organic synthesis

Drugs extracted from plants or other natural sources tend to only contain one optical isomer.
This is because the synthesis of natural compounds in living systems is controlled by enzymes.
Enzymes are biological catalysts with very specific shapes. This means enzymes are likely to only fit one of the optical isomers.

Importance of single isomers

Drugs bind to specific targets in the body.
So, if a drug contained two optical isomers of the same drug, the optical isomers may bind to different targets.
This occurred with the drug thalidomide. One optical isomer treated morning sickness, but the other caused birth defects. The drug has since been banned.
So, to reduce side effects and ensure better therapeutic activity, drug synthesis requires the production of a single optical isomer.

Drawing Enantiomers

You need to be able to draw the enantiomers for chiral molecules.

Step 1 - Find chiral centre

The chiral centre is at the carbon atom that is bonded to four different atoms or groups of atoms.
The molecule shown above has a chiral centre at the carbon atom which is cirlced.

Step 2 - Draw one enantionmer

Draw the four different atoms or groups of atoms in any order around the chiral centre.
Dotted line - shows the bond that's going beneath the paper.
Wedged line - shows the bond that's coming out of the paper. - Normal filled lines - show the bonds that are in the plane of the paper.

Step 3 - Draw other enantiomer

Draw a dotted line to the right of the first enantiomer.
Draw the mirror image on the other side of the dotted line.
You should now have two molecules on either side of the dotted line.
The two molecules will be non-superimposable and they are enantiomers of each other!

Test yourself on Chirality & Drug Synthesis

Optical isomers are most like which body part?
If you draw the mirror image of a molecule with 0 chiral centres, how are the two molecules related?
How many enantiomers does a molecule with one chiral centre have?
The Process of Drawing Enantiomers

Practise this topic for free