Optical Isomerism

Optical Isomerism

• Optical Isomerism is a form of Stereoisomerism - when atoms have the same molecular formula but a different arrangement of atoms in Space. 
• Optical Isomers are like hands - no matter how you rotate your hands, you can never superimpose one hand onto the other. 
• Therefore, we describe Optical Isomers as non-superimposable mirror images - no matter how you rotate the unique groups around the central Carbon atom, you will never be able to superimpose one enantiomer onto another. 
• Optical Isomers must be chiral - this means that there are four different groups surrounding a central, chiral Carbon atom (the centre of chirality).
• To draw enantiomers, draw the first enantiomer and then draw fitted lines to the right of the enantiomer and draw the mirror image of the first enantiomer (thus producing the second enantiomer). 

Plane Polarised Light

• To test which enantiomer is present in a sample, you must shine plane polarised light at the sample. 
• Firstly, a beam of light is shone through a polaroid to polarise it.
• Plane Polarised Light refers to light which is on one plane, or axis.
• Then, shine the plane polarised light through the sample. 
• Use a second polaroid to determine the direction and angle of any rotation of the plane polarised light caused by the enantiomer. 
• For a (+)-isomer, the plane polarised light will be rotated in a clockwise direction.
• However, for a (-)-isomer, the plane polarised light will be rotated in an anti-clockwise direction.

Racemic Mixtures

• A Racemic Mixture refers to a mixture in which two enantiomers are in 50:50 (equal) concentration, therefore meaning that the optical activity of each enantiomer is cancelled out. 
• Racemic Mixtures form due to there being an equal probability of a Carbon atom in a Carbon-Oxygen bond being attacked on either side. 
• Racemic Mixtures do not rotate plane polarised light.

Thalidomide

• Thalidomide was a drug used to treat morning sickness, which is experienced during pregnancy. 
• The R-enantiomer causes sedative effects, but the S-enantiomer led to birth defects. 

Ibuprofen 

• Ibuprofen is a drug used to provide pain relief, particularly from headaches.
• The S+ isomer is used for pain relief, but the R- isomer has unacceptable side effects. 

Naproxen 

• One enantiomer is used to treat chronic arthritis, whilst the other causes liver poisoning.