Optical Isomerism and Chirality | A-level Chemistry | OCR, AQA, Edexcel

Optical Isomerism and Chirality in a Snap!

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The key points covered in this video include:
1. What is an optical isomer?
2. Propoerties of optical isomers
3. Chiral centres

Optical Isomers

In the video on stereoisomerism of complex ions we introduced the idea of optical isomerism. Optical isomers are non-superimposable mirror images of each other. We can illustrate this with a carbon which has four different groups attached. If we try to rotate the molecule on the right to match it up with the one on the left: It’s impossible to realign these different isomers - we call them enantiomers. A macroscopic example of this concept is our hands: This is why we must have the right gloves on the right hands!

Properties of Optical Isomerism

Both enantiomers have the same chemical properties and similar physical properties. They can however have drastically different biological properties. Limonene is a good example of this: One of these isomers smells to us like oranges, whereas the other smells of pine. Another infamous example of optical isomerism is Thalidomide. Thalidomide was a drug that was developed and tested for treating nausea in pregnancy, and one isomer was safe. However the other enantiomer caused birth defects, and even if mothers were given the right one, their bodies converted it to the other. A main difference in physical properties is how one isomer will rotate plane polarised light clockwise and the other will rotate it anticlockwise. A 50:50 mix on the isomers is called a racemic mixture and will have no effect on light as the effects cancel each other out.

Chirality

To form an optical isomer we must have a carbon with four different groups attached - we call these carbons a chiral centre. We can have larger molecules with more than one chiral centre. Chiral centres are asymmetrical so we cannot have symmetry in the molecules - we label chiral centres with a star. All amino acids with the exception of glycine have a chiral centre and are usually made in labs in racemic mixtures.

Summary

An optical isomer is a non-superimposable mirror image of a particular molecule
Optical isomers have different biological properties to each other and will rotate plane polarised light in opposite directions
Chiral centres are the carbon with four different groups attached which makes a molecule optically active
Most amino acids have a chiral centre