A2-Level Advanced Organic Chemistry
Carbon can form four covalent bonds, if a carbon atom is bonded to four different groups there are two possible ways of arranging the groups, creating two possible isomers
Each isomer is a mirror image of the other and both isomers rotate plan polarised light in opposite directions.
If equal amounts of both isomers are present in a mixture, there is no overal rotation of plane polarised light - the mixture is racemic.
Each atom of carbon can make four covalent bonds. The electrons in each covalent bond repel each other and make the bonds move as far away from each other as possible.
The arrangement formed gives each of the four bonds around a carbon atom an angle of 109.5º. If the carbon is bonded to hydrogens, a simple tetrahedral shaped molecule is formed (methane).
If the groups bonded to the carbon are all different, things get interesting. Each bond is ‘locked’ in place around the carbon and cannot move.
There are two different ways you can arrange the four groups, each producing an isomer. The key thing here is the three-dimensional view of each isomer.
If you look carefully at the two isomers, they are mirror images of one another. They are non-superimposable mirror images of each other.
Non-superimposable mirror images
Take your hands and put them out flat in front of you. Place the right hand on top of the left hand. The fingers and thumbs will not line up. The thumb of the right hand is above the little finger of the left hand, and the little finger of the right hand is above the thumb of the left hand. This is what we mean by non-superimposable – two images that cannot be placed on-top of each other and ‘line-up’.
If you repeat the process with two left hands (not actually possible with your own hands!), the hands would superimpose.
If we take our tetrahedral structure we can do the same thing, although it is a bit harder to visualise.
How to draw optical isomers
Keep it simple, every time.
To draw a right hand, you just draw the same shape as a left hand but with the thumb taking the place of the little finger, and so on.
This is drawing a mirror image and it’s the same for molecules. Start with one isomer and imagine it’s your left hand, and the other isomer is the right hand.
Optical isomers are very hard to identify, very often their chemistry is identical, and therefore any chemical analysis on both isomers would give the same results.
Plane Polarised Light
Please note, this is a simplification for A-level purposes to show the idea of rotating plane polarised light to help visualise the effect only.
When light is shined into a molecule, the bonds within the molecule absorb and rotate the light. This means the light leaves the molecule at a different angle to the way it entered.
The exact angles of scattering and the behaviour of light in a molecule are very complicated, but the bonds and the arrangement of atoms in a molecule determine how the light is scattered.
Remember optical isomers have the same types of bonds but they are arranged in a different way, causing light to be scattered in the opposite direction.
Imagine your left and right hand again, hold them out flat. If when light comes down and hits your middle finger it is scattered towards your thumb, so the direction of light being scattered will be different for the left hand compared to the right hand. The light is hitting both hands at the same angle, but it’s being scattered in a different direction.
If you have both hands held out flat, the effect of scattering would be cancelled out as for every light ray that is scattered in one direction, another is scattered in the opposite direction.
However, if you only had a left or right hand, you would be able to detect how the light is being scattered and determine which hand you have.
This, in very simple terms, is how we identify optical isomers. Instead of ‘scattering’ light in different directions, optical isomers actually rotate plane-polarised light in opposite directions.
Light waves are three dimensional, which means the effect of a light ray is felt in three directions around a wave.
When an optical isomer rotates light, it is not possible to observe the effect unless the light is a very thin beam in only two dimensions. To produce this beam, the light has to be plane polarised.
Now, it is easy to see how the light has been rotated and which isomer is responsible for the effect.
If the optical isomers of a compound are present in equal amounts, the effect will be cancelled out, and the plane polarised light will not be rotated overall in either direction.
A mixture that contains equal amounts of both optical isomers is called a racemic mixture.