In this lesson, we will learn:
- How optical isomerism is created from chiral carbon atoms.
- The meaning of the terms chiral centre, enantiomer and racemic mixture.
- How to distinguish enantiomers using the method of polarimetry.
- Understand how optical/polarimetry data is used as evidence for Sn1 or Sn2 reactions.
- We saw in Molecular Geometry that when carbon makes four single bonds to other atoms, it takes a 3d tetrahedral shape. When a carbon atom makes four bonds all to different atoms or entities, it is known as a chiral centre and it creates asymmetry in the whole molecule.
For another example, look at both of your hands in front of you:
- Both hands are made of the same parts – a palm, the back, four fingers, a thumb and the base where your wrist starts.
- Your left and right hands are 3d mirror images of each other.
- It is impossible to make one hand fit the other’s ‘arrangement’ in space in exactly the same way. Your left and right hands are non-superimposable mirror images .
When a carbon chiral centre
exists in a molecule, it will have these properties like being a left or right hand.
The two left-hand / right hand molecules are known as enantiomers
or optical isomers
of one another - any pair of molecules that are non-superimposable mirror images.
See the diagram below:
Molecules 1 and 2 above are made of the same components, but this asymmetry from their arrangement means they are two different chemical compounds with two different sets of chemical properties!
These two different chiral molecules will also interact with other chiral molecules differently (such as in the human body) which has major consequences with some medicinal compounds. This is like shaking hands – it works with two right hands. One right-hand and one left-hand shaking does not fit in the same way!
For this reason, you must be able to use the wedges/dashes to draw 3d molecules correctly – getting them wrong when drawing enantiomers is simply drawing the wrong molecule.
The reason this type of isomerism with chiral centres is called optical isomerism is because enantiomers with a chiral centre are optically active. Enantiomers are able to rotate the plane of polarised light shone at them. The two enantiomers will rotate the plane in opposite directions, so not only will you identify a chiral molecule, you can tell one enantiomer from the other. This method of studying enantiomers is called polarimetry.
Sometimes when creating a chiral product, polarimetry cannot be used. Because the two enantiomers rotate polarised light antagonistically, if there is an equal amount of both enantiomers, the net rotation will be zero. This is what happens with racemic mixtures .
A racemic mixture, or racemate, is any mixture of a pair of enantiomers in equal quantity.
Optical isomerism is a useful to get evidence of the reaction mechanism. In Finding the rate equation, we saw two types of mechanism for a nucleophilic substitution:
- Sn1 reactions, where the rate determining step has only one molecule. In other words, reactant bond breaking and product bond forming (with the nucleophile) happen in separate stages from one another.
- Sn2 reactions, where the rate determining step has two molecules.
We can find evidence of the reaction mechanism because racemization happens in Sn1 reactions
- In Sn1 reactions, when a reactant bond on a carbon atom breaks, the carbocation formed is achiral and trigonal planar in shape. This gives the nucleophile an equal chance of attacking either side of this planar surface, so the product is a racemate – an equal mixture of both enantiomer products. Sn1 reactions can therefore convert enantiopure reactants to a racemate product. /li>
- In Sn2 reactions any stereochemistry is preserved because the bond breaking and making happen at the same time. This means the carbon centre does not become planar, and the nucleophile has only one avenue of attack – directly behind the leaving group. This means that although the stereochemistry gets inverted in the product, racemic mixtures do not appear in Sn2 reactions .
See the image below: