Isomerism Notes
Isomerism in year 2 can be broken down in to the following parts
Optical Isomerism
Plane polarised Light and Racemic Mixtures
Making Optical Isomers from carbonyls
Optical Isomers
When a carbon atom within a molecule has 4 different groups attached to it it is classed as a chiral centre as it produces optical isomers. Sometimes referred to as chirality, the molecules can also be described as enantiomers.
The above molecules have the same molecular formula but they are arranged differently in space, stereoisomers. It may be difficult to notice but these molecules are mirror images of each other and therefore can not supper impose over each other, just like your hands, there is a left hand molecule and right hand molecule.
The different groups could also be different chain lengths, for example the hydrocarbon 3-Methylhexane also has a chiral centre, marked, so has optical isomers.
The highlighted carbon, a chiral centre, is effectively attached to a hydrogen, a methyl, an ethyl, and a propyl group.
Optical isomers have virtually identical chemical properties so it is extremely difficult to separate them from each other.
Plane-Polarised light and Racemic Mixtures
Optical isomers can either be S or R, effectively left or right handed. To test for the enantiomers present plane-polarise light is used. Light can be passed through a filter and become plane polarised, if it then passes through a sample it can rotate the light clockwise or anti-clockwise, the optical isomer of the sample will rotate the light in the opposite direction.
A racemic mixture, sometimes call racemate, is when there is a 1:1 ratio, equal amounts, of each optical isomer. If plane polarised light was to pass through a racemic mixture there would appear to be no affect, this is because half of the mixture would be rotating the light clockwise and the second half of the mixture would be turning it anti-clockwise to the same degree.
Making Optical Isomers from Carbonyls
Some nucleophilic addition reactions will generate enantiomers, with the addition of a nitrile to an aldehyde or asymmetrical ketones. After the reaction for an aldehyde the final carbon will be bonded to a hydrogen, a hydroxy, a nitrile and an aliphatic chain and as all four are different it will have optical isomers.
The nucleophile, CN, can react with the carbonyl from above or below the plane, this will produce either the left or right handed molecule. The direction of the reaction is equally likely so will produce a racemic mixture, which will have no affect on plane polarised light.
Identifying Optical Isomer Products
Which of the following would produce optical isomers if reacted with KCN and then dilute acid.
Propanone
Propanal
Ethanal
Methanal
Pentan-2-one
When trying to work out the above if the molecule is symmetrical at the point of the carbonyl it will not produce optical isomers.
Propanone no isomers as the carbon would have 2 methyls
Propanal Yes, isomers are produced as asymmetrical on the carbonyl
Ethanal Yes, isomers are produced as asymmetrical on the carbonyl
Methanal No, even though it is an aldehyde the carbonyl is symmetrical because both sides have a hydrogen.
Pentan-2-one Yes, the carbonyl has a methyl on one side and an ethyl on the other, so it is asymmetrical and will generate optical isomers.
Worksheets
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