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Summary AQA A Level Chemistry Unit 3.3.7 Optical Isomerism Notes
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Notes relating to Year 13/A2 Organic chemistry topic - optical isomerism.
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Optical IsomerismStereoisomers are molecules that have the same structural
formula but that atoms are arranged differently in a 3D space.
There are t wo types of stereoisomerism:
• Geometric (E/Z)
• Optical
A carbon atom that has 4 different atoms or groups of atoms
attached to it, is called a chiral carbon or chiral centre.
Compounds with a chiral centre (chiral molecules) exist as 2
optical isomers which are also called enantiomers.
To identify a chiral carbon, you need to identify the carbon that
has 4 different groups attached to it.
E.g. 2-Bromobutane ( CH3-CH2-CHBr-CH3 )
• C has 3 Hydrogens around it (same group) so is not the chiral
carbon
• C has 2 Hydrogens around it (same group) so is not the chiral
carbon
• C has a Hydrogen molecule, a methyl group (CH3), a bromine
molecule and an ethyl group (C2H5) around it (different groups)
so it is the chiral carbon
• C has 3 Hydrogens around it (same group) so is not the chiral
carbon
2-Bromobutane is optically active.
, In drawings, the chiral carbon is marked with an asterisk (*).
Drawing Optical Isomers:
• Start by drawing a vertical dotted /dashed line in the center of
the page to represent a line of symmetry
• Draw the chiral carbon with four bonds in a tetrahedral
arrangement
• Make sure t wo bonds lie in the plane of the paper, one bond
comes out (solid wedge) & one bond receding ( hatched/ dotted
wedge)
• Add the four groups. ( Ensure you show the mirror image
sequence of atoms e.g. COOH would be HOOC )
Enantiomers are non-superimposable mirror images of each other.
Their physical and chemical properties are identical but they
differ in their ability to rotate plane polarised light. Hence, why
they are called optical isomers.
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