Isomerism Flashcards
Define structural isomerism.
They have the same molecular formula but a different structural formula.
Define stereoisomerism.
They have the same molecular formula but a different arrangement of atoms in space.
Give the two forms of stereoisomerism.
Geometric (EZ) isomerism and optical isomerism.
Explain what is meant by an optical isomer.
A molecule that is optically active. That is to say is has a chiral centre (is asymmetrical) that has four different groups attached to it. It will rotate plane polarised light.
What do we call a carbon atom that has four different groups attached?
A chiral carbon.
How do we show a chiral carbon on a displayed formula?
With an asterisk.
What are enantiomers?
Either the + or - version of the optical isomer. They cannot be superimposed, and they are mirror images of each other.
How do we draw optical isomers?
Always in a tetrahedral shape, with the chiral carbon in the centre. Then draw the mirror image next to it.
Explain how you would detect an optical isomer.
Project plane polarised light through a sample. Use a polarimeter to detect if, and in which direction, the plane polarised light has rotated.
Define a racemate.
A racemate contains equal quantities of each enantiomer of an optically active compound. They don’t show any optical activity using a polarimeter.
Usually, why is there an equal chance of forming each type of enantiomer in the lab?
Because the carbon being attacked in a mechanism has an equal chance of being attacked above or below the atom.
Why do we not usually modify reactions to produce a single enantiomer?
Because it is difficult and expensive.
Why must pharmaceutical drugs only contain a single optical isomer?
Most drugs work by binding onto an active site of which only one enantiomer will fit. If the wrong optical isomer is used, it may cause harmful side effects (or have no effect).
Give an example of a drug whose optical isomers have different effects.
Thalidomide.
