Stereochemistry Flashcards
What is an isomer?
Isomers are molecules that have the same molecular formula but their atoms are arranged differently. They often have different physical and chemical properties.
Describe the following two types of isomer:
a) Structural isomer
b) Stereoisomer
a) Structural isomer - Atoms are bonded toghether in a different order.
b) Stereoisomer- The order the atoms are bonded together in is the same but the arragment of the atoms in space is different. Can either be geometric or optical.
Describe geometric isomerism.
Geometric isomerism usually occurs in molecules with a double bond, or in saturated rings. It happens as a result of lack of free rotation around a bond. One isomer has both of the attached groups/atoms on one side and so its name will begin with ‘cis’. The other isomer has one attached groups/atoms on each side of the bond and so its name will begin with ‘trans’.
Where does optical isomerism usually occur?
Optical isomerism can occur in compounds where there are 4 different groups arranged tetrahedrally around a central carbon atom. These molecules are described as being chiral.
Describe optical isomerism.
Each optical isomer (also called enatiomer) is the mirror image of the other but the two molecules are non-superimposable which means they do not fit perfectly when placed ontop of eachother (like hands and feet).
How are the effects of optical isomers on plane-polarised light (light in one plane) different?
One of the optical isomer solutions will rotate the plane of the polarised light a certain angle, and the other optical isomer solution of the same concentration (equimolar solution) will rotate the plane of the polarised light by exactly the same angle but in the opposite direction.
What effect would an equimolar (same concentration) mixture of the two optical isomers have on plane-polarised light?
It would have no effect because the rotational effect of one isomer would be cancelled out by the other. This type of mixture is optically inactive and is known as a racemic mixture.
