chemistry - 4.3 Flashcards
What are isomers?
Molecules that have the same molecular formula but different structural formulae.
When do structural isomers occur?
When the atoms are bonded together in a different order in each isomer.
When do stereoisomers occur?
When the order of the bonding in the atoms is the same but the spatial arrangement of the atoms is different in each isomer.
What are the two types of stereoisomer?
Geometric and optical.
When can geometric isomers occur?
When there is restricted rotation around a carbon-carbon double bond or a carbon-carbon single bond in a cyclic compound.
What must geometric isomers have?
Two different groups attached to each of the carbon atoms that make up the bond with restricted rotation.
What can geometric isomers be labelled according to?
Cis or trans according to whether the substituent groups are on the same side (cis) or on different sides (trans) of the bond with restricted rotation.
What do geometric isomers have differences in?
Physical properties, such as melting point and boiling point. They can have differences in chemical properties.
Where do optical isomers occur?
In compounds in which four different groups are arranged tetrahedrally around a central carbon atom (chiral carbon or chiral centre)
What are optical isomers?
Asymmetric and are non-superimposable mirror images of each other.
What can optical isomers be described as?
Enantiomers.
What do optical isomers have in common?
They have identical physical properties, except for their effect on plane- polarised light. They also have identical chemical properties, except when in a chiral environment such as that found in biological systems (only one optical isomer is usually present).
What do optical isomers rotate plane-polarised light by?
The same amount but in opposite directions and so are optically active.
Why, when mixed in equal amounts, are optical isomers optically inactive?
The rotational effect of the plane-polarised light cancels out - this is called a racemic mixture.
