Carbon double bonds are unable to rotate freely like single bonds, they have restricted rotation
Groups or atoms bonded to carbon atoms in a double bond are ‘locked’ into position, and there are two possible ways they can be arranged.
Stereoisomerism occurs when two molecules have the same molecular and structural formula, but atoms within the molecules are arranged in space differently.
In Z-isomers, the highest priority groups bonded to each carbon in the double bond are pointing in the same direction.
In E-isomers, the highest priority groups bonded to each carbon in the double bond are pointing in opposite directions.
Cis and trans isomers are forms of Z and E isomers, but both carbons in the double bond are bonded to the same type of groups.
Stereoisomerism (E and Z, Cis and Trans)
A carbon-carbon double bond cannot rotate within a molecule like a single bond can, it has restricted rotation. If the double bond was to rotate, the merged p-orbitals creating a pi-bond would be placed under great strain and would eventually break – meaning a double bond no longer exists between the carbon atoms.
As a double bond cannot rotate, the three dimensional structures of alkenes can differ depending upon the groups bonded to each carbon within the carbon-carbon double bond.
Molecules with the same molecular formula but different structures are called isomers. In alkenes, the structural formulas can be the same, but the atoms are arranged differently in three-dimensions; they are stereoisomers.
As stereoisomers have different structures they can behave differently as molecules. However, because they have the same structural and molecular formula we have to be able to distinguish between different stereoisomers of a compound. We call two isomers either Z or E isomers.
In order to name either a Z or E isomer, you have to prioritise the groups bonded to each carbon. If the highest priority group on both carbons point in the same direction, they are Z isomers. I always remember this as Z=zame side. (It works for me!).
If the highest priority group on both carbons point in opposite directions, they are E isomers.
To further confuse things, we can also assign a further description.
If all groups on both carbons are the same, we can use ‘cis’ and ‘trans’ as names.
Cis isomers have the same groups bonded to each carbon atom in the double bond, but they are on opposite sides.
Trans isomers have the same groups bonded to each carbon atom in the double bond, but they are on the same side.
One of the most confusing parts of this is that cis and trans are forms of Z and E (respectively) isomers, but Z and E isomers are not necessarily cis and trans isomers.