Isomerism are molecules that have the same molecular formula, but have a different arrangement of the atoms in space. That excludes any different arrangements which are simply due to the molecule rotating as a whole, or rotating about particular bonds.
Stereoisomers are molecules that have the same molecular formula, but the atoms are joined to each other in a different spacial arrangement – they occupy a different position in 3-dimensional space.
- An example of stereoisomerism found in some alkenes
- C=C bonds in alkenes have restricted rotation.
- while single covalent bonds can rotate easily, therefor alkanes cannot have geometrical isomers
- Found in two forms CIS and TRANS
- CIS: Groups/atoms are on the same side of the double bond.
- TRANS: Groups/atoms are on opposite sides across the double bond.
Questions regarding geometrical isomerism are often seen in AS/A-level Chemistry exam! 👨🏫 Don't mix up CIS and Trans!
- Trans isomers have higher melting points than cis isomers.
- However, cis isomers have higher boiling points than trans isomers.
- Another form of stereoisomerism
- Occurs when compounds have non-superimposable mirror images
- The two isomers are known as optical isomers or enantiomers
- They occur when molecules have a chiral centre
- A chiral centre contains an asymmetric carbon atom
- An asymmetric carbon atom has four different atoms (or groups) arranged tetrahedrally around it.
- If a carbon has more than one of any atom/group attached, it cannot be chiral
Identical physical properties (except for polarised light)
Identical chemical properties (unless reacting with other optical isomers when the orientation of the two molecules could have a bearing on the final result)
Drafted by Cherry (Chemistry)