Cis-trans isomerism is one type of stereoisomerism and is commonly also known as geometric isomerism. Stereoisomers have the same structure and functional groups but differ in the way their atoms are arranged in space (orientated differently with respect to each other). The other type of stereoisomerism is known as enantiomerism or optical isomerism.
Cis-trans isomerism occurs in compounds where free rotation is restricted by the presence of:
- multiple bonds
- a ring structure
- other steric factors i.e. large atoms
A single σ covalent bond can be rotated freely without any bond breaking. For example, the C-C bond in ethane can be rotated with minimal energy. The hydrogen atoms bonded to the carbon atom are constantly rotating along C-C bond axis.
However, this rotation will be restricted in either the presence of multiple bonds (double or triple bonds) or the presence of ring structures.
Let’s take a look closer by covering it in 2 sections:
- Cis-Trans Isomerism in Alkenes
- Cis-Trans Isomerism in Ring Structures
1. Cis-Trans Isomerism in Alkenes
Not all alkenes will exhibit cis-trans isomerism.
Criteria for Cis-Trans Isomerism
Two criteria must be satisfied in order for alkenes to exhibit cis-trans isomerism:
- Restricted rotation about