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 C=C double bond (due to the presence of π bond)
- The 2 groups bonded to any one carbon in C=C must not be identical
This will give rise to two isomers: cis isomer and trans isomer.
Let’s consider the molecule of but-2-ene:
But-2-ene satisfied the two criteria for cis-trans isomerism. As such, but-2-ene exists as two distinct and separate isomers.
- The isomer where the same groups are on the same side of the C=C double bond is known as the cis isomer.
- The isomer where the same groups are on opposite side of the C=C double bond is known as the trans isomer.
Cis-trans isomers have similar (but not identical) chemical properties but different physical properties
Cis isomer has a higher boiling point because of its higher polarity.
Cis isomer has lower melting point because of its lower symmetry. It has lower packing efficiency in the solid state than the trans isomer. It packs less closely in the solid state than the trans isomer and have more empty spaces between its molecules. The molecules of the cis isomer are further apart, and hence the attraction between the molecules is weaker. Less energy is needed to separate them during melting.
What if all 4 groups on C=C bond are different?
We can also have different groups attached to the 2 carbon atoms of the C=C double bond and still have cis-trans isomerism.
In this case, we need to consider the Group Priority (formally known as E/Z notation). Their naming convention is not covered under the A-Level H2 Chemistry syllabus and we will not be discussing it in this blog.
Maximum Possible Number of Cis-Trans Isomers
Maximum number of isomers = 2n whereby n is the number of C=C double bonds in the alkene molecule
The above formula gives the maximum possible number of cis-trans isomers. To see whether is it correct, you need to draw out all the isomers and check whether there are any identical isomers.
How many cis-trans isomers can hepta-2,4-diene have? The skeletal formula is given below.
Try it out and leave your answer in the Comment Section below.
Do note that though deuterium (D) is an isotope of hydrogen (H), they are considered as two different groups.
Also, be very clear about what you write in Chemistry examinations. Do note write double bond. You must write C=C double bond because there are many types of double bonds.
2. Cis-Trans Isomerism in Ring Structures
Similar to the C=C bonds in alkenes, the restricted rotation of a covalent bond due to a ring structure allows for cis-trans isomerism.
Cis isomer is the one where the same groups exist on the same side with respect to the plane of the ring.
Trans isomer is the one where the same groups exist on the opposite sides of the plane of the ring.
Cyclooctene is a cycloalkene with an eight-membered ring. It is notable because it is the smallest cycloalkene that can exhibit as either the cis- or trans-isomer.
Usually, cycloalkenes just have the cis isomer. The trans isomer is not possible due to significant ring strain in the molecule.
So for any cycloalkene molecules to exhibit cis-trans isomerism, it must have at least 8 carbon atoms in the ring in order to minimise ring strain.
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