Explain Cis-trans isomerism in detail with examples and diagram

Answer 1

Cis-trans isomerism, also known as geometric isomerism, is a type of stereoisomerism that occurs when different groups or atoms are arranged differently around a double bond or a ring in a molecule. These isomers have the same molecular formula but differ in the spatial arrangement of atoms.

In cis-trans isomerism, the positioning of substituents can be on the same side of the double bond or ring (cis) or on opposite sides (trans). This difference in arrangement affects the physical and chemical properties of the isomers.

To illustrate this, let's consider an example of cis-trans isomerism in a simple molecule called 2-butene (C4H8). 2-butene has a double bond between the second and third carbon atom. The two carbon atoms adjacent to the double bond are labeled as carbon 1 and carbon 4.

In the cis form of 2-butene, both methyl groups are on the same side of the double bond. So, carbon 1 is attached to two hydrogen atoms and two methyl groups, while carbon 4 is attached to one hydrogen atom and one methyl group. The structural formula of cis-2-butene can be represented as:

H H
│ │
H ─ C ─ C = C ─ H
│ │
H CH3

In the trans form of 2-butene, the two methyl groups are on opposite sides of the double bond. So, carbon 1 is attached to two hydrogen atoms and one methyl group, while carbon 4 is attached to one hydrogen atom and two methyl groups. The structural formula of trans-2-butene can be represented as:

H H
│ │
H ─ C = C ─ C ─ H
│ │
H CH3

As you can see, the cis and trans isomers have different spatial arrangements of their substituents, resulting in distinct physical and chemical properties. For example, cis-2-butene has a higher boiling point compared to trans-2-butene because the cis isomer has a dipole moment due to the asymmetrical distribution of substituents.

Cis-trans isomerism can also occur in cyclic compounds, such as cycloalkanes and cycloalkenes. In these cases, the cis form refers to substituents on the same side of the ring, while the trans form refers to substituents on opposite sides.

In summary, cis-trans isomerism arises when different groups or atoms are arranged differently around a double bond or ring in a molecule. This isomerism has significant implications for the physical and chemical properties of compounds.