2 butene show geometrical isomerism due to restricted rotation about the double bond and the geometrical isomers of 1-butene are cis-2-butene and trans-2-butene

Geometrical isomerism in 2-butene:

Geometrical isomerism is a type of stereoisomerism that arises due to the restricted rotation about a double bond or a ring. In the case of 2-butene, the presence of a double bond between the second and third carbon atoms (C2 and C3) leads to the possibility of geometrical isomerism.

Explanation:

Rotation about a double bond:
- The double bond in 2-butene consists of a sigma bond and a pi bond.
- The sigma bond allows free rotation around it, but the pi bond restricts the rotation.
- The pi bond consists of a side-by-side overlap of p orbitals, which are perpendicular to the plane of the molecule.
- This pi bond creates a barrier to rotation, making it difficult for the two groups attached to the double bond to freely rotate around it.

Geometrical isomerism:
- Geometrical isomerism occurs when there are two different groups attached to each carbon atom of a double bond.
- In 2-butene, the two groups attached to the C2 and C3 carbon atoms can be either both on the same side of the double bond (cis isomer) or on opposite sides (trans isomer).
- The cis isomer has both methyl groups on one side of the double bond, while the trans isomer has the methyl groups on opposite sides of the double bond.
- Due to the restricted rotation about the double bond, these two isomers cannot interconvert by a simple rotation.
- Therefore, they exist as separate entities and show geometrical isomerism.

Chiral carbon:
- Chiral carbon is a carbon atom that is bonded to four different groups.
- It is not applicable to 2-butene because both carbon atoms (C2 and C3) of the double bond have only three different groups attached to them.

Conclusion:
The reason for geometrical isomerism in 2-butene is the restricted rotation about the double bond. The presence of a double bond restricts the rotation, leading to the formation of cis and trans isomers that exhibit geometrical isomerism.