geomerical isometism happens due to.......... around p bond

Answer

### Step-by-Step Text Solution:

1. **Understanding Geometrical Isomerism**:
Geometrical isomerism, also known as cis-trans isomerism, occurs when there are different spatial arrangements of atoms or groups around a double bond (pi bond).

2. **Conditions for Geometrical Isomerism**:
For geometrical isomerism to occur, two main conditions must be met:
- The molecule must be planar, meaning that all atoms involved in the double bond must lie in the same plane.
- There must be restricted rotation around the double bond. This is because the presence of a pi bond creates a situation where the atoms cannot rotate freely as they can around a single bond.

3. **Identifying the Double Bond**:
The double bond (which can be a carbon-carbon double bond or a nitrogen-nitrogen double bond) is crucial for geometrical isomerism. The pi bond formed in a double bond prevents the rotation of the atoms attached to the double-bonded carbons or nitrogens.

4. **Cis and Trans Isomers**:
- In a **cis isomer**, similar groups (like hydrogen atoms or alkyl groups) are on the same side of the double bond.
- In a **trans isomer**, similar groups are on opposite sides of the double bond.

5. **Example of Geometrical Isomerism**:
Consider a molecule with a carbon-carbon double bond where one carbon is attached to a hydrogen atom and a methyl group:
- In the **cis configuration**, both the hydrogen and the methyl group are on the same side of the double bond.
- If we attempt to rotate around the double bond, we cannot do so freely due to the pi bond, leading to a change in configuration to the **trans isomer**, where the hydrogen and methyl group are on opposite sides.

6. **Conclusion**:
Therefore, geometrical isomerism occurs due to **restricted rotation around the pi bond**.