Geometrical Isomerism

Geometrical Isomerism is a type of stereoisomerism where compounds differ in spatial arrangement around a double bond or ring, such as cis and trans forms.

Definition of Geometrical Isomerism:

• Geometrical isomerism is a type of stereoisomerism (same molecular formula and bonding order, but different spatial arrangements of atoms or groups).
• It occurs due to the restricted rotation around double bonds or within cyclic compounds, especially when two different groups are attached to the atoms involved in the restriction.

Why rotation is restricted?

• In a C=C double bond, one bond is sigma (σ), and the other is pi (π).
• The π-bond is formed by the sideways overlap of unhybridized p orbitals.
• Rotating around the double bond would break the π-bond, which requires energy.
• Therefore, rotation is restricted, and different spatial arrangements are possible.

Conditions for geometrical isomerism:

1. There must be a rigid structure like:
   • C=C double bond
   • C=N double bond (in oximes)
   • Ring systems (cyclic compounds)
2. Each of the doubly bonded carbon atoms (or the atoms with restricted rotation) must be attached to two different groups.

Examples:

• Alkenes: 2-butene (CH₃CH=CHCH₃) – exists in cis and trans forms.
• Cyclic compounds: 1,2-dichlorocyclohexane – can show cis and trans forms based on the position of Cl atoms.
• Oximes: CH₃CH=NOH (aldoximes and ketoximes)

For example:

• Swapping the positions of H and CH₃ on one carbon leads to a different isomer.

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