Geometrical Isomerism

Geometrical isomerism is a form of stereoisomerism that arises from the restriction of rotation around a bond, typically a double bond or a ring structure, leading to isomers with different spatial arrangements of groups attached to the atoms involved in the bond. These isomers are called geometrical isomers or cis-trans isomers.

Key Concepts

• Stereoisomerism: Isomers that have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in space.
• Geometrical Isomerism: A type of stereoisomerism where isomers have different spatial arrangements due to the restricted rotation around a bond.
• Cis-Trans Isomerism: The most common type of geometrical isomerism, where the two substituents can either be on the same side (cis) or opposite sides (trans) of a double bond or a ring.

Conditions for Geometrical Isomerism

Geometrical isomerism occurs under the following conditions:

1. There must be restricted rotation around the bond. This is typically found in alkenes (with a C=C double bond) and cyclic compounds.
2. Each carbon of the double bond (or the relevant atoms in a ring) must have two different substituents.

Differences Between Cis and Trans Isomers

Property	Cis Isomer	Trans Isomer
Position of Groups	Same side of the double bond or ring	Opposite sides of the double bond or ring
Dipole Moment	Usually higher due to the additive effect of the dipoles	Usually lower due to the canceling effect of the dipoles
Symmetry	Less symmetrical	More symmetrical
Melting Point	Lower due to less efficient packing	Higher due to more efficient packing
Boiling Point	Higher due to stronger dipole-dipole interactions	Lower due to weaker dipole-dipole interactions
Stability	Generally less stable	Generally more stable

Examples of Geometrical Isomerism

Alkenes

For alkenes, the simplest example is but-2-ene:

• Cis-but-2-ene: The two methyl groups (CH₃) are on the same side of the double bond.

• Trans-but-2-ene: The two methyl groups are on opposite sides of the double bond.

Cyclic Compounds

For cyclic compounds, consider 1,2-dichlorocyclohexane:

• Cis-1,2-dichlorocyclohexane: The two chlorine atoms are on the same side of the ring.

• Trans-1,2-dichlorocyclohexane: The two chlorine atoms are on opposite sides of the ring.

Determining Geometrical Isomers

To determine if a compound can exhibit geometrical isomerism, one must look at the structure and apply the following rules:

1. Identify the double bond or ring structure.
2. Check if each carbon of the double bond (or relevant atoms in a ring) has two different substituents.
3. If both conditions are met, the compound can exhibit geometrical isomerism.

E-Z Notation

For alkenes with more complex substituents, the cis-trans notation is not sufficient. Instead, the E-Z notation is used based on the Cahn-Ingold-Prelog priority rules:

• E (Entgegen): The higher priority groups are on opposite sides of the double bond.
• Z (Zusammen): The higher priority groups are on the same side of the double bond.

To assign E or Z:

1. Assign priorities to the substituents based on atomic number (higher atomic number = higher priority).
2. Compare the groups on each carbon of the double bond.
3. Use the E-Z notation to describe the isomer.

Conclusion

Geometrical isomerism is an important concept in organic chemistry that affects the physical and chemical properties of compounds. Understanding the differences between cis and trans isomers, as well as how to determine and name them, is crucial for students and professionals in the field.