One shot ISOMERISM 02 (G.I) ORGANIC CHEMISTRY CRASH COURSE Class 11th/12th/JEE MAINS /NEET2025

One shot ISOMERISM 02 (G.I) ORGANIC CHEMISTRY CRASH COURSE Class 11th/12th/JEE MAINS /NEET2025
Complete Geometrical Isomerism
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Geometrical Isomerism: A Comprehensive Explanation
Introduction
Isomerism is a fundamental concept in chemistry that describes compounds with the same molecular formula but different structures. One of the most significant types of isomerism is stereoisomerism, which occurs when compounds have the same connectivity but differ in the spatial arrangement of their atoms. Among stereoisomers, geometrical isomerism (also known as cis-trans isomerism or E-Z isomerism) is a key subclass that arises due to restricted rotation around a bond, typically a double bond (C=C) or in cyclic structures.
Understanding Geometrical Isomerism
Definition
Geometrical isomerism is a type of stereoisomerism in which molecules have the same molecular and structural formulas but differ in the spatial arrangement of atoms or groups due to restricted rotation around a bond.
The most common causes of restricted rotation are:
1. Double Bonds (C=C): In alkenes and similar compounds, the presence of a π-bond prevents free rotation.
2. Cyclic Structures: In rings, the rigidity of the cyclic system prevents free rotation of attached groups.
As a result, two different spatial arrangements arise, leading to geometrical isomers.
Types of Geometrical Isomerism
1. Cis-Trans Isomerism
Cis-trans isomerism is the simplest form of geometrical isomerism and is commonly observed in alkenes and cyclic compounds.
Cis-isomer → Identical or similar groups are on the same side of the double bond or ring.
Trans-isomer → Identical or similar groups are on opposite sides of the double bond or ring.
Examples of Cis-Trans Isomerism
(a) Alkenes (C=C Compounds)
Alkenes commonly exhibit cis-trans isomerism because the π-bond in the double bond prevents free rotation.
Example: But-2-ene (C₄H₈)
Cis-But-2-ene: The two methyl (-CH₃) groups are on the same side of the double bond.
Trans-But-2-ene: The two methyl (-CH₃) groups are on opposite sides of the double bond.
(b) Dihaloalkenes
Example: 1,2-Dichloroethene (C₂H₂Cl₂)
Cis-1,2-Dichloroethene: Both Cl atoms are on the same side.
Trans-1,2-Dichloroethene: The Cl atoms are on opposite sides.
(c) Cyclic Compounds
Cyclic compounds also exhibit cis-trans isomerism due to restricted rotation within the ring.
Example: 1,2-Dimethylcyclohexane (C₆H₁₂)
Cis-1,2-Dimethylcyclohexane: Both methyl (-CH₃) groups are on the same side of the ring.
Trans-1,2-Dimethylcyclohexane: The methyl (-CH₃) groups are on opposite sides of the ring.
2. E-Z Isomerism (Cahn-Ingold-Prelog Priority Rules)
Cis-trans nomenclature becomes ineffective when there are more than two different groups attached to the double-bonded carbons. To address this, the E-Z system is used based on Cahn-Ingold-Prelog priority rules.
E-isomer ("Entgegen" - Opposite) → The higher priority groups are on opposite sides.
Z-isomer ("Zusammen" - Together) → The higher priority groups are on the same side.
Example: 2-Bromo-2-butene (CH₃-CH=C(Br)-CH₃)
E-2-Bromo-2-butene: The Br and CH₃ with the highest priority are on opposite sides.
Z-2-Bromo-2-butene: The Br and CH₃ with the highest priority are on the same side.
Conditions for Geometrical Isomerism
For a compound to exhibit geometrical isomerism, the following conditions must be met:
1. Restricted Rotation: There must be a double bond (C=C) or a cyclic structure that prevents free rotation.
2. Each Carbon Must Have Two Different Groups: If one of the doubly bonded carbons has identical groups, geometrical isomerism is not possible.
Example: Propene (CH₃-CH=CH₂) does NOT exhibit geometrical isomerism because one of the doubly bonded carbons has two hydrogen atoms.
Physical and Chemical Properties of Geometrical Isomers
1. Physical Properties
Boiling Point:
Cis-isomers generally have higher boiling points due to stronger dipole-dipole interactions.
Trans-isomers have lower boiling points due to their symmetrical shape, leading to weaker intermolecular forces.
Melting Point:
Trans-isomers have higher melting points because of better packing in the solid state.
Polarity:
Cis-isomers tend to be more polar, whereas trans-isomers are often nonpolar due to symmetry.
2. Chemical Properties
Cis-isomers are usually more reactive due to the proximity of functional groups.
Trans-isomers tend to be more stable due to lower steric strain.
Importance and Applications of Geometrical Isomerism
1. Biological Significance
Retinal (Vision Process): The cis-trans isomerization of retinal in the eye is critical for vision.
Fats and Lipids: Cis-fats are natural and healthy, while trans-fats (found in processed foods) are harmful.
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