Geometrical Isomerism in Alkenes: The CIS-TRANS and E-Z Nomenclature
Master the art of distinguishing between different spatial arrangements in alkenes and predict geometrical isomers with confidence.
Why Geometrical Isomerism Matters
Geometrical isomerism arises due to restricted rotation around double bonds in alkenes. These isomers have the same molecular formula but different spatial arrangements, leading to distinct physical and chemical properties.
🎯 JEE Importance
Geometrical isomerism questions appear in every JEE paper, testing your understanding of stereochemistry, priority rules, and isomer prediction. Mastering this topic can secure 2-4 easy marks.
🚀 Quick Navigation
1. Conditions for Geometrical Isomerism
Essential Requirements
Double bond prevents free rotation, locking groups in position
Each carbon of double bond must have two different substituents
Examples
Shows Geometrical Isomerism
CH₃-CH=CH-CH₃
Different groups on both carbons
No Geometrical Isomerism
CH₂=CH-CH₃
Same groups on one carbon
2. CIS-TRANS Nomenclature System
The Traditional Approach
CIS-TRANS system works when each carbon of the double bond has at least one identical group.
CIS Configuration
Definition: Similar groups are on the same side of the double bond
CIS-2-Butene
Both CH₃ groups on same side
Higher boiling point
TRANS Configuration
Definition: Similar groups are on the opposite sides of the double bond
TRANS-2-Butene
CH₃ groups on opposite sides
Lower boiling point
💡 Physical Property Differences
- CIS isomers have higher boiling points (more polar)
- TRANS isomers have lower melting points (better packing)
- CIS isomers are less stable due to steric hindrance
- TRANS isomers are more thermodynamically stable
3. E-Z Nomenclature System (IUPAC)
The Comprehensive Approach
E-Z system works for all alkenes using Cahn-Ingold-Prelog priority rules to assign configurations.
Cahn-Ingold-Prelog Priority Rules
Rule 1: Atomic Number
Higher atomic number gets higher priority
Br (35) > Cl (17) > S (16) > O (8) > N (7) > C (6) > H (1)
Rule 2: Isotopes
Higher mass number gets higher priority
D (²H) > H (¹H), T (³H) > D > H
Rule 3: Sequential Comparison
If first atoms are same, compare second atoms
-CH₂CH₃ > -CH₃ (C,C,H vs C,H,H)
Rule 4: Multiple Bonds
Treat multiple bonds as duplicate atoms
-CH=O treated as C(O,O), -C≡N as C(N,N,N)
Assigning E/Z Configuration
Step 1: Assign priorities to groups on each carbon using CIP rules
Step 2: Check positions of high priority groups
Step 3: Assign configuration:
E Configuration (Entgegen)
High priority groups OPPOSITE
Z Configuration (Zusammen)
High priority groups TOGETHER
Example: 1-Bromo-1,2-dichloroethene
Step 1: Assign priorities
Left carbon: Br (35) > Cl (17) → Priority: Br > Cl
Right carbon: Cl (17) > H (1) → Priority: Cl > H
Step 2: Check positions
High priority groups are TOGETHER → Z configuration
Correct name: (Z)-1-bromo-1,2-dichloroethene
4. Predicting Number of Geometrical Isomers
Systematic Approach
For Simple Alkenes: C=C
Formula: 2ⁿ where n = number of carbons with two different groups
| Alkene Type | Formula | n value | Isomers |
|---|---|---|---|
| abC=Cab | 2² | 2 | 4 (2 pairs of enantiomers) |
| abC=Cac | 2¹ | 1 | 2 (CIS & TRANS) |
| abC=Ccd | 2² | 2 | 4 (EE, EZ, ZE, ZZ) |
For Polyenes
Independent double bonds: Multiply possibilities for each double bond
Cumulated dienes: Consider overall molecule symmetry
Conjugated dienes: s-CIS and s-TRANS conformers possible
Practice: Predict isomers for CH₃-CH=CH-CH=CH-CH₃
Step 1: Identify double bonds
Two double bonds: C₂=C₃ and C₄=C₅
Step 2: Analyze each double bond
C₂=C₃: CH₃, H on C₂ and H, CH on C₃ → Shows geometrical isomerism
C₄=C₅: CH, H on C₄ and H, CH₃ on C₅ → Shows geometrical isomerism
Step 3: Calculate total isomers
Each double bond can have E or Z configuration
Total possibilities = 2 × 2 = 4 geometrical isomers
Possible isomers: EE, EZ, ZE, ZZ
5. Practice Problems
Test Your Understanding
Problem 1: Assign E/Z configuration to Br-CH=CH-Cl
Problem 2: How many geometrical isomers for CH₃-CH₂-CH=CH-CH₃?
Problem 3: Assign configuration to HOOC-CH=CH-COOH
Problem 4: Predict isomers for CH₃-CH=CH-CH=CH-COOH
Need Help?
Draw the molecules, assign priorities using CIP rules, and determine relative positions of high priority groups.
📋 Quick Reference Guide
CIP Priority Order
- I > Br > Cl > S > P > F > O > N > C > H
- -CH₂OH > -C≡CH > -CH=CH₂ > -CH₂CH₃
- -COOH > -CHO > -CH₂OH > -C₆H₅
- R > H (always)
Memory Aids
- E = Enemy (opposite sides)
- Z = Zame Zide (same side)
- CIS = Same side (like siblings)
- TRANS = Across (like transformers)
Common Mistakes to Avoid
🎯 JEE Exam Strategy
First check if geometrical isomerism is possible
Always use CIP rules step-by-step for E/Z assignment
Verify your answer by checking both double bond carbons
Draw the molecule to visualize group positions
Ready to Master More Stereochemistry?
Continue your journey through organic chemistry with optical isomerism and conformational analysis