---
title: "Geometrical isomerism; trans stability; propene"
subject: "Engineering Chemistry"
chapter: 9
key: "semester_2/engineering_chemistry:geometrical-isomerism"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/geometrical-isomerism
site: BCT Engineering
---

# Geometrical isomerism; trans stability; propene

Engineering Chemistry · Chapter 9: Stereochemistry

- Asked in: 2074 Bhadra, 3 marks
- Video: [Constitutional Isomers and Cis and Trans Isomers- Org Chem One (1) Lecture Series Video 9](https://www.youtube.com/watch?v=hsdtpQgQs0A) — All In with Dr Betts
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/geometrical-isomerism
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-9

## Question

What is geometrical isomerism? Why is trans-isomer more stable than cis-isomer? Why is geometrical isomerism not possible in propene?

## Short answer

Geometrical isomerism arises from restricted rotation about a double bond when each carbon has two different groups. The trans isomer is more stable because bulky groups are further apart, reducing steric repulsion. Propene (CH3–CH=CH2) shows no geometrical isomerism because one carbon carries two identical H atoms.

## Answer

**Geometrical (cis–trans) isomerism** is a type of stereoisomerism in which isomers have the same structural formula but differ in the spatial arrangement of groups about a double bond (or ring) because rotation about it is restricted.

Condition: each doubly bonded carbon must carry two **different** groups (a≠b, c≠d in abC=Ccd).

#### Why trans is more stable than cis
In the cis isomer the two bulky groups are on the same side, so they come close and there is van der Waals (steric) repulsion, which raises the energy and gives a more strained molecule. In the trans isomer they are far apart, so there is little strain and lower energy. Example: trans-but-2-ene is more stable than cis-but-2-ene by about 4 kJ mol⁻¹.

#### Why propene shows no geometrical isomerism
Propene is CH₃–CH=CH₂. The terminal carbon holds two identical hydrogen atoms (not two different groups), so interchanging them gives the same compound. Hence no cis–trans isomers exist.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why is the trans isomer more stable than cis?
   A. Bulky groups are closer
   B. Bulky groups are far apart, so there is less steric repulsion
   C. The trans isomer has stronger double bonds
   D. The cis isomer has less strain
   Answer: B. In cis, groups on the same side repel, raising energy by about 4 kJ/mol for but-2-ene. (Key point: Stability)

2. Why does propene show no geometrical isomerism?
   A. It has no double bond
   B. It has a chiral carbon
   C. Rotation is free about C=C
   D. One doubly bonded carbon holds two identical H atoms
   Answer: D. Interchanging identical H atoms gives the same compound. (Key point: Propene)

3. What is required for geometrical isomerism?
   A. Two identical groups on one carbon
   B. Two different groups on each doubly bonded carbon
   C. A triple bond
   D. A chiral centre
   Answer: B. Plus restricted rotation about the bond. (Key point: Condition)

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