---
title: "Cis and trans isomers; relative stability"
subject: "Engineering Chemistry"
chapter: 9
key: "semester_2/engineering_chemistry:cis-trans-stability"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/cis-trans-stability
site: BCT Engineering
---

# Cis and trans isomers; relative stability

Engineering Chemistry · Chapter 9: Stereochemistry

- Asked in: 2072 Magh, 2 marks
- Video: [The Relative Stability and Comparison of Alkene Isomers : Cis/Trans, Hyperconjugation, and More](https://www.youtube.com/watch?v=qYsxvcJfq1g) — ChemTalk Education
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/cis-trans-stability
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-9

## Question

What are Cis and trans-isomers? Which of these two isomers is more stable and why?

## Short answer

Cis and trans isomers are geometrical isomers where similar groups are on the same side (cis) or opposite sides (trans) of a double bond. The trans isomer is more stable because bulky groups are farther apart, minimizing steric repulsion.

## Answer

**Cis and trans isomers** are geometrical isomers of a compound with a C=C double bond (or ring). In the **cis** isomer, similar groups are on the **same side** of the double bond; in the **trans** isomer, they are on **opposite sides**.
```text
 CH₃     CH₃      CH₃     H
    \   /            \   /
     C=C              C=C
    /   \            /   \
   H     H          H     CH₃
cis-but-2-ene   trans-but-2-ene
```
**The trans isomer is more stable.** In the cis form the two bulky CH₃ groups are close together and repel each other (steric strain), which raises the energy. In the trans form they are far apart, so the repulsion is minimum. Trans-but-2-ene has a heat of hydrogenation about 4 kJ mol⁻¹ lower than the cis form. Similarly, fumaric acid (trans) is more stable than maleic acid (cis), and maleic acid changes into fumaric acid on heating.

## Check yourself

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

1. What is a cis isomer?
   A. Similar groups on opposite sides
   B. A mirror image of trans
   C. An isomer with a chiral carbon
   D. Similar groups on the same side of the double bond
   Answer: D. Trans has them on opposite sides. (Key point: Definition)

2. Why is trans-but-2-ene more stable?
   A. Methyl groups attract each other in trans
   B. Methyl groups are far apart, so steric repulsion is minimum
   C. It has a stronger double bond
   D. It has hydrogen bonding
   Answer: B. Cis has CH3 groups close together causing strain. (Key point: Stability reason)

3. What evidence shows trans-but-2-ene is more stable?
   A. Its heat of hydrogenation is higher
   B. It has a lower boiling point
   C. It is optically active
   D. Its heat of hydrogenation is about 4 kJ/mol lower
   Answer: D. Lower heat of hydrogenation means lower energy. (Key point: Evidence)

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