---
title: "Geometrical isomerism with Z and E notation"
subject: "Engineering Chemistry"
chapter: 9
key: "semester_2/engineering_chemistry:geometrical-isomerism-ze"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/geometrical-isomerism-ze
site: BCT Engineering
---

# Geometrical isomerism with Z and E notation

Engineering Chemistry · Chapter 9: Stereochemistry

- Asked in: 2070 Bhadra, 2 marks; 2068 Bhadra, 2 marks (asked 2 times)
- Video: [Cis Trans Isomers and E and Z Notation](https://www.youtube.com/watch?v=HcD0DWrn1zY) — chemistryinaminute
- 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-ze
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-9

## Question

Define the geometrical isomerism and write an example with Z and E notation.

## Short answer

Geometrical isomerism arises from restricted rotation about a double bond when each carbon bears two different groups. In but-2-ene, the isomer with CH3 groups on the same side is Z, opposite side is E, based on CIP priority (higher atomic number first).

## 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).

#### Example: but-2-ene, CH₃–CH=CH–CH₃
```text
  CH₃     CH₃              CH₃     H
     \   /                    \   /
      C=C                      C=C
     /   \                    /   \
    H     H                  H     CH₃
  (Z)-but-2-ene           (E)-but-2-ene
   (cis)                    (trans)
```
**CIP priority rules for E/Z:** on each doubly bonded carbon, rank the two attached groups by the atomic number of the atom directly attached (higher atomic number = higher priority; if they are the same, compare the next atoms). If the two higher-priority groups lie on the **same side**, the configuration is **Z** (zusammen); if they are on **opposite sides**, it is **E** (entgegen). Here CH₃ has higher priority than H on each carbon, so the isomer with both CH₃ groups on the same side is (Z) and the one with them on opposite sides is (E).

## Check yourself

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

1. What causes geometrical isomerism?
   A. A carbon with four different groups
   B. Different functional groups
   C. Restricted rotation about a double bond (or ring)
   D. Free rotation about C-C
   Answer: C. Restricted rotation gives different spatial arrangements. (Key point: Definition)

2. What condition is required for geometrical isomerism in abC=Ccd?
   A. Each doubly bonded carbon has two different groups
   B. Both carbons have identical groups
   C. One carbon has two identical groups
   D. The molecule must be cyclic
   Answer: A. a must differ from b, and c from d. (Key point: Condition)

3. When is a configuration Z?
   A. When they are on opposite sides
   B. When both groups are hydrogen
   C. When the two higher-priority groups are on the same side
   D. When the lower priority groups are opposite
   Answer: C. Z = zusammen (together), E = entgegen (opposite). (Key point: Z/E)

## Related in Engineering Chemistry

- [Geometrical isomerism; trans stability; propene](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Ageometrical-isomerism) — Engineering Chemistry, chapter 9