---
title: "Cis, trans, Z, E notation for but-2-enedioic acid"
subject: "Engineering Chemistry"
chapter: 9
key: "semester_2/engineering_chemistry:butenedioic-ez"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/butenedioic-ez
site: BCT Engineering
---

# Cis, trans, Z, E notation for but-2-enedioic acid

Engineering Chemistry · Chapter 9: Stereochemistry

- Asked in: 2073 Bhadra, 2.5 marks; 2072 Asoj, 3 marks; 2070 Magh, 2 marks (asked 3 times)
- Video: [Using Cis/Trans versus E/Z to Describe Double Bonds](https://www.youtube.com/watch?v=p6AZtX9WsUQ) — jOeCHEM
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/butenedioic-ez
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-9

## Question

Write Cis, Trans and Z, E notation for the possible isomer of but-2-enedioic acid.

## Short answer

Maleic acid is the (Z) isomer with COOH groups on the same side; fumaric acid is the (E) isomer with COOH groups opposite. Priority: COOH > H, so Z when higher groups are same side.

## Answer

But-2-enedioic acid, HOOC–CH=CH–COOH, has two isomers because each doubly bonded carbon has two different groups (H and COOH).
```text
      cis / Z                         trans / E
 HOOC       COOH                 HOOC        H
    \      /                        \      /
     C == C                          C == C
    /      \                        /      \
   H        H                      H        COOH
 maleic acid                    fumaric acid
```
- **cis (Z)-isomer, maleic acid:** the two COOH groups are on the same side.
- **trans (E)-isomer, fumaric acid:** the two COOH groups are on opposite sides.

**Z/E assignment:** on each carbon, COOH has higher priority than H (C > H). When the two COOH groups are on the same side, it is Z (zusammen); on opposite sides, it is E (entgegen).

Here cis = Z and trans = E. Maleic acid has m.p. 130 °C; fumaric acid has m.p. about 287 °C and is more stable.

## Check yourself

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

1. Why does but-2-enedioic acid show geometrical isomerism?
   A. It has a chiral carbon
   B. It has a triple bond
   C. Each double-bonded carbon has two different groups (H and COOH)
   D. Rotation about C=C is free
   Answer: C. Different groups on each carbon plus restricted rotation give cis/trans isomers. (Key point: Why isomers)

2. Which isomer is maleic acid?
   A. cis (Z), with both COOH on the same side
   B. trans (E)
   C. cis (E)
   D. trans (Z)
   Answer: A. Maleic acid is cis = Z; fumaric acid is trans = E. (Key point: Maleic acid)

3. On each carbon of HOOC-CH=CH-COOH, which group has higher priority?
   A. H, because it is smaller
   B. Both are equal
   C. COOH, because C has higher atomic number than H
   D. H, because it is lighter bonded
   Answer: C. COOH wins since carbon (6) > hydrogen (1). (Key point: CIP priority)

4. Which is more stable and has the higher m.p.?
   A. Fumaric acid (E), m.p. about 287 °C
   B. Maleic acid, 287 °C
   C. Both have equal m.p.
   D. Maleic acid, since it is Z
   Answer: A. Fumaric (trans) melts at about 287 °C vs 130 °C for maleic. (Key point: Stability)

## Related in Engineering Chemistry

- [Cis and trans isomers; relative stability](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Acis-trans-stability) — Engineering Chemistry, chapter 9
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