---
title: "Mechanism of E1 and E2 reactions"
subject: "Engineering Chemistry"
chapter: 10
key: "semester_2/engineering_chemistry:e1-e2-mechanism"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/e1-e2-mechanism
site: BCT Engineering
---

# Mechanism of E1 and E2 reactions

Engineering Chemistry · Chapter 10: Reaction Mechanism in Organic reactions

- Asked in: 2070 Magh, 3+2 marks
- Video: [E1 and E2 Elimintaion Reactions | Mechanism](https://www.youtube.com/watch?v=L9VtDeIkPCg) — Najam Academy
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/e1-e2-mechanism
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-10

## Question

Describe the reaction mechanism of E₁ and E₂ with the suitable example.

## Short answer

E1 proceeds via carbocation intermediate, e.g. (CH3)3CBr → (CH3)2C=CH2, rate = k[RX]. E2 is concerted with simultaneous β-H removal and C–Br clevage, e.g. CH3CH2Br → CH2=CH2, rate = k[RX][base].

## Answer

#### E1 mechanism
**Mechanism: E1 (two steps).**
```text
Step 1 (slow): ionisation to carbocation
 (CH₃)₃C—Br  ---slow--->  (CH₃)₃C⁺ + Br⁻

Step 2 (fast): base removes a β-hydrogen
 OH⁻ + H—CH₂—C⁺(CH₃)₂ ---fast---> H₂O + CH₂=C(CH₃)₂
```
Overall: (CH₃)₃C–Br + KOH(alc) → (CH₃)₂C=CH₂ + KBr + H₂O (2-methylpropene)
- Rate = k[RX]: first order (unimolecular), as only the halide takes part in the slow step.
- Carbocation intermediate, so rearrangement is possible; competes with SN1.

#### E2 mechanism
**Mechanism: E2 (one step, concerted).**
```text
 B:⁻  H—CH₂—CH₂—Br   →  [B···H···CH₂···CH₂···Br]⁻‡  →  B–H + CH₂=CH₂ + Br⁻
```
The base removes the β-hydrogen while the C–Br bond breaks and the π bond forms at the same time (the H and Br are anti-periplanar).
Example: CH₃CH₂Br + KOH(alc) → CH₂=CH₂ + KBr + H₂O
- Rate = k[RX][Base]: second order (bimolecular).
- Needs a strong base and a high concentration of it; there is no intermediate.

## Check yourself

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

1. How does E1 dehydrohalogenation of (CH3)3C-Br proceed?
   A. Slow ionisation to a carbocation, then fast removal of a beta-hydrogen by the base
   B. One concerted step in which base removal of H and loss of Br happen together
   C. Fast attack of OH- on carbon, then slow loss of Br-
   D. Slow removal of H by the base, then fast loss of Br-
   Answer: A. E1 is two steps: the halide ionises slowly to a carbocation, then the base quickly removes a beta-H to form the alkene. (Key point: E1 mechanism steps)

2. How does E2 of CH3CH2Br with alcoholic KOH proceed?
   A. Two steps via a carbocation
   B. The base first attacks carbon and replaces Br
   C. Br- leaves slowly then H is lost fast
   D. One step: base removes beta-H as Br leaves, forming ethene
   Answer: D. E2 is concerted with an anti-periplanar H and Br. (Key point: E2 mechanism)

3. What is the rate law for E1 elimination?
   A. Rate = k[RX][Base]
   B. Rate = k[RX], independent of base concentration
   C. Rate = k[Base]
   D. Rate = k[RX]^2
   Answer: B. Only the halide takes part in the slow ionisation step, so E1 is first order in RX and independent of the base. (Key point: E1 rate law)

4. What is the rate law for E2 elimination?
   A. Rate = k[RX], first order
   B. Rate = k[Base], first order
   C. Rate = k[RX][Base], second order
   D. Rate = k[RX]^2
   Answer: C. Base and substrate both take part in the single step, so E2 is bimolecular and second order. (Key point: E2 rate law)

## Related in Engineering Chemistry

- [Elimination reactions; E1 vs E2 mechanism](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Ae1-vs-e2) — Engineering Chemistry, chapter 10, asked 2×
- [Dehydrohalogenation; mechanism and kinetics of E1 [partly illegible]](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Ae1-kinetics) — Engineering Chemistry, chapter 10, asked 2×
- [Dehydrohalogenation of t-butyl bromide; E1 vs E2](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Atbutyl-dehydrohal-e1-e2) — Engineering Chemistry, chapter 10
- [Unimolecular elimination mechanism; difference from E2](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Ae1-mechanism-differ-e2) — Engineering Chemistry, chapter 10, asked 2×

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- [Dielectric boundary, E1 given, find E2 D2 P2](https://www.bctengineer.com/question.md?key=semester_3%2Felectromagnetism%3Adielectric-boundary-find-e2-d2) — Electromagnetism, chapter 2