--- title: "Dehydrohalogenation; mechanism and kinetics of E1 [partly illegible]" subject: "Engineering Chemistry" chapter: 10 key: "semester_2/engineering_chemistry:e1-kinetics" source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/e1-kinetics site: BCT Engineering --- # Dehydrohalogenation; mechanism and kinetics of E1 [partly illegible] Engineering Chemistry · Chapter 10: Reaction Mechanism in Organic reactions - Asked in: 2071 Magh - Video: [alkyl halide to alkene dehydrohalogenation vi E1 mechanism](https://www.youtube.com/watch?v=wb6LxYfarbo) — Ferdinand Solis - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/e1-kinetics - Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-10 ## Question What is dehydrohalogenation of alkyl halide? Give the mechanism and kinetics of E1 reaction. ## Short answer Dehydrohalogenation is elimination of H and X from adjacent carbons yielding an alkene. The E1 mechanism involves slow carbocation formation then fast base removal of a β-hydrogen; kinetics are first order, Rate = k[RX]. ## Answer #### Dehydrohalogenation An **elimination reaction** is a reaction in which two atoms or groups are removed from adjacent carbon atoms of a molecule, forming a double (or triple) bond, without being replaced by other groups. **Dehydrohalogenation** of an alkyl halide is the elimination of H and X (as HX) from adjacent carbons, usually by heating with a base such as alcoholic KOH, to form an alkene. #### 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) - The carbocation is an intermediate, so rearrangement is possible, and E1 competes with SN1. - E1 is favoured by 3° halides, a weak base (or low base concentration) and a polar protic solvent. #### Kinetics Only the alkyl halide takes part in the slow, rate-determining step (carbocation formation), so: Rate = k [R–X] The reaction is first order overall (unimolecular) and its rate does not depend on the concentration of the base. The rate increases with carbocation stability: 3° > 2° > 1°. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What happens in an elimination reaction? A. A group is replaced by another group on the same carbon B. Two molecules add together to form a larger saturated molecule C. A hydrogen is exchanged between two carbons without forming a pi bond D. Two atoms or groups are removed from adjacent carbons, forming a double or triple bond Answer: D. Elimination removes two groups from neighbouring carbons without replacing them, so a C=C (or triple) bond forms. (Key point: Elimination reaction) 2. 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) 3. Why is E1 first order? A. The base takes part in the slow step B. Both halide and base form the transition state C. Only the halide takes part in the slow carbocation-forming step D. The reaction is a single step Answer: C. Rate = k[RX], independent of base concentration. (Key point: E1 kinetics) 4. How does E1 rate vary with substrate? A. 1 degree > 2 degree > 3 degree B. 3 degree > 2 degree > 1 degree, as carbocation stability increases C. All equal D. Only primary halides react Answer: B. Stable carbocations form faster. (Key point: Substrate reactivity in E1) ## Close variants set in other exams - [Dehydrohalogenation of 1° alkyl halide](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Adehydrohal-primary) — Engineering Chemistry, chapter 10, asked 4× ## Related in Engineering Chemistry - [Mechanism and kinetics of SN2; stereochemical inversion [partly illegible]](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Asn2-kinetics-inversion) — Engineering Chemistry, chapter 10 - [Elimination reaction; dehydrohalogenation of tertiary halide](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Ae1-tertiary-dehydrohal) — Engineering Chemistry, chapter 10, asked 3× - [Mechanism for primary alkyl halide in aqueous medium; solvent](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Asn2-primary-solvent) — Engineering Chemistry, chapter 10 - [SN reactions; hydrolysis of 3° alkyl halide](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Asn-tertiary-hydrolysis) — Engineering Chemistry, chapter 10, asked 2× - [Tertiary alkyl halide with aq. NaOH / alc. KOH](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Atert-halide-naoh-koh) — Engineering Chemistry, chapter 10 - [3° alkyl halide with alcoholic KOH; solvent](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Atert-alc-koh-solvent) — Engineering Chemistry, chapter 10