- 2070 Bhadra · 3+2 marks
Dehydrohalogenation of t-butyl bromide; E1 vs E2
Short answer
Tertiary butyl bromide undergoes E1 via slow carbocation formation then fast base removal, yielding 2-methylpropene; Rate = k[RX]. Unlike E1, E2 is a concerted one-step mechanism with rate = k[RX][base], requiring strong base and anti-coplanar H and X.
Explain the reaction mechanism for the dehydrohalogenation of tertiary butyl bromide. Differentiate between E¹ and E² mechanism.
Dehydrohalogenation of tert-butyl bromide
tert-Butyl bromide, (CH₃)₃C–Br, is a tertiary halide. When heated with alcoholic KOH (or in a polar protic solvent such as ethanol), it undergoes E1 elimination through a stable 3° carbocation to give 2-methylpropene.
Mechanism: E1 (two steps).
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.
- The carbocation is an intermediate, so rearrangement is possible, and E1 competes with SN1.
Difference between E1 and E2
| Basis | E1 | E2 |
|---|---|---|
| Steps | Two steps, carbocation intermediate | One step, concerted |
| Rate law | Rate = k[RX], first order | Rate = k[RX][Base], second order |
| Molecularity | Unimolecular | Bimolecular |
| Substrate | 3° > 2° (stable carbocation) | 3° > 2° > 1° |
| Base | Weak base is enough | Strong, concentrated base needed |
| Solvent | Polar protic | Less polar / alcohol |
| Rearrangement | Possible | Not possible |
| Stereochemistry | No special geometry | Anti-periplanar H and X |
| Example | (CH₃)₃CBr in ethanol | CH₃CH₂Br + alc. KOH |
✓ Answer checked by Claude Opus 5.5 (high effort)
Related questions
- Elimination reaction; dehydrohalogenation of tertiary halide · Engineering Chemistry, chapter 10 · asked 3×
- Dehydrohalogenation; mechanism and kinetics of E1 [partly illegible] · Engineering Chemistry, chapter 10 · asked 2×
- Elimination reactions; E1 vs E2 mechanism · Engineering Chemistry, chapter 10 · asked 2×
- Mechanism of E1 and E2 reactions · Engineering Chemistry, chapter 10
- Nucleophile vs electrophile; dehydrohalogenation mechanism · Engineering Chemistry, chapter 10
- Elimination reaction; dehydrohalogenation; Saytzeff's rule · Engineering Chemistry, chapter 10 · asked 2×
- Dielectric boundary, E1 given, find E2 D2 P2 · Electromagnetism, chapter 2
- FDMA vs TDMA; T1 and E1 hierarchy · Communication Systems, chapter 5