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Nucleophile vs electrophile; dehydrohalogenation mechanism

Short answer

Nucleophiles are electron-rich species (Lewis bases) that donate electrons to electrophiles, which are electron-deficient (Lewis acids). Dehydrohalogenation of 2-bromo-2-methylpropane (tertiary halide) follows E1: slow C–Br cleavage giving (CH₃)₃C⁺, then OH⁻ removes a β-H to yield 2-methylpropene.

Differentiate between nucleophile and electrophile. Explain the mechanism of dehydrohalogenation of 2-Bromo-2-methylpropane.

Nucleophile vs electrophile

BasisNucleophileElectrophile
NatureElectron-rich species with a lone pair; donates electronsElectron-deficient species; accepts an electron pair
Meaning"Nucleus-loving", attacks positive centres"Electron-loving", attacks negative centres
ChargeNegative or neutralPositive or neutral
Acts asLewis baseLewis acid
ExamplesOH⁻, CN⁻, Cl⁻, NH₃, H₂OH⁺, NO₂⁺, AlCl₃, carbocations, BF₃

Dehydrohalogenation of 2-bromo-2-methylpropane

The substrate is a tertiary halide, (CH₃)₃CBr. With alcoholic KOH it reacts by E1.

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.
  • Carbocation intermediate, so rearrangement is possible; competes with SN1.

✓ Answer checked by Claude Opus 5.5 (high effort)

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