Dehydrohalogenation of 1° alkyl halide

A primary alkyl halide undergoes dehydrohalogenation with alcoholic KOH by the E2 mechanism (one step), because a primary carbocation is too unstable for E1.

Mechanism: E2 (one step, concerted).

 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.