Conversion of ethene to ethyne

Conversion of Ethene (CH₂=CH₂) to Ethyne (HC≡CH)

The conversion of ethene (CH₂=CH₂) into ethyne (HC≡CH) is an important transformation in organic chemistry. It involves halogenation followed by double dehydrohalogenation. Let us understand the mechanism step by step.

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Step 1: Halogenation of Ethene

Ethene undergoes electrophilic addition with bromine (Br₂ in CCl₄) to give 1,2-dibromoethane.

CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br

Step 2: First Dehydrohalogenation

On treatment with alc. KOH, one molecule of HBr is eliminated from 1,2-dibromoethane, forming bromoethene (vinyl bromide).

CH₂Br–CH₂Br → CH₂=CHBr

Step 3: Second Dehydrohalogenation

With a stronger base such as NaNH₂ in liquid NH₃, the second HBr molecule is eliminated to form the triple bond.

CH₂=CHBr → HC≡CH

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Overall Reaction:

CH₂=CH₂ → (Br₂) → CH₂Br–CH₂Br → (alc. KOH) → CH₂=CHBr → (NaNH₂/liq. NH₃) → HC≡CH

🔍 Key Mechanism Points:

• Electrophilic addition of Br₂ forms 1,2-dibromoethane.
• Elimination with alc. KOH removes one HBr (E2 mechanism).
• Further elimination with NaNH₂/liquid NH₃ removes the second HBr, creating a triple bond.
• Final product is ethyne (acetylene).