Question

How would you convert the following compounds into benzene? 

1. Ethyne
2. Ethene
3. Hexane

Answer 1

Conversions into benzene:

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(i) Ethyne → Benzene

When ethyne is passed through a red-hot iron tube at about 873 K, three molecules of ethyne combine to form benzene.

Chemical equation:
3C₂H₂ → C₆H₆

(Reaction: Trimerisation of ethyne)

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(ii) Ethene → Benzene

Ethene is first converted into ethyne, which is then converted into benzene.

Step 1: Conversion of ethene to ethyne
Ethene + Br₂ → 1,2-dibromoethane

1,2-dibromoethane + alcoholic KOH → Ethyne

Step 2: Conversion of ethyne to benzene
3C₂H₂ → C₆H₆

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(iii) Hexane → Benzene

n-Hexane is converted into benzene by catalytic reforming (dehydrocyclisation).

Chemical equation:
C₆H₁₄ → C₆H₆ + 4H₂

Conditions:
Catalyst: Pt or Cr₂O₃/Al₂O₃
Temperature: 773 K

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Final Summary:

• Ethyne → Benzene by trimerisation
• Ethene → Ethyne → Benzene
• Hexane → Benzene by dehydrocyclisation