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Organic conversion

Benzene to benzoic acid

2 steps: Friedel-Crafts alkylation, then oxidation

Class 11, Chapter 9 Hydrocarbons; Class 12, Chapter 8 Aldehydes, Ketones and Carboxylic Acids

  1. BenzeneC₆H₆Arene
  2. Step 1

    CH₃Cl, anhydrous AlCl₃

    Friedel-Crafts alkylation
    i

    Friedel-Crafts alkylation; the product is more reactive than benzene, so polyalkylation occurs unless benzene is in excess.

    TolueneC₆H₅CH₃Arene
  3. Step 2

    KMnO₄, KOH, heat; then H₃O⁺

    i

    Any alkyl side chain with a benzylic hydrogen is oxidised to COOH.

    OHO
    Benzoic acidC₆H₅COOHCarboxylic acid

Exam answer

  1. Step 1: Benzene to toluene: CH₃Cl, anhydrous AlCl₃.
  2. Step 2: Toluene to benzoic acid: KMnO₄, KOH, heat; then H₃O⁺.
Alternative routes (4)
  • Benzene → Ethylbenzene → Benzoic acid
    1. Benzene to ethylbenzene: CH₃CH₂Cl, anhydrous AlCl₃.
    2. Ethylbenzene to benzoic acid: KMnO₄, KOH, heat; then H₃O⁺.
  • Benzene → Benzaldehyde → Benzoic acid
    1. Benzene to benzaldehyde: CO + HCl, anhydrous AlCl₃ / CuCl.
    2. Benzaldehyde to benzoic acid: KMnO₄, KOH, heat; then H₃O⁺.
  • Benzene → Chlorobenzene → Toluene → Benzoic acid
    1. Benzene to chlorobenzene: Cl₂, anhydrous FeCl₃.
    2. Chlorobenzene to toluene: CH₃Cl, Na, dry ether.
    3. Toluene to benzoic acid: KMnO₄, KOH, heat; then H₃O⁺.
  • Benzene → Bromobenzene → Phenylmagnesium bromide → Benzoic acid
    1. Benzene to bromobenzene: Br₂, anhydrous FeBr₃.
    2. Bromobenzene to phenylmagnesium bromide: Mg, dry ether.
    3. Phenylmagnesium bromide to benzoic acid: (i) CO₂ (dry ice), dry ether (ii) H₃O⁺.
Why these reagents

Reagent and reason, step by step

  • Step 1: Benzene to toluene: CH₃Cl, anhydrous AlCl₃ Friedel-Crafts alkylation; the product is more reactive than benzene, so polyalkylation occurs unless benzene is in excess.
  • Step 2: Toluene to benzoic acid: KMnO₄, KOH, heat; then H₃O⁺ Any alkyl side chain with a benzylic hydrogen is oxidised to COOH.
Questions

Benzene to benzoic acid: quick answers

How many steps does it take to convert benzene to benzoic acid?

2 steps: Benzene, then toluene, then benzoic acid.

Which named reaction is used to convert benzene to benzoic acid?

Friedel-Crafts alkylation. Benzene reacts with an alkyl halide in the presence of anhydrous AlCl3 to give an alkylbenzene.

What is the reagent for step 1 of benzene to benzoic acid?

Benzene to toluene: CH₃Cl, anhydrous AlCl₃. Friedel-Crafts alkylation; the product is more reactive than benzene, so polyalkylation occurs unless benzene is in excess.

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