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Organic conversion

Benzene to salicylic acid

3 steps: substitution, then substitution, then Kolbe's reaction

Class 12, Chapter 6 Haloalkanes and Haloarenes; Class 12, Chapter 7 Alcohols, Phenols and Ethers

  1. BenzeneC₆H₆Arene
  2. Step 1

    Cl₂, anhydrous FeCl₃

    i

    Electrophilic substitution; the Lewis acid polarises Cl₂.

    Cl
    ChlorobenzeneC₆H₅ClHaloarene
  3. Step 2

    (i) NaOH, 623 K, 300 atm (ii) H⁺

    i

    Haloarenes are unreactive to NaOH under mild conditions; this needs high temperature and pressure.

    HO
    PhenolC₆H₅OHPhenol
  4. Step 3

    (i) NaOH (ii) CO₂, 400 K, 4 to 7 atm (iii) H⁺

    Kolbe's reaction
    i

    Kolbe's reaction puts a COOH group ortho to the OH through the phenoxide ion.

    OHOOH
    Salicylic acido-HOC₆H₄COOHCarboxylic acid

Exam answer

  1. Step 1: Benzene to chlorobenzene: Cl₂, anhydrous FeCl₃.
  2. Step 2: Chlorobenzene to phenol: (i) NaOH, 623 K, 300 atm (ii) H⁺.
  3. Step 3: Phenol to salicylic acid: (i) NaOH (ii) CO₂, 400 K, 4 to 7 atm (iii) H⁺.
Alternative routes (2)
  • Benzene → Cumene → Phenol → Salicylic acid
    1. Benzene to cumene: CH₃CH₂CH₂Cl, anhydrous AlCl₃.
    2. Cumene to phenol: (i) O₂ (ii) H₃O⁺.
    3. Phenol to salicylic acid: (i) NaOH (ii) CO₂, 400 K, 4 to 7 atm (iii) H⁺.
  • Benzene → Benzene sulphonic acid → Phenol → Salicylic acid
    1. Benzene to benzene sulphonic acid: fuming H₂SO₄ (oleum).
    2. Benzene sulphonic acid to phenol: (i) NaOH (molten), heat (ii) H⁺.
    3. Phenol to salicylic acid: (i) NaOH (ii) CO₂, 400 K, 4 to 7 atm (iii) H⁺.
Why these reagents

Reagent and reason, step by step

  • Step 1: Benzene to chlorobenzene: Cl₂, anhydrous FeCl₃ Electrophilic substitution; the Lewis acid polarises Cl₂.
  • Step 2: Chlorobenzene to phenol: (i) NaOH, 623 K, 300 atm (ii) H⁺ Haloarenes are unreactive to NaOH under mild conditions; this needs high temperature and pressure.
  • Step 3: Phenol to salicylic acid: (i) NaOH (ii) CO₂, 400 K, 4 to 7 atm (iii) H⁺ Kolbe's reaction puts a COOH group ortho to the OH through the phenoxide ion.
Questions

Benzene to salicylic acid: quick answers

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

3 steps: Benzene, then chlorobenzene, then phenol, then salicylic acid.

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

Kolbe's reaction. Sodium phenoxide reacts with carbon dioxide at 400 K and 4 to 7 atm, and acidification gives salicylic acid.

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

Benzene to chlorobenzene: Cl₂, anhydrous FeCl₃. Electrophilic substitution; the Lewis acid polarises Cl₂.

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