Organic conversion

Benzene to picric acid

3 steps: substitution, then substitution, then substitution

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

BenzeneC₆H₆Arene
Step 1
Cl₂, anhydrous FeCl₃
i
Electrophilic substitution; the Lewis acid polarises Cl₂.
ChlorobenzeneC₆H₅ClHaloarene
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.
PhenolC₆H₅OHPhenol
Step 3
conc. HNO₃
i
The activated ring takes three nitro groups (2,4,6-trinitrophenol).
Picric acidC₆H₂(NO₂)₃OHPhenol

Alternative routes (2)

Benzene → Cumene → Phenol → Picric acid
1. Benzene to cumene: CH₃CH₂CH₂Cl, anhydrous AlCl₃.
2. Cumene to phenol: (i) O₂ (ii) H₃O⁺.
3. Phenol to picric acid: conc. HNO₃.
Benzene → Benzene sulphonic acid → Phenol → Picric 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 picric acid: conc. HNO₃.

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 picric acid: conc. HNO₃ The activated ring takes three nitro groups (2,4,6-trinitrophenol).

Questions

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

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

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

None of the steps has a name. They are substitution and substitution and substitution, so write the reagent and the condition for each step.

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

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