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Aromatic Compound Reaction Pathways

25min Part 4 / Ch8 / Lesson 2
Prerequisites: 4-8-1 , 4-7-2

Objectives

  • Understand synthetic routes from benzene to phenol and aniline
  • Know the systematic reaction map of aromatic compounds
  • Understand the basics of structural determination

Synthetic Routes from Benzene

Benzene C₆H₆Nitrobenzene C₆H₅NO₂HNO₃+H₂SO₄Aniline C₆H₅NH₂Sn+HCl(reduce)Acetanilide+CH₃COClSulfonic acid C₆H₅SO₃Hconc. H₂SO₄Phenol C₆H₅OHNaOH fusionSalicylic acid+CO₂(Kolbe)Aspirin+acetic anhydrideMethyl salicylate+CH₃OH
Aromatic compound reaction map

Benzene → Nitrobenzene → Aniline

Nitration: C6H6+HNO3→H2SO4C6H5NO2+H2O\mathrm{C_6H_6 + HNO_3 \xrightarrow{H_2SO_4} C_6H_5NO_2 + H_2O}

Reduction: C6H5NO2+3Sn+7HCl→C6H5NH3Cl+3SnCl2+2H2O\mathrm{C_6H_5NO_2 + 3Sn + 7HCl \rightarrow C_6H_5NH_3Cl + 3SnCl_2 + 2H_2O}

→ Add NaOH to liberate: C6H5NH3Cl+NaOH→C6H5NH2+NaCl+H2O\mathrm{C_6H_5NH_3Cl + NaOH \rightarrow C_6H_5NH_2 + NaCl + H_2O}

Aniline properties:

  • Weak base (aqueous solution is nearly neutral)
  • Red-purple with bleaching powder → detection test
  • Diazotization → precursor to azo dyes

Phenol Synthesis and Derivatives

Cumene process (industrial):

Benzene + propylene → cumene → phenol + acetone

Derivatives from phenol:

ReactionProductUse
+ NaOHSodium phenoxide—
+ CO₂ (Kolbe–Schmitt)Salicylic acidPharmaceutical precursor
Salicylic acid + acetic anhydrideAcetylsalicylic acid (aspirin)Analgesic/antipyretic
Salicylic acid + CH₃OHMethyl salicylateAnti-inflammatory

Structural Determination Basics

To determine the structure of an aromatic compound:

  1. Find the molecular formula (elemental analysis → empirical formula → molecular formula from MW)
  2. Calculate degree of unsaturation: U=2C+2−H+N2U = \frac{2C + 2 - H + N}{2} (benzene ring = 4)
  3. Identify functional groups from reactions (FeCl₃ test, silver mirror test, NaHCO₃ reaction, etc.)

Check Your Understanding

Q1 What is produced when nitrobenzene is reduced?

Q2 Aspirin is made from...

Q3 What reagent detects aniline?

Exercises

Q1. Write the two-step synthesis of aniline from benzene using chemical equations.

Solution

Step 1 (Nitration):

C6H6+HNO3→conc. H2SO4C6H5NO2+H2O\mathrm{C_6H_6 + HNO_3 \xrightarrow{conc.\,H_2SO_4} C_6H_5NO_2 + H_2O}

Step 2 (Reduction):

C6H5NO2→Sn+HClC6H5NH2\mathrm{C_6H_5NO_2 \xrightarrow{Sn + HCl} C_6H_5NH_2}

(In practice, aniline hydrochloride C₆H₅NH₃Cl forms first; NaOH is added to liberate free aniline.)