Introduction to Phenols, Aromatic Amines & Aromatic Acids
Unit 2 covers three key classes of aromatic compounds. Phenols (highly acidic and antimicrobial), Aromatic Amines (basic, source of diazonium salts used to synthesize dyes and drugs), and Aromatic Acids (acidity affected by ring substituents). Diazonium salt reactions (Sandmeyer) are especially important for drug synthesis.
Syllabus & Topics
- 1Phenols: Acidity – phenol is more acidic than aliphatic alcohols due to resonance stabilization of phenoxide ion.
- 2Effect of substituents on acidity of phenols: Electron-withdrawing groups (NO2, halogens) increase acidity; Electron-donating groups (CH3, OH) decrease acidity.
- 3Qualitative tests for phenols: FeCl3 test (violet color), Bromine water test (white precipitate of 2,4,6-tribromophenol).
- 4Structure and uses of Phenol (antiseptic), Cresols (Lysol – disinfectant), Resorcinol (antifungal, acne), Naphthols (α and β – antiseptic, dye intermediate).
- 5Aromatic Amines: Basicity – Aniline is LESS basic than aliphatic amines because the lone pair is delocalized into the benzene ring.
- 6Effect of substituents on basicity: Electron-donating groups (OCH3, alkyl) increase basicity; Electron-withdrawing groups (NO2, Cl) decrease basicity.
- 7Aryl Diazonium Salts: Formation (Aniline + NaNO2 + HCl, 0-5°C = Diazotization).
- 8Synthetic uses of diazonium salts: Sandmeyer reaction (ArCl, ArBr, ArCN using Cu salts), Balz-Schiemann (ArF), Gattermann (using Cu powder).
- 9Azo coupling: Diazonium salt + phenol/amine → Azo dye (-N=N-).
- 10Benzoic Acid: Acidity stronger than aliphatic acids due to inductive effect.
- 11Effect of substituents on acidity of benzoic acid.
- 12Important reactions: Formation of benzoyl chloride, ester, amide; Kolbe reaction, electrophilic substitution.
Learning Objectives
Frequently Asked Questions (FAQs)
Q1. Why is Phenol more acidic than Ethanol?
When phenol loses H⁺, the phenoxide ion formed is stabilized by resonance, as the negative charge is delocalized into the benzene ring (multiple resonance structures).
In contrast, the ethoxide ion formed from ethanol has no resonance stabilization.
👉 Greater stability of the conjugate base = stronger acid.
Q2. Why is Aniline less basic than Cyclohexylamine?
In aniline, the lone pair on nitrogen is delocalized into the benzene ring by resonance, making it less available to accept a proton.
In cyclohexylamine, the lone pair remains localized on nitrogen, making it fully available for protonation.
👉 Therefore, cyclohexylamine is more basic than aniline.
Q3. What is the Sandmeyer Reaction?
The Sandmeyer reaction converts an aryl diazonium salt into an aryl halide (Cl, Br) or aryl cyanide (CN) using cuprous salts.
General reaction:
ArN₂⁺ + CuCl → ArCl + N₂ + Cu⁺
It is widely used to introduce halogens into aromatic rings without direct halogenation.
Q4. What conditions are required for Diazotization?
Reaction:
ArNH₂ + NaNO₂ + HCl (at 0–5°C) → ArN₂⁺Cl⁻ (Diazonium salt)
Low temperature (0–5°C) is essential because diazonium salts are unstable and decompose rapidly at higher temperatures.
Q5. What is Azo Coupling and its pharmaceutical significance?
Azo coupling reaction:
Diazonium salt + activated aromatic compound (phenol or amine) → R–N=N–Ar (Azo compound)
Pharmaceutical and industrial significance:
Azo dyes are used as diagnostic stains
Used as food colorants and indicators
The chemistry principles are applied in certain sulfonamide antibiotics