Unit 11: Phenols

Introduction and Nomenclature

Phenol, also known as carbolic acid, is an aromatic compound in which a hydroxyl (-OH) group is directly attached to a benzene ring. The IUPAC name is phenol (C₆H₅OH), while the common name carbolic acid reflects its early use as a disinfectant. The presence of the hydroxyl group on an aromatic system imparts unique chemical behaviour, notably increased acidity compared with aliphatic alcohols.

Preparation of Phenol

Phenol can be prepared on both laboratory and industrial scales via several routes. Three important methods are discussed below.

1. Dow Process (from Chlorobenzene)

In the Dow process, chlorobenzene undergoes nucleophilic substitution with aqueous sodium hydroxide under high temperature and pressure to give sodium phenoxide, which is acidified to phenol.

C₆H₅Cl + NaOH ⟶[high T, P] C₆H₅ONa + NaCl

C₆H₅ONa + HCl ⟶ C₆H₅OH + NaCl

The reaction proceeds via an addition‑elimination mechanism (benzyne intermediate) because the C–Cl bond of chlorobenzene is resistant to simple SN2 substitution.

2. From Diazonium Salt

Phenol is obtained by hydrolysis of benzene diazonium chloride. The diazonium group is replaced by a hydroxyl group when warmed with water.

C₆H₅N₂⁺Cl⁻ + H₂O ⟶ C₆H₅OH + N₂↑ + HCl

This method is useful in the laboratory for converting anilines to phenols via diazotisation.

3. From Benzene Sulphonic Acid

Benzene sulphonic acid undergoes fusion with solid sodium hydroxide to give sodium phenoxide, which upon acidification yields phenol.

C₆H₅SO₃H + 2 NaOH ⟶[fusion] C₆H₅ONa + Na₂SO₃ + H₂O

C₆H₅ONa + HCl ⟶ C₆H₅OH + NaCl

The harsh conditions (high temperature) are required to break the strong C–S bond.

Physical Properties

• State: White crystalline solid at room temperature; may appear slightly pinkish due to oxidation.
• Odour: Characteristic antiseptic, medicinal smell.
• Solubility: Moderately soluble in water (~8.2 g/100 mL at 20 °C) because of hydrogen bonding; readily soluble in aqueous NaOH forming sodium phenoxide.
• Melting Point: 40–42 °C.
• Boiling Point: 181.7 °C.
• Toxicity: Poisonous; can cause skin irritation and systemic effects.

Chemical Properties

The hydroxyl group attached to an aromatic ring influences both the acidity of phenol and its reactivity toward electrophiles.

Acidic Nature

Phenol is a weak acid (Ka = 1.3 × 10-10, pKa ≈ 9.99). It is more acidic than aliphatic alcohols (pKa ≈ 16–18) but less acidic than carboxylic acids (pKa ≈ 4–5). The increased acidity arises from resonance stabilization of the phenoxide ion.

C₆H₅OH ⇌ C₆H₅O⁻ + H⁺

The negative charge on the oxygen is delocalised over the ortho and para positions of the benzene ring, lowering the energy of the conjugate base.

Reaction with Ammonia

Phenol reacts with ammonia under pressure to give aniline and water (the Bucherer‑Bergs reaction is a related transformation).

C₆H₅OH + NH₃ ⟶[high T, P] C₆H₅NH₂ + H₂O

Reaction with Zinc Dust

Upon heating with zinc dust, phenol undergoes deoxygenation to yield benzene.

C₆H₅OH + Zn ⟶ C₆H₆ + ZnO

Reaction with Sodium Metal

Like alcohols, phenol reacts with sodium to liberate hydrogen gas and form sodium phenoxide.

2 C₆H₅OH + 2 Na ⟶ 2 C₆H₅ONa + H₂↑

Reaction with Benzene Diazonium Chloride (Azo Coupling)

In alkaline medium, phenol couples with benzene diazonium chloride at the para position to give p‑hydroxyazobenzene, an orange‑red dye.

C₆H₅OH + C₆H₅N₂⁺Cl⁻ ⟶[NaOH] C₆H₅–N=N–C₆H₄‑OH + HCl

Reaction with Phthalic Anhydride (Phenolphthalein Formation)

Phenol condenses with phthalic anhydride in the presence of concentrated sulfuric acid to give phenolphthalein, a widely used pH indicator.

2 C₆H₅OH + C₆H₄(CO)₂O ⟶[H₂SO₄] C₂₀H₁₄O₄ (phenolphthalein) + H₂O

Acylation

Phenol undergoes acylation with acyl chlorides (or anhydrides) in the presence of a base (e.g., pyridine) to give phenyl esters.

C₆H₅OH + CH₃COCl ⟶[pyridine] C₆H₅OCOCH₃ (phenyl acetate) + HCl

Kolbe’s Reaction

When phenol is treated with sodium hydroxide and carbon dioxide under pressure, carboxylation occurs preferentially at the ortho position to give salicylic acid (2‑hydroxybenzoic acid).

C₆H₅OH + NaOH + CO₂ ⟶[125 °C, 100 atm] C₆H₄(OH)COONa + H₂O

C₆H₄(OH)COONa + HCl ⟶ C₆H₄(OH)COOH (salicylic acid) + NaCl

Reimer–Tiemann Reaction

Phenol reacts with chloroform in alkaline medium to introduce an aldehyde group at the ortho position, yielding salicylaldehyde (2‑hydroxybenzaldehyde).

C₆H₅OH + CHCl₃ + 3 NaOH ⟶ C₆H₄(OH)CHO + 3 NaCl + 3 H₂O

The mechanism involves dichlorocarbene (:CCl₂) generated from chloroform and base, which electrophilically attacks the phenoxide ion.

Electrophilic Aromatic Substitution (EAS)

The hydroxyl group activates the benzene ring toward electrophilic substitution, directing incoming groups to the ortho and para positions.

• Nitration: Phenol + dilute HNO₃ → o‑nitrophenol and p‑nitrophenol (major). With concentrated nitric acid, oxidation occurs.
• Sulphonation: Phenol + H₂SO₄ (room temp) → o‑phenolsulphonic acid; at higher temperature → p‑phenolsulphonic acid.
• Bromination: Phenol + Br₂ (in CS₂ or water) → 2,4,6‑tribromophenol (white precipitate) due to strong activation.
• Friedel‑Crafts Alkylation: Phenol + R‑Cl + AlCl₃ → ortho/para alkyl phenols (often low yields due to polymerization).

Qualitative Tests for Phenol

1. Ferric Chloride Test: Adding a few drops of neutral FeCl₃ solution to phenol gives a violet‑blue colour due to formation of a complex phenolate‑Fe³⁺ species.
2. Aqueous Bromine Test: Phenol decolourises bromine water and yields a white precipitate of 2,4,6‑tribromophenol.
3. Libermann Test: Phenol + NaNO₂ + conc. H₂SO₄ → deep red colour (nitroso‑phenol complex) which turns blue upon addition of alkali.

Uses of Phenol

• Antiseptic and Disinfectant: Phenol’s ability to denature proteins makes it effective in throat sprays, mouthwashes, and surface disinfectants (e.g., Lysol).
• Phenolic Resins: Reaction with formaldehyde yields Bakelite and other thermosetting plastics used in adhesives, laminates, and electrical insulators.
• Intermediate in Synthesis: Precursor to dyes (azo dyes), drugs (aspirin via salicylic acid), herbicides, and explosives.
• Production of Caprolactam: Phenol is a starting material for nylon‑6 via the cumene process.
• Other Applications: Manufacturing of pharmaceuticals, cosmetics, and as a reagent in laboratory organic synthesis.