Phenol

1.0Introduction

Phenol, commonly called carbolic acid, was initially extracted from coal tar in the early 1800s.Today, it is mainly manufactured synthetically on an industrial scale. In laboratories, phenol and its derivatives are typically synthesized from benzene compounds.

• Phenol, also called monohydroxy benzene or carbolic acid, is the simplest hydroxyl derivative of benzene and is also its IUPAC name.
• Methyl-substituted phenols are known as cresols.
• When two hydroxyl groups are present on a benzene ring, the compounds are called benzene diols (like 1,2-, 1,3-, and 1,4-benzenediol).
• In phenols, the –OH group is bonded directly to an sp²-hybridized carbon atom of the aromatic ring.
• The bond angle in phenol around the oxygen is approximately 109°.

The C–O bond length in phenol is about 136 pm, slightly shorter than that in methanol. This is due to the partial double bond character resulting from resonance between the lone pair of electrons on oxygen and the π-system of the benzene ring.

2.0Physical Properties

• Phenol is a colorless crystalline solid with a melting point of 43°C and a boiling point of 182°C. It turns pink when exposed to air and light due to partial oxidation.
• Phenol has a higher boiling point than the arenes or haloarenes or ethers of the same molecular weight. It is due to the formation of the intermolecular hydrogen bonds.
• Compared to pure aromatic hydrocarbons, phenols are more soluble in water due to their ability to form hydrogen bonding with water.
• As the hydrocarbon part increases in size and mass, the solubility decreases due to the increasing hydrophobic nature.

3.0Preparation of Phenol

(a) From Haloarenes – Dow Process

Chlorobenzene is heated with aqueous sodium hydroxide at 350°C under high pressure, producing sodium phenoxide, which on acidification gives phenol.

This is called the Dow Process.

The presence of electron-withdrawing groups (–NO₂, –CN, etc.) at the ortho and para positions increases the rate of nucleophilic substitution

Reactivity order (towards nucleophilic substitution):
Nitro-substituted haloarenes > chlorobenzene

(b) From Diazonium Salts

Phenol is formed when benzene diazonium salt is treated with warm water.
This reaction replaces the diazonium group (–N₂⁺) with –OH.

(c) From Benzene Sulphonic Acid

Benzene sulphonic acid is fused with NaOH at high temperature, forming sodium phenoxide, which gives phenol on acidification.

(d) From Cumene Hydroperoxide (CHP Method)

In this commercial method, cumene (isopropylbenzene) is oxidized to cumene hydroperoxide, which on acid-catalyzed hydrolysis gives phenol and acetone.

4.0Electrophilic Aromatic Substitution in Phenol

The –OH group in phenol is an activating and ortho/para-directing group due to resonance, increasing electron density at those positions.

1. Nitration

• With dilute HNO₃ → Forms ortho- and para-nitrophenols.
• With conc. HNO₃ → Forms 2,4,6-trinitrophenol (Picric acid).
• Yield is poor due to oxidation by nitric acid.
• Modern method: Treat phenol with conc. H₂SO₄, then HNO₃.

2. Reaction with Ammonia

• Heating phenol with NH₃ under pressure or ZnCl₂ gives aniline.

3. Halogenation

• In non-polar solvents (CHCl₃, CS₂) → Forms monobromophenol.
• In water with Br₂ → Forms 2,4,6-tribromophenol (white precipitate).
• No Lewis acid needed as phenol activates Br₂ via phenoxide formation.

4. Sulphonation

• With dil. H₂SO₄ → Forms o-sulphonic acid.
• With conc. H₂SO₄ → Forms p-sulphonic acid.
• Reaction is reversible.

5. Reimer–Tiemann Reaction

• Phenol + CHCl₃ + NaOH (refluxed at 60°C) → Forms o- and p-hydroxybenzaldehydes.
• Predominantly gives o-isomer unless it is blocked.

• Used to prepare vanillin from guaiacol.

6. Kolbe Reaction

• Phenol + NaOH + CO₂ (125°C, high pressure) → Sodium salicylate → Acidification → Salicylic acid.
• Electrophilic substitution at ortho-position.

• With KOH, the para-isomer is favored.

Acidic Nature and Reactions of Phenol

Phenol exhibits weakly acidic behavior due to the ability of the –OH group to lose a proton.

(a) Phenol can donate a proton (H⁺) from its –OH group, forming the phenoxide ion (C₆H₅O⁻).

(b) It reacts with aqueous sodium hydroxide (NaOH) to form sodium phenoxide (C₆H₅ONa) and water:

C₆H₅OH+NaOH→C₆H₅ONa+H₂O(c) Phenol does not react with sodium carbonate (Na₂CO₃) or sodium bicarbonate (NaHCO₃) because it is less acidic than carbonic acid and carboxylic acids, hence it cannot displace CO₂ from these salts.