Phenol

Quick Notes

• Phenol is an aromatic compound with an –OH group attached directly to a benzene ring.

• Preparation: Phenols can be formed from diazonium salts
  (phenylamine + NaNO₂/ with HCl <10°C → diazonium salt → phenol + N₂ + H₂O on warming).
• Reacts with bases like NaOH to form sodium phenoxide (soluble).
• Reacts with sodium metal to form sodium phenoxide and H₂ gas.
• Forms azo dyes with diazonium salts in alkaline conditions.
• Undergoes nitration with dilute HNO₃ at room temp to give 2-nitro and 4-nitrophenol.
• Reacts with Br₂(aq) to form 2,4,6-tribromophenol (white precipitate).
• Phenol is more acidic than water and ethanol, due to delocalisation of negative charge on phenoxide ion, enabling phenol to lose a H⁺ ion.
• The –OH group is activating and 2,4,6-directing, influencing substitution to 2-, 4-, and 6- positions.

Full Notes

Preparation of Phenol from Phenylamine

Phenol can be synthesised from phenylamine in two steps:

• Step 1:
  React phenylamine with HNO₃ + HCl (or NaNO₂ + HCl) below 10°C to form a diazonium salt (C₆H₅N₂⁺Cl⁻).
  • C₆H₅NH₂ + HNO₂ + HCl → C₆H₅N₂⁺Cl⁻ + 2H₂O
• Step 2:
  Gently warm the diazonium salt with water to form phenol, liberating nitrogen gas.
  • C₆H₅N₂⁺ + H₂O → C₆H₅OH + N₂↑

Reactions of Phenol

There are some common reactions of phenol you need to know.

With Bases (e.g. NaOH)

Phenol reacts with aqueous NaOH to form sodium phenoxide, a water-soluble salt.

This reaction confirms the acidic nature of phenol and shows how phenol can act as an acid (donating a H+ ion).

• C₆H₅OH + NaOH → C₆H₅O⁻Na⁺ + H₂O

With Sodium Metal

Phenol reacts with sodium metal to release hydrogen gas and form sodium phenoxide:

• C₆H₅OH + Na → C₆H₅O⁻Na⁺ + 0.5H₂

With Diazonium Salts to Form Azo Compounds

In alkaline solution (NaOH(aq)), phenol reacts with diazonium salts to form azo dyes. Azo dyes are colourful aromatic compounds used in industry to colour fabrics.

Nitration with Dilute HNO₃

Phenol undergoes nitration at room temperature with dilute nitric acid to form nitrophenol.

• Forms a mixture of 2-nitrophenol and 4-nitrophenol.
• C₆H₅OH + HNO₃ → NO₂–C₆H₄–OH + H₂O

Bromination with Br₂(aq)

Phenol reacts rapidly with bromine water without the need for a catalyst and forms 2,4,6-tribromophenol.

• 2,4,6-tribromophenol is a white precipitate.
• C₆H₅OH + 3Br₂ → C₆H₂Br₃OH + 3HBr

Acidity of Phenol

Phenol is weakly acidic, it can lose a H⁺ from the OH group:

• C₆H₅OH ⇌ C₆H₅O⁻ + H⁺
• The phenoxide ion is stabilised by delocalisation of the negative charge into the aromatic ring, making phenol more acidic than alcohols (see below).

Comparing Acidities: Water, Phenol, Ethanol

Ethanol is less acidic than water and phenol.

Phenol is more acidic than both due to resonance stabilisation of phenoxide.

• Order of acidity: Ethanol < Water < Phenol

Differences in Reactivity Compared to Benzene

Phenol is more reactive than benzene.

This is due to the lone pair of electrons on the oxygen delocalising into the π-system.

This increases electron density, activates the ring towards electrophilic substitution.

Directing Effects of the –OH Group

The hydroxyl (OH) group activates the benzene ring and directs substitution to the 2nd (ortho), 4th, and 6th (para) positions.

This explains the pattern of substitution in nitration and bromination.

Application to Other Phenols (e.g. Naphthol)

Phenol chemistry applies similarly to related compounds like naphthol:

They also undergo electrophilic substitution easily, show acidic behaviour and can form azo dyes with diazonium salts.

Summary

• Phenol is an aromatic compound with an OH group attached directly to the ring.
• Prepared from phenylamine via diazonium salts.
• Reacts with NaOH to form sodium phenoxide, with Na metal to form phenoxide + H₂, and with diazonium salts to form azo dyes.
• Undergoes nitration and bromination easily without a catalyst.
• Phenol is more acidic than water and ethanol, due to resonance stabilisation of phenoxide ion.
• OH group increases reactivity and directs substitution to 2, 4, 6 positions.