Phenols

Full Notes

Phenols are a class of aromatic compound. The simplest (commonly referred to as just phenol) has a formula of C₆H₅OH. It is essentially a benzene ring with a hydroxy (OH) group.

Acidity of Phenol

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

Delocalisation of the negative charge into the ring stabilises the ion, making phenol more acidic than alcohols but weaker than carboxylic acids.

For Example Phenol can react with aqueous NaOH to form sodium phenoxide, a water-soluble salt.

C₆H₅OH + NaOH → C₆H₅O⁻Na⁺ + H₂O
This reaction confirms the acidic nature of phenol.

Phenol Reactions

Phenol reacts with bromine (Br₂) and dilute nitric acid (HNO₃). These reactions proceed more readily than with benzene due to the activating effect of the lone pair on the –OH group.

Nitration with Dilute HNO₃

Phenol undergoes nitration at room temperature with dilute nitric acid.

C₆H₅OH + HNO₃ → C₆H₄NO₂OH + H₂O

Note that both 2-nitrophenol and 4-nitrophenol get formed.

Bromination with Br₂(aq)

Phenol reacts rapidly with bromine water without the need for a catalyst.

C₆H₅OH + 3Br₂ → C₆H₂Br₃OH + 3HBr

Forms 2,4,6-tribromophenol, a white precipitate.

Reactivity of Phenol vs. Benzene

Phenol is more reactive than benzene due to the lone pair on the oxygen delocalising into the π-system.

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

As a result, phenol reacts under milder conditions than benzene (e.g. nitration and bromination without a catalyst, unlike benzene).

Directing Effects and Substitution Products

Substituents already attached to the benzene ring affect both reactivity and position of further substitutions.

Activating Groups (such as –OH, –NH₂ and –R groups): Increase electron density in the ring, directing new groups to positions 2 and 4.

Deactivating Groups (such as –NO₂, –COOH and –COR groups): Withdraw electron density from the ring, reducing reactivity and directing to position 3.

Directing Effects of the –OH Group in Phenol

The hydroxyl group is an activating group and activates the benzene ring (see above), directing substitutions to occur at the 2, 4 and 6-positions in the ring.

This explains the pattern of substitution in nitration and bromination and the products formed (such as 2,4,6-tribromophenol).

Summary

• Phenol is weakly acidic and reacts with NaOH but not carbonates.
• Phenol reacts readily with bromine water and dilute nitric acid, forming substituted products without catalysts.
• Phenol is more reactive than benzene due to the activating effect of the OH group increasing ring electron density.
• Activating groups direct to 2 and 4 positions, deactivating groups direct to the 3 position.