Chemical Properties

Phenol reacts at two sites: its –OH group, which is weakly acidic and can be acylated, and its benzene ring, which the –OH group makes very reactive towards electrophilic substitution at the ortho and para positions. This lesson covers its acidity, esterification, halogenation, nitration, sulphonation, the Kolbe and Reimer–Tiemann reactions, and the tests used to identify phenol.

Phenol is more acidic than alcohols because the phenoxide ion is stabilised by resonance: it reacts with sodium and with sodium hydroxide, but it is too weak an acid to release carbon dioxide from sodium hydrogencarbonate. Electron-withdrawing groups such as –NO₂ increase its acidity, and electron-donating groups such as –CH₃ decrease it. The –OH group activates the ring, so phenol decolourises bromine water at once to give a white precipitate of 2,4,6-tribromophenol, gives o- and p-nitrophenols with dilute nitric acid, forms salicylic acid in Kolbe's reaction and salicylaldehyde in the Reimer–Tiemann reaction. Phenol is reduced to benzene by zinc dust, oxidised to benzoquinone by chromic acid, and identified by the violet colour it gives with neutral ferric chloride.

Two reactive sites

• The –OH group: its O–H bond can break, so phenol behaves as a weak acid, and it can be acylated to form esters.
• The benzene ring: a lone pair on the oxygen is delocalised into the ring, raising the electron density at the ortho and para positions. The –OH group is therefore strongly activating and ortho-para directing in electrophilic substitution.

A. Acidic character

• With sodium: 2C₆H₅OH + 2Na → 2C₆H₅ONa + H₂
• With sodium hydroxide: C₆H₅OH + NaOH → C₆H₅ONa + H₂O (sodium phenoxide). Alcohols do not react with aqueous NaOH in this way.
• With sodium hydrogencarbonate: no reaction. Phenol is weaker than carbonic acid and cannot release CO₂, whereas carboxylic acids can.

Why phenol is more acidic than ethanol: in the phenoxide ion, the negative charge on oxygen is spread by resonance over the ortho and para carbons of the ring, which stabilises the ion. In the ethoxide ion the charge stays on oxygen. The carbon carrying the –OH group in phenol is also sp² hybridised and more electronegative than an sp³ carbon, which helps release the proton.

Substituents change the acidity (a lower pKa means a stronger acid):

Electron-withdrawing groups such as –NO₂ at the ortho and para positions stabilise the phenoxide ion further and increase acidity; electron-donating groups such as –CH₃ destabilise it and decrease acidity.

B. Esterification (acylation)

Phenol reacts with acid chlorides or acid anhydrides, usually in the presence of a base such as pyridine:

• C₆H₅OH + CH₃COCl → CH₃COOC₆H₅ + HCl (phenyl ethanoate)
• Acetylation of salicylic acid with acetic anhydride gives acetylsalicylic acid, the drug aspirin.

C. Electrophilic substitution in the ring

1. Halogenation. With bromine water, phenol gives a white precipitate of 2,4,6-tribromophenol at once: C₆H₅OH + 3Br₂ → C₆H₂Br₃OH + 3HBr. With bromine in a solvent of low polarity, such as CS₂ or CHCl₃, at low temperature, mainly p-bromophenol forms, with some o-bromophenol.
2. Nitration. With dilute nitric acid at about 298 K, phenol gives a mixture of o-nitrophenol and p-nitrophenol, which are separated by steam distillation. With concentrated nitric acid it gives 2,4,6-trinitrophenol (picric acid), but the yield is poor because the acid also oxidises phenol; picric acid is therefore made by first sulphonating phenol with concentrated sulphuric acid and then treating the product with concentrated nitric acid.
3. Sulphonation. With concentrated sulphuric acid, phenol gives mainly o-phenolsulphonic acid at a lower temperature and mainly p-phenolsulphonic acid at about 373 K.
4. Kolbe's reaction. Sodium phenoxide is heated with carbon dioxide at about 400 K and 4–7 atm; acidifying the product gives salicylic acid (2-hydroxybenzoic acid). The phenoxide ion is even more reactive than phenol, so it is attacked even by a weak electrophile such as CO₂.
5. Reimer–Tiemann reaction. Phenol is heated with chloroform and aqueous sodium hydroxide at about 340 K; acidification gives salicylaldehyde (2-hydroxybenzaldehyde). A –CHO group is introduced at the ortho position.

D. Other reactions

• Reduction with zinc dust: on heating, C₆H₅OH + Zn → C₆H₆ + ZnO, converting phenol to benzene.
• Oxidation: chromic acid (sodium dichromate with sulphuric acid) oxidises phenol to benzoquinone. Air slowly oxidises phenol to coloured products, which is why stored phenol turns pink.
• Coupling: in mildly alkaline solution, benzenediazonium chloride couples with phenol at the para position to form p-hydroxyazobenzene, an orange dye.

E. Tests for phenol

• Neutral ferric chloride solution gives a violet colour with phenol.
• Bromine water is decolourised and a white precipitate of 2,4,6-tribromophenol forms.

Ethanol gives neither result, so these tests distinguish phenol from ethanol.