7.9.5 Phenol

C₆H₅OH OH

Phenol is a pale pink crystalline solid (m.p. 42oC. b.p. 181oC), slightly soluble in water, with a characteristic ‘carbolic’ smell. It is very poisonous and blisters skin on contact.

Chemistry of production of phenol from benzene (via cumene).

Production of Phenol

Propene and benzene (both derived cheaply from petroleum by cracking and catalytic reforming ) are compressed and passed over a heated phosphoric acid catalyst.

_30atms/250o_C

C₆H₆ + CH₃CH=CH₂ C₆H₅CH(CH₃)₂

^{phosphoric acid cumene}

The cumene is oxidised by air to its peroxide, which decomposes into phenol and propanone on treatment with sulphuric acid.

CH₃ CH₃

| |

C₆H₅-C-H + O₂ C₆H₅-C-O-O-H C₆H₅OH + CH₃COCH₃

| | ^phenol

CH₃ CH₃

Weakly acidic nature of phenol.

Acidity of Phenol

Phenol is weakly acidic dissociating in water

C₆H₅OH ó C₆H₅O^- + H⁺ K_a=1.3 x 10^-10 mol dm^-3

Phenol is too weak an acid to affect litmus paper or liberate carbon dioxide from carbonates and hydrogencarbonates, but it dissolves readily in alkalis.

Reactions of phenol with ethanoyl chloride, iron(III) chloride solution, bromine water (comparison with benzene), sodium hydroxide and sodium hydrogencarbonate solution (comparison with ethanoic acid).

(Discussion of activation and directive effects in benzene ring not required).

Reactions of Phenol

1. Ethanoly chloride CH₃COCl.

Phenol reacts readily giving the ester

C₆H₅OH + CH₃COCl CH₃COO C₆H₅ + HCl

2. Iron(III) chloride solution.

An aqueous solution of phenol produces an intense purple coloured salt complex.

3. Bromine water.

An aqueous solution of phenol readily decolourises the bromine and gives an immediate white precipitate with bromine water. This is a substitution reaction.

OH OH