Preparation of Phenol from Benzene Sulphonic Acid

Benzene is sulphonated with oleum (fuming sulphuric acid, H2SO4 with dissolved SO3) to give benzenesulphonic acid; heating this with molten (solid, not aqueous) NaOH at 623 K gives sodium phenoxide, a process called alkali fusion, which is then acidified with dilute HCl to liberate phenol: C6H5-SO3H + NaOH (623 K, fused) gives C6H5-ONa, then + HCl gives C6H5-OH. This was historically the earliest industrial route to phenol, later superseded by …