Preparation of Phenol -- Dow's Process

In Dow's process, chlorobenzene is hydrolysed with 6-8% NaOH at high pressure (about 300 bar) and 633 K in a closed vessel to give sodium phenoxide, which on acidification with dilute HCl liberates phenol: C6H5-Cl + NaOH (633 K, 300 bar) gives C6H5-ONa + NaCl; then C6H5-ONa + HCl gives C6H5-OH + NaCl. The harsh conditions (high temperature and pressure) are needed because the C-Cl bond of chlorobenzene is much stronger, and far less reactive toward nucleophilic substitution, than the C-X bond of a simple alkyl halide -- partial double-bond character from conjugation of a chlorine lone pair int …