Chlorobenzene is nitrated with conc. $\mathrm{HNO}{}_3$/conc. $\mathrm{H}{}_2\mathrm{SO}{}_4$, the para isomer is separated and heated with aqueous $\mathrm{NaOH}$ at 443 K, and the mixture is then acidified. The final product is

Nitration is ortho/para directing, giving 1-chloro-4-nitrobenzene as the separated para isomer. The para nitro group activates the ring toward nucleophilic substitution, so $\mathrm{OH}{}^{-}$ displaces $\mathrm{Cl}{}^{-}$ at 443 K via the Meisenheimer complex to give sodium 4-nitrophenoxide. Acidification gives 4-nitrophenol.