Phenol is manufactured by heating chlorobenzene with 10% aqueous NaOH solution at about 623K under 300 atm and in the presence of copper salt acting as a catalyst to form sodium phenoxide. The sodium salt when treated with dil. HCl, gives phenol. This method is known as Dow's process. 

When phenol is heated with ${\mathrm{CHCl}}_3$ in the presence of aqueous NaOH, an aldehydic group (−CHO) get introduced in the ring at a position ortho to the phenolic group. This reaction is known as the Reimer-Tiemann reaction.