Phenol is treated with one equivalent of $\mathrm{Br}{}_2$ in $\mathrm{CS}{}_2$ at $0{}^{\circ }\mathrm{C}$. The major product is

In water, phenol ionises partly and the phenoxide is so reactive that 2,4,6-tribromophenol forms. In a non-polar solvent at low temperature, ionisation is suppressed and one equivalent of Br2 gives monobromination, with the para isomer favoured over ortho by sterics.