Nitrobenzene is treated with (i) $\mathrm{Sn}/\mathrm{HCl}$; (ii) $\mathrm{NaNO}{}_2$ / dil. $\mathrm{HCl}$ at 273-278 K; (iii) phenol in aqueous $\mathrm{NaOH}$ (pH 9-10). The major product is

Step (i) gives aniline, step (ii) gives benzenediazonium chloride. In step (iii) the phenoxide ion is attacked by the diazonium ion at the para position, giving 4-hydroxyazobenzene. 4-Aminoazobenzene would need aniline as the coupling partner at pH 4-5.