Aniline is treated successively with: (i) $(\mathrm{CH}{}_3\mathrm{CO}){}_2\mathrm{O}$, (ii) $\mathrm{Br}{}_2$ in ethanoic acid, (iii) aqueous $\mathrm{HCl}$ and heat, (iv) $\mathrm{NaNO}{}_2/\mathrm{HCl}$ at 273-278 K, (v) $\mathrm{H}{}_3\mathrm{PO}{}_2$. The final major product is:

Steps (i)-(iii) give 4-bromoaniline as the major product. Step (iv) converts it into 4-bromobenzenediazonium chloride, and $\mathrm{H}{}_3\mathrm{PO}{}_2$ replaces the diazonium group with hydrogen (deamination), giving bromobenzene. This is a way of making a halobenzene where the amino group was used only as a directing group.