Aniline reacts with mathrm‌NaNO2,mathrm‌HCl at 273−278‌mathrm‌~K to give benzene diazonium salt left(mathrm‌C6‌mathrm‌H5‌mathrm‌~N2+‌overline‌mathrm‌Cl=X‌right) which on hydrolysis left(mathrm‌H2‌mathrm‌
O
Delta
‌right) gives phenol. Complete reaction is as follows