N-Methylaniline is treated with $\mathrm{NaNO}{}_2$ and dilute $\mathrm{HCl}$ at 273-278 K. The product is:

Secondary amines, aliphatic or aromatic, react with nitrous acid at the nitrogen to give N-nitrosoamines, which are yellow oils. Diazonium salts need a primary aromatic amine, and the 4-nitroso product is the result with tertiary aromatic amines.