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Formation of Diazonium Salts from Aromatic Amines

Description: Treatment of aromatic amines with acid and nitrous acid (or NaNO2) leads to the formation of diazonium salts.

Notes: NaNO2 may be used instead of HNO2; it is converted into HNO2 in the presence of acid. The reaction is only practical for aromatic amines (alkyl diazoniums are extremely unstable).


Notes: These examples show the use of either HNO2 or NaNO2. Also, instead of H2SO4, H+ is often written.

Mechanism: There are two stages to this reaction. The first is formation of the nitrosonium ion, which is a very good electrophile. Protonation of HNO2 (Step 1, arrows A and B) leads to formation of the conjugate acid, from which water is then eliminated to form the nitrosonium ion (Step 3, arrows C and D).

In the second stage of the reaction, the amine attacks the nitrosonium ion to form a N-N bond (Step 1, arrows E and F). This is followed by a proton transfer (Step 2, arrows G and H) and the drawing of the resonance form with a N=N bond (Step 3, arrows I and J). A second proton transfer (Step 4, arrows K and L) then frees up a lone pair on nitrogen, which then expels water (Step 5, arrows M and N) to give the diazonium ion.


Notes: The proton transfer steps could be shown through two step mechanisms instead of the one-step mechanism shown here. Other acids besides H2SO4 could alternatively be used;  counter-ion for the diazonium ion here is HSO4 (-), although it’s not shown.