When a primary aromatic amine dissolved or suspended in cold aqueous mineral acid is treated with sodium nitrite , there is formed a diazonium salt
`underset("amine")underset(1^(@)"aromatic")(Ar-NH_(2))+NaNO_(2)2HX overset("cold")rarrAr-underset("A dizonium salt")(N-=overset(+)(N: )X^(-)+NaX+2H_(2)O`
This Process is called diazotization Since diazonium salts slowly decompose even at ice-bath temperatures the solution is used immediately after preparation Initially a nitrosonium cation is formed from nitrous acid in acid solution.
`HO-NO+H^(+)overset("fast")hArrH_(2)overset(+)O-NOoverset("fast")hArrH_(2)O+NO^(+)` Nucleophilic attack by the electron pair of the amine on the nitrosonium ion produces an N-nitrosamine It undergoes a series of proton transfers to yied a diazonium cation
`ArNH_(2)+NO^(+)overset(slow)rarrAroverset(+)NH_(2) NO underset(fas t)overset(-H^(+))rarr`
`underset(N -nitrosami n e)(ArNHNO)underset((ream))overset(fas t)rarrAroverset(..)N underset(Diazoic aci d)(=overset(..)N-OH)underset(fas t)overset(H^(+))rarr`
In weakly aqueous acid solutions (and also is more strongly acid solutions) the rate controlling step is nitrosation of the free amine to a primary nitrosamine .
