Why is ammonium salt, such as RNH3+, can not react with nucleophiles like HI to give RI and NH4+? The reaction is pretty much like the substitution of alcohol in acid condition.
In my opinion, NH3 (rather than NH2-) is a very good leaving group, so it can be substituted by good nucleophiles like I-. In order to prevent the resulting ammonia from attacking back, we can add more acids like HI to drive the equilibrium to the right direction.
I haven't found any signs of this reaction in textbooks, so I thought this reaction cannot happen. If it is a common reaction, where can I find more information? Thank you all in advance.