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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.

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    $\begingroup$ RNH3+ protonates any nucleophile that approaches it destroying the nucleophilicity. $\endgroup$ – Waylander Jul 21 '18 at 7:38
  • $\begingroup$ @Waylander On the one hand, the scope of this reaction is indeed limited, but at least I-(Iodide) can act as a good nucleophile which cannot be protonated. On the other hand, If the ammonium salt is tertiary, such as R4N+, the substitution is still possible, just as the cleavage of ether. $\endgroup$ – liwt31 Jul 21 '18 at 8:38
  • $\begingroup$ Quaternary ammonium salts are well etablished as good leaving groups but I can find no examples of a primary ammonium hydroiodide salt reacting to form a primary iodide as you suggest. Perhaps someone with database access can search this better than me. $\endgroup$ – Waylander Jul 21 '18 at 9:45
  • $\begingroup$ @liwt31 Quaternary maybe? $\endgroup$ – Oscar Lanzi Jul 21 '18 at 9:46
  • $\begingroup$ @OscarLanzi yes, thank you for your correction. $\endgroup$ – liwt31 Jul 22 '18 at 4:37

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