After finding out that the following reaction takes place in aqua regia :

$\ce {HNO_3 + 3HCl -> 2H_2O + Cl_2 + NOCl}$,

I quickly got curious about what the mechanism for this interesting reaction would look like. Upon doing some thinking on my own, I obtained the following:

$\ce {Step 1: HNO_3 + HCl -> H_2NO_3^ + Cl^-}$

$\ce {Step 2: H_2NO_3^+ -> NO_2^+ + H_2O}$

$\ce {Step 3: NO_2^+ + Cl^- -> NO_2Cl}$

$\ce {Step 4: NO_2Cl + HCl -> NOClOH^+ + Cl^-}$

$\ce {Step 5: NOClOH^+ + Cl^- -> Cl_2 + HNO_2}$

$\ce {Step 6: HNO_2 + HCl -> H_2NO_2^+ + Cl^-}$

$\ce {Step 7: H_2NO_2^+ + Cl^- -> NOCl + H_2O}$

I wasn't able to find any research papers online which published a mechanism for the reaction so I asked my friends to verify if the mechanism was plausible. They agreed that it was plausible and when asked which they thought was the rate-determining step, they replied Step 1 because nitric acid is acting as base. However, I disagree with them and would like to propose Step 5 since it is probably not easy for the chloride ion to abstract a chlorine cation from the intermediate to form dichlorine.

I would like to ask if anyone could find any steps in my mechanism which are unlikely to occur and also do state the reason for why they think so in their response. Also, do share your thoughts on what the rate-determining step would be.

Actually, I am most doubtful about Steps 1 and 2. I was wondering if the nitronium ion forms at all as they could just occur in a concerted manner.

  • 1
    $\begingroup$ I'm afraid the biggest problem here is that you're assuming there's just a single reaction there, which is not the case. $\endgroup$ – Mithoron Jan 4 '18 at 16:23

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