A question came up in our worksheet to predict the reaction product of nitrous acid and concentrated nitric acid with benzene.

I quickly recognized it to be similar to the nitration reaction. Following this ChemGuide page, which gives the reaction of sulphonation as:

$$\ce{HNO3 + 2H2SO4 -> NO2+ + 2HSO4- + H3O+}$$

I wrote this reaction:

$$\ce{HNO2 + 2HNO3 -> NO+ + 2NO3- + H3O+}\label{a}\tag{1}$$

and thought that this electrophilic $\ce{NO+}$ would then perform EAS on the benzene to give nitrosobenzene (aka nitrosation reaction).

However, the answer given in my worksheet is nitrobenzene instead. I am unsure if the following reaction (for the generation of the electrophile $\ce{NO2+}$):

$$\ce{HNO3 + 2HNO2 -> NO2+ + NO2- + H3O+}\label{b}\tag{2}$$

is even valid. To me, reaction $\ref{a}$ seems more favorable because it involves the formation of the more stable $\ce{NO3-}$. Then why is reaction $\ref{b}$ favored instead? Why is nitrobenzene the product of this reaction, and not nitrosobenzene?


You were absolutely correct when you said that nitrosation takes place.

What actually happens is that after nitrosation occurs, concentrated $\ce{HNO3}$ oxidises the nitroso group to form nitrobenzene.

A Guidebook To Mechanism In Organic Chemistry; Chapter 6, section 6.2;Peter Sykes

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An Alternate view, NOT mechanistically correct, but just to help remember it is that NO+ is a weaker nucleophile than NO2- as in one of the resonating structures of NO+, all the atoms have a complete octet and therefore it is relatively more stable. So, NO2- being the stronger nucleophile attacks.

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  • 2
    $\begingroup$ If it’s not mechanistically correct, it has no place on this site. $\endgroup$ – Jan Jul 2 at 16:05
  • $\begingroup$ As I mentioned in the answer, it is a helpful way for people to remember. I can delete the answer if you would like me to do so. Sorry mate. $\endgroup$ – Chaitanya Gambali Jul 3 at 1:49

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