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The following question was given in my FIITJEE study material:

$$\ce{Benzene + \text{conc.} HNO_3 + HNO_2 ->}$$ Final product of the reaction is:

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I started with the solution by deciding the Lewis acid out of $\ce{HNO_3}$ and $\ce{HNO_2}$. I guess $\ce{HNO_3}$ should be Lewis acid as O.S. of N is higher in it. Hence, I wrote the reaction to generate electrophile:

$$\ce{HNO_3 + HNO_2 -> NO_3^- + H_2O + NO^+}$$

Now, it is a bit clear who's the electrophile. Hence, I thought that first option would be the final product. But, according to the answer key it was wrong.

What I am doing wrong?


The correct answer was:

Second one (nitrobenzene)

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    $\begingroup$ You have conc. HNO3 there, and HNO2 even can't be concentrated. It's basically adding water to mixture doesn't change anything besides kinetics. $\endgroup$ – Mithoron Oct 6 '19 at 22:50
  • $\begingroup$ Concentrated HNO3 is a nasty oxidising agent and will take the nitroso (NO) group to a nitro (NO2) group. Then again, I'm not even sure how good the reaction of HNO2 with an unactivated benzene ring is. I've personally only seen this reaction mentioned once and it was in the context of phenols. I will happily be corrected, though. $\endgroup$ – orthocresol Oct 6 '19 at 23:22
  • $\begingroup$ @orthocresol Nitrosation is known with toluene and xylenes in strong acids (J. Chem. Soc., Perkin Trans. 2, 1999, 699–705), so nitrosation of benzene does seem reasonable here. But yes, the nitrosyl group does not last long in annixidizing acid (the above reference uses nonoxidizing acids). $\endgroup$ – Oscar Lanzi Oct 7 '19 at 0:44
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    $\begingroup$ Never thought I’d see the day where a FIITJEE question would make it to HNQ. $\endgroup$ – Certainly not a dog Oct 7 '19 at 7:51
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Nitrosyl cation, $\ce{NO^+}$, is a better electrophile than molecular $\ce{HNO3}$ (you don't get $\ce{NO_2^+}$ in this system without sulfuric acid or other strong auxiliary acid) and so you get nitrosobenzene in the substitution reaction. But then the nitrosyl group with its nonbonding electron pair is an attractive target for oxidation by the nitric acid and thereby is converted to $\ce{NO_2}$. Thus nitrobenzene. Then, the nitrobenzene, deactivated by the nitro group, will not react with the relatively mild electrophile $\ce{NO^+}$ anymore, meaning products C and D are never reached (and their nitrosyl groups would not have survived anyway).

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  • $\begingroup$ Well, someone doesn't like this. Care to propose an alternative mechanism? Didn't think so. $\endgroup$ – Oscar Lanzi Oct 10 '19 at 22:53

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