Say you take a substituted benzene* and react it with oleum and nitric acid. Would it result in a nitrated ring, a sulphonated ring, or some sort of a combination?

Thinking about it in terms of the first step, the stronger base of $\ce{SO3}$ and $\ce{HNO3}$ seems likelier to get protonated first, making the preferred protonated electrophile (say it's A) less reactive than the other possible intermediate (say that one's B). But then the next step, the r.d.s, requires a pretty strong, reactive electrophile to attack the ring, which would make B the probable winner.

How do I decide between these factors, if such a general assumption can be made?

*I assumed a substituent wouldn't make a difference in this comparison. The answer, if so, would be more widely applicable than if the question were just about benzene. If there are differences, a note on them would be appreciated.

  • 4
    $\begingroup$ 100% dry nitric acid is not possible and thus the oleum would dissolve in that water to form sulphuric acid. And it is widely know that a mixture of sulphuric acid and nitric acid causes nitration. $\endgroup$ – Nisarg Bhavsar May 6 at 6:35
  • 2
    $\begingroup$ Wikipedia says: "Oleum is a harsh reagent, and is highly corrosive. One important use of oleum as a reagent is the secondary nitration of nitrobenzene. The first nitration can occur with nitric acid in sulfuric acid, but this deactivates the ring towards further electrophilic substitution. A stronger reagent, oleum, is needed to introduce the second nitro group onto the aromatic ring." $\endgroup$ – Nilay Ghosh May 6 at 6:37
  • $\begingroup$ @NisargBhavsar: great reasoning there. But does the added water always suffice to turn all the SO3 into sulfuric acid? If not, there's a chance for sulfonation to occur. $\endgroup$ – harry May 6 at 6:52
  • $\begingroup$ @NilayGhosh: thanks, that explains how nitration works. Isn't sulfonation possible, though? $\endgroup$ – harry May 6 at 6:54

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.