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Generation of Nitryl Cation in the Nitration of Benzene watch

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    Okay, so I know that to nitrate benzene, it has to be done with conc. nitric acid, and using conc. sulfuric acid as the catalyst. Next step is to see how the nitryl electrophile is generated. This is the summary that my book gives:

    1. H_2SO_4+HNO_3\Rightarrow HSO_4^-+H_2NO_3^+
    2. H_2NO_3^+\Rightarrow H_2O+NO_2^+
    3. H_2SO_4+H_2O\Rightarrow H_3O^++HSO_4^-

    Net reaction:

    2H_2SO_4+HNO_3\Rightarrow 2HSO_4^-H_3O^++NO_2^+

    Okay, now this is my understanding of it. The first and second step can be summarised as:
    H_2SO_4+HNO_3\Rightarrow HSO_4^-+H_2O+NO_2^+
    Then, that H2O reacts with more sulfuric acid, so it is not present in the net equation. So I agree with pretty much everything that the book says, but how did they get 2HSO_4^-H_3O^+ as one of the products? Why is there ionic bonding between the two molecules?

    Please explain .
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    Okay, this doesn't make any sense at all >< . I just checked my other textbook and it has the following:

    HNO_3+H_2SO_4\Rightarrow NO_2^++H_2SO_4^-+H_2O

    That makes sense, since after the nitration, the H+ reacts with the H2SO4 from above, reforming sulfuric acid, whereas in the net reaction from my first book (first post) - that doesn't happen.

    Which is correct, and why? ><
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    (Original post by ViralRiver)
    Okay, this doesn't make any sense at all >< . I just checked my other textbook and it has the following:

    HNO_3+H_2SO_4\Rightarrow NO_2^++H_2SO_4^-+H_2O

    That makes sense, since after the nitration, the H+ reacts with the H2SO4 from above, reforming sulfuric acid, whereas in the net reaction from my first book (first post) - that doesn't happen.

    Which is correct, and why? ><
    Here's the complete scheme:

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    (Original post by charco)
    Here's the complete scheme:

    Thank-you, you have no idea how much more sense that makes to me.

    Just one question, I now have this as the complete, net reaction, is it correct:?

    C_6H_6+HNO_3\xrightarrow[H_2SO_4]{\text{conc.}}C_6H_5NO_2+H_2O
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    Also, how would I explain that in an exam?

    The sulfuric acid dissociates, producing a proton (H+) and the hydroxysulphate ion (HSO4-). Nitric acid accepts this proton, forming H2NO3+ (what's the name of this cation?), which then dehydrates to form the intermediate of water (H2O) and the nitryl cation (NO2+). The nitryl cation then acts as the electrophile, in the electrophilic substitution of benzene, forming nitrobenzene.

    One extra thought; nitric acid accepts the proton from sulfuric acid, doesn't that mean it's acting as a base?
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    (Original post by ViralRiver)
    Also, how would I explain that in an exam?

    The sulfuric acid dissociates, producing a proton (H+) and the hydroxysulphate ion (HSO4-). Nitric acid accepts this proton, forming H2NO3+ (what's the name of this cation?), which then dehydrates to form the intermediate of water (H2O) and the nitryl cation (NO2+). The nitryl cation then acts as the electrophile, in the electrophilic substitution of benzene, forming nitrobenzene.

    One extra thought; nitric acid accepts the proton from sulfuric acid, doesn't that mean it's acting as a base?
    yes, that's pretty much it. You don't need to mention water as being an intermediate - it is absorbed by the conc sulphuric acid (which is a good dehydrating agent). The only important intermediate is the nitronium ion.

    And yes, the nitric acid is behaving as a Bronsted Lowry base in this reaction.
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    (Original post by charco)
    yes, that's pretty much it. You don't need to mention water as being an intermediate - it is absorbed by the conc sulphuric acid (which is a good dehydrating agent). The only important intermediate is the nitronium ion.

    And yes, the nitric acid is behaving as a Bronsted Lowry base in this reaction.
    Okay, thanks a lot .
 
 
 
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