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#1 2009-02-23 16:09:49


Nitration of pyrrole, preferred product wrote:

Why does pyrrole nitrate to form mostly 2-nitropyrrole instead of 3-nitropyrrole?

That is a good question, because logically the 3-nitropyrrole might be expected. It is furtherest from the electron withdrawing nitrogen and the electron movements mimic an enamine alkylation. If that is not the case, why does nitration occur at C2?

I don't know the reason, so I will present three possibilities. One method of analysis uses the resonance structures of the intermediate cations. In that analysis, the nitration at C2 has two resonance structures while attack at C3 has one. For that reason, nitration at C2 may be expected.

I don't know which argument should be more true. The first is the simplest.

However, I am uncomfortable with this resonance argument unless a reaction is clearly reversible so that the resonance structures can shift an equilibrium toward the greater resonance contributor. While that may be the case here, in other reactions, that is often not the case.

If we were to look at the electron densities of the resonance structures of pyrrole, we would find that the charge densities are greater at C3 (-0.11) than at C2 (-0.035) or nitrogen (+0.28). If we consider electronic contributions of the ground state, then we could argue that the negative charge at C2 should be greater than that at C3 due to a smaller charge separation.

*edit* The charge calculations above were Huckel MO calculations. I found another calculation, an orbital partial charge contribution based upon atom electronegativities. It results in N (-0.0902), C2 (-0.1196) and C3 (-0.0535). I don't know this proves anything except to discount the value of calculations.

Finally, it has been thought that nitration occurs by an ipso attack of a nitronium ion on a benzene ring. That analogy for pyrrole should occur on the nitrogen atom. Migration of the nitro group would place the nitro group at C2 if the elimination reaction were faster than migration to C3.



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