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Topic: Why is the " Pyrolle like " is electron withdrawing group?  (Read 955 times)

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Offline sharbeldam

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Why is the " Pyrolle like " is electron withdrawing group?
« on: December 28, 2019, 07:07:33 AM »
I read that amines can be EDG if conjugated since they contribute their electrons by resonance and EWG if not conjugated when they cant contribute their electrons in resonance so the electronegativity of nitrogen dominates.

But if we take a look at imidazole, it is known that the " pyrolle like " nitrogen acts as an electron withdrawing group despite of it being conjugated and there are 2 resonance structures.

I'd be very appreciative if someone could clear that up.

Thanks.
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Offline hollytara

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Re: Why is the " Pyrolle like " is electron withdrawing group?
« Reply #1 on: December 28, 2019, 11:16:33 PM »
Are you sure about this?  the reactivity toward electrophilic aromatic substitution is pyrrole> imidazole > benzene > pyridine.  This makes it appear that the "pyrrole-like" N is an electron donor, but the "pyridine-like" N is an electron withdrawing group. 

Offline sharbeldam

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Re: Why is the " Pyrolle like " is electron withdrawing group?
« Reply #2 on: December 29, 2019, 02:52:09 AM »
I'm sure I read it but Im not sure it's true, Please take a look here
https://www.quora.com/Why-is-imidazole-less-basic-than-pyrazole
I read that also in researchgate site, the attached photo is from a book as well.

What confused me initially is to know why imidazole is more basic than pyrazole, they have both 1 available pair of electrons that does not include in the aromatic ring electrons, and they both have resonance structures that stabilizes the conjugate acid (added the resonance structures in attached picture since there is a youtube video that says that there is no resonance structures for pyrazole after it takes proton), that link says that imidazole is more basic because that pyrolle like nitrogen acts as a EWG and increase the positive charge on the conjugate acid which makes imidazole more basic since the pyrolle nitrogen is further away than pyrazole, it's a good explanation of the bascicity but im not convinced as to why it is an EWG as you seem to agree with me.

If it is not true, and they both have resonance structures-delocalization as shown below then why indeed is imidazole more basic?


Thanks .
« Last Edit: December 29, 2019, 05:49:24 AM by sharbeldam »
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Offline hollytara

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Re: Why is the " Pyrolle like " is electron withdrawing group?
« Reply #3 on: December 30, 2019, 01:11:35 AM »
So there are 5 resonance structures for both imidazole and pyrazole as conjugate acids.

Pyrazole: number N's and 2. 
structures:  double bonds 1=2 and 3=4, lone pair on 5  this has 2 + charges on N and a - charge on C5 (not good)
                  double bonds 2=3 and 4=5, lone pair on 1  this has + charge on N2
                  double bonds 3=4 and 5=1 lone pair on 2  this has + charge on N1
                  double bonds 4=5 and 1=2 lone pair on 3 this has 2 + charges on Ns and a - charge on C3 (not good)
                  double bonds 5=1 and 2=3 lone pair on 4 this has 2 + charge on Ns and a - on C4 (not good)

can you do the ones for imidazole?  They are always better since the two + charges aren't adjacent!

Offline sharbeldam

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Re: Why is the " Pyrolle like " is electron withdrawing group?
« Reply #4 on: December 30, 2019, 01:50:36 AM »
Big thanks!! i dont know why was i close to minded/lazy to draw only 2 resonance structures.
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