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Topic: How should I explain this pKa order?  (Read 3132 times)

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Offline kanonsviel

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How should I explain this pKa order?
« on: September 24, 2010, 02:55:40 AM »
I know that a stronger acid corresponds to a weaker conjugate base.
But why the 3H-pyrrole(I'm not sure if it's named like this) is a weaker base than pyridine?
Is that because it is readily converted to 1H-pyrrole which is an aromatic compound, donating it's electron pair to the aromatic system? ???

Offline Schrödinger

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Re: How should I explain this pKa order?
« Reply #1 on: September 24, 2010, 05:04:24 AM »
Yes. When one proton from the 3rd position in the pyrrole ring leaves the ring, it leaves behind a negative charge. That -ve charge will be in conjugation with the 2 other pi bonds... That would result in aromaticity...hence the acidity.

Whereas in the case of pyridine, the system is aromatic irrespective of whether the Nitrogen is protonated or not
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Offline kanonsviel

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Re: How should I explain this pKa order?
« Reply #2 on: September 24, 2010, 08:32:03 AM »
Got it, thanks for your help ! :)

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