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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: mana on June 11, 2018, 04:28:05 AM

Title: comparing acidity of pyridinium and imidazolium cations
Post by: mana on June 11, 2018, 04:28:05 AM

hi all
why pyridinium cation is stronger acid than  imidazolium cation although the resonance structures of pyridinium cation are more than imidazolium cation.
I mean, if imidazolium cation is unstable, so it must donate a proton immediately to return to its stability :P

Title: Re: comparing acidity of pyridinium and imidazolium cations
Post by: Babcock_Hall on June 11, 2018, 10:44:00 AM

I would think in terms of the Lewis octet rule.

Title: Re: comparing acidity of pyridinium and imidazolium cations
Post by: wildfyr on June 11, 2018, 01:47:09 PM

I would look at it as an aromaticity question. Remember that all resonance forms are not equal, some are way bigger contributors than others. Do you think the resonance structures where the aromaticity of pyridine is broken are big contributors?

Title: Re: comparing acidity of pyridinium and imidazolium cations
Post by: mana on June 12, 2018, 01:21:45 AM

Quote from: wildfyr on June 11, 2018, 01:47:09 PM

I would look at it as an aromaticity question. Remember that all resonance forms are not equal, some are way bigger contributors than others. Do you think the resonance structures where the aromaticity of pyridine is broken are big contributors?

The only rule I know in this case is the positive charge on electronegative atom causes instability :-[ and here only the first structure in pyridine obeys from this, is there any more instability factors?

Title: Re: comparing acidity of pyridinium and imidazolium cations
Post by: Babcock_Hall on June 12, 2018, 10:44:14 AM

I think that you may be misunderstanding or misapplying the rule that you mentioned, and you are also disregarding other rules.  A positive charge on a nitrogen atom that has a full octet is frequently observed in various Lewis structures.  All else held equal, I would rather put a positive charge onto an atom having a full octet than a positive charge onto an atom that did not.  There is also the question of aromaticity that wildfyr mentioned.  Perhaps you can start with these two ideas and explain why some of the resonance structures are minor contributors.

Title: Re: comparing acidity of pyridinium and imidazolium cations
Post by: mana on June 13, 2018, 02:35:50 AM

Quote from: Babcock_Hall on June 12, 2018, 10:44:14 AM

I think that you may be misunderstanding or misapplying the rule that you mentioned, and you are also disregarding other rules.  A positive charge on a nitrogen atom that has a full octet is frequently observed in various Lewis structures.  All else held equal, I would rather put a positive charge onto an atom having a full octet than a positive charge onto an atom that did not.  There is also the question of aromaticity that wildfyr mentioned.  Perhaps you can start with these two ideas and explain why some of the resonance structures are minor contributors.

So  imidazulium cation is a weeker acid because it has 2 octet forms of resonance structure but pyridine just has one octet form of resonance structure, am I right?

Title: Re: comparing acidity of pyridinium and imidazolium cations
Post by: Babcock_Hall on June 14, 2018, 09:03:09 AM

Yes, I believe that you are correct.

Title: Re: comparing acidity of pyridinium and imidazolium cations
Post by: mana on June 14, 2018, 12:08:56 PM

thanks for all the answers :)