DATBOOT CAMP Verification

[DrRoyal Pains]

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I am on datbootcamp's question of the day email list, and the question today asked which was more acidic. I narrowed it down to either benzene or propyne. The answer was propyne. I understand that the compound is sp hybridized and the lone pair of electrons are closer to the nucleus and more stable, and I understand that benzene is sp2 hybridized.But benzene has resonance and the alkyne does not. Following Chad's CARDIO pneumonic, wouldn't this make benzene a stronger acid because it is aromatic and has resonance?

 

[Jordwin]

That's a great question, I was thinking the same thing.

 

[Vistabane]

The benzene would have resonance once a hydrogen atom was lost, but because benzene is aromatic it will strongly oppose having a hydrogen removed because that will break the aromaticity of the ring. For that reason, I think that propyne is the stronger acid because it has both a stable conjugate base due to the triple bond and a more favorable loss of the hydrogen atom compared to the benzene ring.

 

[paymaant]

I am on datbootcamp's question of the day email list, and the question today asked which was more acidic. I narrowed it down to either benzene or propyne. The answer was propyne. I understand that the compound is sp hybridized and the lone pair of electrons are closer to the nucleus and more stable, and I understand that benzene is sp2 hybridized.But benzene has resonance and the alkyne does not. Following Chad's CARDIO pneumonic, wouldn't this make benzene a stronger acid because it is aromatic and has resonance?

Alkynes do have resonance. I believe propyne is more acidic than benzene because benzene loses its aromaticity when it loses a proton.

 

[DrRoyal Pains]

Even if benzene looses its aromaticity, it still has more resonance than the alkyne. Wouldn't this make it more acidic?

 

[Illfavor]

I didn't even use cardio. pka of a terminal alkyne is 25. First and foremost, alkenes are much less acidic than alkynes in general. So that's case #1 for the alkyne. Secondly, breaking aromaticity is one of the least favorable things you can do. It's incredibly unfavorable. Benzene's pka has got to be greater than the standard alkene, putting it in the 40s.

EDIT: As a corollary here, consider why cyclopentadiene is such a great acid! (when compared to other alkenes) It wants to lose a proton to become aromatic. Or, why do we have to use catalysts in reactions with benzene?

Point is, CARDIO isn't fool-proof (as Chad admits), so use all of your knowledge when making these kinds of choices.

 

[Glimmer1991]

I just gotta interject this because so many people have this misconception--it's mnemonic, not "pneumonic." We aren't talking about pneumonia here. 🙂

 

[DrRoyal Pains]

Alright. Thanks for the clarification. I know alkynes are more acidic than alkenes, that is basically common sense. I was just confused on the resonance/breaking aromaticity portion. And to glimmer, thanks haha. Good to know.