Which Proton is More Acidic?

[densaugeo]

Hi everyone, I was wondering if someone could explain which proton is more acidic for this problem:

Also, for the next problem, is this an exception to the CARDIO rule? Would proton #2 be more acidic due to the sp character even though proton #1 has resonance structures?

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[FeralisExtremum]

Hi everyone, I was wondering if someone could explain which proton is more acidic for this problem:
View attachment 189798

I'll make an attempt on this one, but hopefully someone more knowledgeable can confirm:

I think proton #1 is more acidic. Notice the OH at #1 and #2 both have another OH group ortho relative to each other, so that doesn't distinguish them in terms of acidity much. But there is also a carbonyl group (recall these are electron withdrawing, therefore they increase acidity) and its position is different relative to each: it is meta to #2 and para to #1. When it comes to the amount of effect a group has based on its position: ortho > para > meta. Therefore it has the greatest effect (in this case, increased acidity) relative to the OH group at #1 where it is para, so #1 is the most acidic proton.

For your second question, I have no idea.

 

[DAT Destroyer]

Hi everyone, I was wondering if someone could explain which proton is more acidic for this problem:
View attachment 189798

Also, for the next problem, is this an exception to the CARDIO rule? Would proton #2 be more acidic due to the sp character even though proton #1 has resonance structures?

View attachment 189799

Problem #1
For the Phenols.....Proton #1 is more acidic. The OH group para to the carbonyl wins. The conjugate base phenoxide anion has more important resonance forms. If you draw them out, you would see the substituent carbonyl group participating in the resonance.

Problem #2
Resonance is the best stabilizing factor......Very rarely does another factor trump resonance !!!!!
However........this one is interesting. The pKa of the triple bond is about 25......and the pKa of the H coming off the ring flanked by the alkene group is about 37. Resonance usually wins....but not here. The resonance form that is achieved by removal of the benzylic H does not count all that much, since the benzene ring would be disturbed. Delocalization on a carbon is only fair.

A really great book that has many p Ka examples is by Clayden if you want to look at more.

I hope this helps......

Dr. Jim Romano

 

[FeralisExtremum]

Problem #2
Resonance is the best stabilizing factor......Very rarely does another factor trump resonance !!!!!
However........this one is interesting. The pKa of the triple bond is about 25......and the pKa of the H coming off the ring flanked by the alkene group is about 37. Resonance usually wins....but not here. The resonance form that is achieved by removal of the benzylic H does not count all that much, since the benzene ring would be disturbed. Delocalization on a carbon is only fair.

I was wondering Dr. Romano, if we were to deprotonate that H at position 2 (at the alkyne), is there any resonance/conjugation with the nearby alkene group? Can alkynes even be part of a conjugated pi system?

 

[DAT Destroyer]

I was wondering Dr. Romano, if we were to deprotonate that H at position 2 (at the alkyne), is there any resonance/conjugation with the nearby alkene group? Can alkynes even be part of a conjugated pi system?

Alkynes can indeed be part of a conjugated system.......why not ? They have p orbitals.....but only uses one for resonance since they are in different planes. There is no significant resonance with the alkene group that you drew.....thus the acidity effect is due to the fact that the electron on the acteylide anion lies in a stable sp orbital.....with significatnt s character.