Acidity Ranking Question

[Quinque]

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Can someone please explain to me why compound B is more acidic than compound D?

I know that both substituents on compound B and D are electron donating, which would stabilize the negative charge on the conjugate base. I also know that compound D has a stronger donator.

BUT, I also know that B's substituent can create another very very minor resonance structure if I de-prononate the hydroxyl H (2 neg charges on two carbons and a positive charge on oxygen). Is this why even though compound D is a stronger donator, compound B is more acidic? Thank you.

 

[Bersabeh]

View attachment 210674

Can someone please explain to me why compound B is more acidic than compound D?

I know that both substituents on compound B and D are electron donating, which would stabilize the negative charge on the conjugate base. I also know that compound D has a stronger donator.

BUT, I also know that B's substituent can create another very very minor resonance structure if I de-prononate the hydroxyl H (2 neg charges on two carbons and a positive charge on oxygen). Is this why even though compound D is a stronger donator, compound B is more acidic? Thank you.

yes they both donators, but here its mostly due to stereochemistry, causing steric hindrance, causing it to be more acidic or ready to leave.

 

[DAT Destroyer]

View attachment 210674

Can someone please explain to me why compound B is more acidic than compound D?

I know that both substituents on compound B and D are electron donating, which would stabilize the negative charge on the conjugate base. I also know that compound D has a stronger donator.

BUT, I also know that B's substituent can create another very very minor resonance structure if I de-prononate the hydroxyl H (2 neg charges on two carbons and a positive charge on oxygen). Is this why even though compound D is a stronger donator, compound B is more acidic? Thank you.

This is not so easy ! Yes , B would be more acidic than D. Why ? We must consider the anion. The anion in B is better stabilized. If you removed the H from the OH group in each, and examined the RESONANCE forms in the anion......you would note that there is a bit of DESTABILIZATION in D because a positive charge would reside on this resonance hybrid. We get a positive charge on the nitrogen atom. This is not seen in B, where we have a methoxy group. Thus, B is better stabilized. An anion that has greater stabilization translates into a more acidic compound, hence a lower pKa. Literature values for 3-methoxyphenol and 4-aminophenol are 9.65 and 10.30 respectively. Resonance theory works nicely here . I hope this helps on a difficult, but delightful question.

Dr. Romano