orgo, question about acidity

Question

Vocaloid · Answer

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sorry if this is a dumb question but why is phenol more acidic than the carboxylic anhydride?

Vocaloid · Answer

Please do not comment unless you are making a serious attempt to answer the question.

Tranquility · Answer

I'm struggling to find anything on this. I'm trying to find a list I've seen but it goes something like
sulfonic acid > carboxylic acid > phenol > thiol > alcohol > aldehyde > ketone > esters

forgot where acid anhydride going in that

But by looking at pKa, we know that 
a phenol (pKa of 10) is MORE acidic than a ketone (pKa of 20) and acid anhydride (pKa of ~17)

phenol > acid anhydride > ketone
in terms of acidity

A phenol is more acidic because it has an aromatic ring so when you lose the hydrogen ion, its conjugate base is stabilized because the negative charge on the oxygen is delocalized around the ring structure.

Tranquility · Answer

@imqwerty

Vocaloid · Answer

Oh

(I just realized I drew a ketone, pretend the first one is a carboxylic acid)

Tranquility · Answer

So the first thing that comes to mind is that acid anhydride can't be a very strong acid since it doesn't have the electronegative O-H or S-H bonds. It does have the alpha hydrogens which are slightly acidic.

But I think the main reason why phenols are more acidic is due to the resonance stabilization of the conjugate base because the negative charge can be delocalized.

Hopefully, qwerty can contribute more to this discussion :)

Tranquility · Answer

attachment

Tranquility · Answer

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imqwerty · Answer

the third figure is 2 4-pentanedione btw but ig it's supposed to be carboxylic anhydride instead

imqwerty · Answer

I agree with traq + the acidic hydrogen from phenol is attached to oxygen which is more electro-negative and can release H+ more easily as compared the the less electro-negative carbon in carboxylic anhydride

also $Ph-O^{-}$ will have a larger resonance as compared to the carboxylic anhydride's conjugate base
so since phenol's conjugate base is more stable than carboxylic anhydride's conj. base, the H+ releasing tendency will be more for phenol

Tranquility · Answer

That's the IUPAC name, acid anhydride is like the structure name of a diketone with a carbon in between.

Like you can have a functional group of a carbonyl

But thank you for answering qwerty!

imqwerty · Answer

but they have an oxygen in the middle

Tranquility · Answer

Oh you're right, not sure how I missed that

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