[navalava]

I attached the structure for Vitamin C.
I thought it would be the first -OH because it's close to the double bond with oxygen, so I thought there would be more electron withdrawal from the oxygen atom due to its electronegativity.

But the correct answer is actually the second -OH, which I don't really understand.
There was something about delocalization of electrons, but I don't know what causes that in this structure or why delocalization plays a greater role in increasing acidity than electron withdrawal.
I'd really appreciate any help on this!

[Dan]

Try drawing some resonance structures for the potential anions you could form by deprotonation. Consider the stability of these resonance structures and decide which anion is the most stable.

[Schrödinger]

It is the 2nd one. Consider the conjugate base that arises out of a proton abstraction by say, some other base.

The conjugate base has a negative charge on the O atom which can be delocalized thanks to the two pi bonds in conjugation with each other (one being the C=C and the other being C=O)

[OrgoTutor]

I don't know [...] why delocalization plays a greater role in increasing acidity than electron withdrawal.

"Electron withdrawal," which I usually think of synonymous with the "inductive effect" is a pull of the electron which toward a more electronegative atom through the sigma bond framwork--electron density doesn't get pulled through sigma bonds very well, at least not as well it can through a pi bond system. My guess as to why electons flow much more smoothly through a pi system is that the p orbitals which form the pi system are contiguous and overlapping orbitals--therefore they form a large low-energy Molecular Orbital (MO) that spans the whole pi system. Contrast that with the sigma bond framework which consists of mostly isolated sigma bonds--not one big MO--so the pull of any electron density through these sigma bonds will mostly be due to electrostatic effects, unlike the pi system which has electrostatic and MO effects to lower the energy of the electrons. I imagine there's a more accurate and detailed MO-based explanation for all this, but at this point I don't know it. However, the general rule is "whenever inductive effects and delocalization (or resonance) effects compete, the resonance effects always win."