Acid/base: electron-donating vs electron-withdrawing

[TheRelevant]

Hey guys. Hope your MCAT studying is going well! I was hoping one of you might know the reason behind the following phenomenon:

One of these oxygens acts as an electron-withdrawing group whereas the other acts as an electron-donating group. But both have lone pairs of electrons to donate to the resonance structures. But they still have idiosyncratic effects?

The only reason I could think of, and it doesn't seem very convincing, is that the lone pairs of electrons in the ether are right beside the pi-bond. So they can directly donate electron density into the resonance delocalization. In the p-hydroxybenzaldehyde however, there is no adjacent pi-bond to donate the lone pair to? And oxygen being electronegative, it causes the alpha carbon to attain a partial positive charge, which acts as an electron-withdrawing group from the resonance structure. Any input would be greatly appreciated! Thank you 😀

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[JMMTB]

Drawing out the resonance structures helps a ton when deciding whether something is electron donating or withdrawing.

With the ether/alcohol a pair of electrons "jumps" (imagine electron arrow pushing) down to form a double bond between the oxygen and the carbon, but in order to do so it must force the electrons in the adjacent pi bond to jump out and go up onto the carbon. This then adds electron density to the ring and charges it. For the benzene ring drawn resonance leads to a lone pair on the carbons ortho and para to the alcohol.

As for the carbonyl you are right that the oxygen is more electronegative and will cause the electrons in the pi bond to jump up onto the oxygen. This leaves a positive charge on the carbon. The pi bond adjacent to the carbonyl can then "spread" the positive charge over several carbons. It is this delocalizing effect that stabilizes it. These positive charges are also found ortho and para to the carbonyl.

So for example with an ester, it can be either electron donating or withdrawing depending on which side it attached to the ring.

Kind of hard to explain without seeing the arrows being drawn but hope that helps!

 

[DrknoSDN]

Going out on a limb... Are you asking about the pKa comparison? An aldehyde is withdrawing while a ether is donating.
Probably easier to just memorize the withdrawing and donating groups.

Reference to the benzene specifically:
http://chemwiki.ucdavis.edu/Organic..._on_resonance_effects_on_acidity_and_basicity

 

[TheRelevant]

The pKa comparison wouldn't really be helpful (unless you used relative comparisons with acrylic acid and phenol). I guess I am having trouble figuring out how oxygen can act as an electron-donating to another oxygen in one instance, but as electron-withdrawing in another. And my resonance sucks. Would anyone care to show me where I am going wrong?