Excellent question. The short version is, “experiments tell us resonance is more important for O and N”.

Acidity is all about the stability of the conjugate base, which for esters and ketones results in a species known as an “enolate”.

The more electron-poor the C=O bond, the better lone pair on carbon (i.e. the negative charge of the conjugate base) is able to donate into it, forming a new pi-bond and resulting in a (more stable) negative charge on oxygen.

Oxygen and nitrogen substituents have two effects which operate in opposite directions.

On one hand, these atoms are electronegative, which means that they will withdraw electron density through the single bond. On the other hand, they are capable of *donating* electron density through forming pi-bonds with adjacent electron acceptors (e.g. C=O) via resonance, an effect known as pi donation.

So what effect wins? It’s an easy question to ask, but hard to predict just from first principles alone. What we observe from experiments is that for oxygen and nitrogen, the pi-donation effect outweighs the inductive effect.

This means that the C=O is *less* electron poor in esters and (especially) amides because of donation from the adjacent oxygen and nitrogen. This means that the enolates of these species are less stabilized, which is reflected in their weaker acidity.

A similar effect is responsible for why O and N substituents are “activating” in electrophilic aromatic substitution reactions – the pi-donation of O and N outweighs the electron withdrawing effect.

(with chlorine, the electron-withdrawing effect outweighs the pi-donation effect).

I’d be happy to elaborate – text is a tough medium through which to explain these things :-)

My guess would be that induction effects possibly only really matter with halogens, since they are highly electronegative. Or that the electron pair in the pi bond is already delocalized in resonance with the other single bonded oxygen, meaning the effects of resonance are less in the ester compared to the ketone. Or both? I would appreciate anyone with more knowledge than me helping me out here, every day it feels like I find something that goes against everything I thought I knew 🙏

That’s about the right number.

I suggest not dealing with pKb. Instead I suggest thinking about the pKaH, the “pKa of the conjugate acid”. For example the pKaH of the ethoxide ion CH3CH2O(-) is 16.

They are around the same.

You are correct, fixed!

Great catch. Thank you. I fixed the image and uploaded a replacement. Thanks!!!

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