Basicity and Pka

[sos5]

From Mprep question of the day
Acetone (on the left) and methyl acetate (on the right) are shown below. Regarding deprotonation of a hydrogen from the left methyl group (for both molecules), which molecule will have the higher pKa and why?

A
The molecule on the left will have the higher pKa and destabilize the conjugate base
B
The molecule on the left will have the higher pKa and stabilize the conjugate base
C
The molecule on the right will have the higher pKa and destabilize the conjugate base
D
The molecule on the right will have the higher pKa and stabilize the conjugate base

Answer is C. I thought the answer would be D. I don't understand how methyl acetate is destabilized upon loss of a proton. Isn't there more resonance once it is deprotonated so more stabilized?

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[Neuroplasticity]

1. A higher pka corresponds to a weaker acid, which corresponds to a stronger conjugate base, or a less stable conjugate base. HF has a higher pka than Cl-, so F- is a stronger base than Cl- because F- is less stable as an anion.

2. More resonance forms does not mean something is more stable. The ketone anion will have 2 resonance forms, one with the (-) on the C, and one with the (-) on the oxygen. The ester will have 3 resonance forms, one with the (-) on the C and the other 2 oxygens neutral, one with the (-) on the C and upper oxygen (-) lower oxygen (+), and one with the C neutral with a (-) on upper oxygen. So the ketone has the (-) on the carbon in 1/2 resonance forms, while the ester has the (-) on the carbon in 2/3 resonance forms, making the ester less stable.

3. The ester group is more electron rich than the ketone. Depronating adds more electron density, so the ester doesn't depronate as readily. (You are probably familiar with electron withdrawing groups lowering pka, so electron donating groups raise pka)

 

[NextStepTutor_3]

That's a great answer. Just wanted to mention that I wouldn't even worry about resonance for this question - we can answer it very quickly through elimination. Stronger acids have more stable conjugate bases (which explains why they lose a proton so readily). Weaker acids (like HF) have less stable conjugate bases, making them less likely to be deprotonated.

From this alone, we already know that a stronger acid = a lower pKa = a more stable conjugate base, and a weaker acid = a higher pKa = a less stable conjugate base. This eliminates choices B and D, since a molecule with a higher pKa wouldn't have a stable conjugate.