AAMC CBT 7 Question 98 BS

[MedChallenge]

http://www.e-mcat.com/ITDVersions/5...mid=19&language=ENU&urid=PT7P/Pass03_Fig2.GIF

I have a problem at a very basic level on understanding the acidity of certain protons. Why are the protons (in Compound 7) at carbons (c) and (d) more acidic than the proton at (a)? Can't the proton at (a) take part in resonance with the double bond and also be stabilized by the CO2Et group?

I have a feeling the answer involves the positive charge not being able to transfer to the carbonyl or CO2Et group, but why is that?

Any help is greatly appreciated!

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[Rabolisk]

There is no resonance with the double bond. The double bond would have to be shifted over once more for there to be resonance. There is a difference between vinylic proton and allylic proton.

There is some stabilization due to the CO2Et group, but this effect is not as big as a regular CO2R group. The single-bonded oxygen (the ester oxygen) has lone pairs that will donate some electron density, resulting in less stabilization of the negative charge. Remember that conjugate bases of neutral molecules have negative, not positive charge.

http://www.chem.ucalgary.ca/courses/351/Carey5th/Ch21/ch21-2.html

 

[MedChallenge]

Ah. I made two very fundamental mistakes in my thought process. I'm still thinking of carbocation stabilization in Sn1 rxns which wouldn't help for a conjugate base with negative charge! lol

Thanks for the clarification.