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Topic: effect of ester on acidity  (Read 5189 times)

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HoustonS

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effect of ester on acidity
« on: November 22, 2005, 01:07:46 PM »
HELLO,

Can you tell me why an ester decreases the acidity of protons on ketones/aldehydes?

My confusion is about the resonance structures. When I draw them I see more structures when there is an ester. I must be doing it wrong. Any hints?

Thanks a lot!

S
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Re:effect of ester on acidity
« Reply #1 on: November 22, 2005, 01:26:37 PM »
Think about the resonance structures you have before deprotonation.  Would you say that the carbonyl group of the ester is more or less electron deficient than the carbonyl group in a ketone?  How would this affect the acidity of the alpha protons?
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Offline Albert

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Re:effect of ester on acidity
« Reply #2 on: November 22, 2005, 01:27:13 PM »
In my opinion, the presence of two O-atoms in esters (which attract electrons) make them less suitable of losing an H+.

Moreover, I think a base (like OH-) is more liable to give this reaction:

RCOOR + OH- -> RCOOH + RO-
« Last Edit: November 22, 2005, 02:11:00 PM by Albert »
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HoustonS

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Re:effect of ester on acidity
« Reply #3 on: November 22, 2005, 03:02:08 PM »
somehow i still don't get it...i am trying...

any more hints?

i mean the ester has more resonance than the ketone/aldehyde alone, right? if so, shouldn't that make the alpha protons more acidic? does it have to do with the alkyl groups on the ester?

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Re:effect of ester on acidity
« Reply #4 on: November 22, 2005, 05:14:19 PM »
Think in terms of pure electron density.  Since you can draw more resonance structures with the ester, then the charges are more diffuse, right?

Maybe try thinking about it this way: why is it favorable to deprotonate alpha to a carbonyl?
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