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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Mikez on April 19, 2011, 06:44:14 PM

Title: Acidity
Post by: Mikez on April 19, 2011, 06:44:14 PM

Why is the ketone more acidic than the ester?? Similary where does aldehyde fit in?

Deprotonation of the alpha hydrogen--- ester has more resonance structures but my prof said that the ketone is more acidic???

THANK YOU!!!

Title: Re: Acidity
Post by: Honclbrif on April 19, 2011, 08:29:25 PM

The carbonyl stabilizes the anion by moving the electron density to the more electronegative oxygen atom where it is more stable, and away from the less electronegative carbon where it is less stable. Its that second part that really plays a role here.

Considering the ketone enolate: how many resonance structures have a charge on the alpha-carbon?

Now consider the ester enolate: how many resonance structures have a charge on the alpha-carbon?

Title: Re: Acidity
Post by: Mikez on April 20, 2011, 11:00:17 AM

doesn't the ester have more resonance structures? which means its more stable and more acidic?

and the aldehyde has even fewer resonance structures than the ketone enolate, yet the aldheyde is more more acidic?

thanks

Title: Re: Acidity
Post by: Honclbrif on April 20, 2011, 12:20:23 PM

You're right that more resonance structures usually means more stable, but that is a generalization and you sometimes have to look at things more closely. In the case of the ester enolate some of those resonance structures transfer electron density back to carbon, destabilizing the enolate. Draw them out and you'll see.

As for ketone vs. aldehyde acidity: for a simple case consider the electron donating ability of a hydrogen vs. an alkyl group.