Acidities of Alpha Protons

[virtualmaster999]

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Hi everyone!

I'm a little confused on ranking acidities for compounds with alpha carbons. So I understand that the more electron donating the groups are, the less acidic. So I get how aldehydes for example would be more acidic than ketones and esters, but why are diketones more acidic than aldehydes? Wouldn't two R groups be very electron donating, or does the fact that there are multiple alpha hydrogens make a difference?

Thank you in advance!

 

[DAT Destroyer]

Hi everyone!

I'm a little confused on ranking acidities for compounds with alpha carbons. So I understand that the more electron donating the groups are, the less acidic. So I get how aldehydes for example would be more acidic than ketones and esters, but why are diketones more acidic than aldehydes? Wouldn't two R groups be very electron donating, or does the fact that there are multiple alpha hydrogens make a difference?

Thank you in advance!

A diketone such as 2, 4-hexane dione is indeed more acidic than an aldehyde or ketone. Removal of the double alpha hydrogen would form an enolate anion that is very very well stabilized,,,,,,Why ? It is flanked by TWO carbonyl groups that can participate in anion stabilization. We can write 3 resonance forms....A simple aldehyde or ketone gives only 2 resonance forms.....More resonance forms means greater anion stabilization, hence the hydrogen is easier to come off. This is reflected in the pKa value of about 9......versus 17 or 18 for aldehydes and ketones. This is a HUGE concept. If you still need clarity...refer to a standard organic text such as Wade, McMurray, Solomons, or Klein .

Hope this helps.
Dr. Romano