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Topic: Acidic Hydrogen of Aldehyde vs Ketone  (Read 2453 times)

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Offline rj729

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Acidic Hydrogen of Aldehyde vs Ketone
« on: October 23, 2017, 03:11:38 PM »
Can anyone explain to me why this mechanism has the less acidic proton of the ketone being deprontonated vs the aldehyde? https://ibb.co/jEc5Y6
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Offline sjb

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Re: Acidic Hydrogen of Aldehyde vs Ketone
« Reply #1 on: October 23, 2017, 04:44:52 PM »
What are the relevant pKas - I don't think the -CHO is that acidic?
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Offline rj729

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Re: Acidic Hydrogen of Aldehyde vs Ketone
« Reply #2 on: October 23, 2017, 11:04:04 PM »
Everything that I've read says an aldehyde is more acidic than a ketone. According to this site http://www.chem.ucalgary.ca/courses/350/Carey5th/Ch21/ch21-2.html the pka of an aldehyde is 17 vs that of a ketone which is 20.
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Offline spirochete

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Re: Acidic Hydrogen of Aldehyde vs Ketone
« Reply #3 on: October 24, 2017, 01:01:55 AM »
Those pKa's are only true if an aldehyde actually has acidic alpha hydrogens, such as in acetaldehyde. Benzaldehyde lacks any acidic alpha hydrogens. It only has the carbonyl C-H, which is much less acidic. The conjugate base for that one doesn't have any good resonance structures where the negative charge is shared by oxygen.
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Offline rj729

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Re: Acidic Hydrogen of Aldehyde vs Ketone
« Reply #4 on: October 24, 2017, 08:02:21 AM »
Oh wow. Makes sense. Thank you!
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