Acidity of alpha hydrogens ranking

[brownhamster]

This might be out of the scope of the mcat, but I once knew it, and can't find the right explanations.

This question is based on # 606 in examkrackers ochem.

pka of alpha H in aldehyde < pka of alpha H in ketone.
How would you explain that?

Also, pka of alpha H in ester's seem to be the least reactive among the carboxylic acid derivatives. Why is that? I remember it had something to do with the lone pairs on the ester oxygen competing for resonance.

Also the alpha H of an amide is the most acidic among the carboxylic acid derivatives. Again, I can't find an explanation.

And finally, how would you rank the acidity of the alpha hydrogen of an acid chloride, and what would be the reasoning?

Thanks.

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[t1me]

According to this link and many other internet sources:

http://www.chem.ucalgary.ca/courses/350/Carey/Ch21/ch21-2.html

pKa of aldehyde < pKa ketone

Alpha H of amide is more acidic because imines are more stable than alkenes (as part of the resonance structure of the conjugate base) due to nitrogen being more electronegative.

When trying to see what's the most acidic, consider the conjugate base stability. Determine stability from possible resonance, hybridization, and inductive effects.

 

[chemtopper]

aldehyde is far more acidic with a pKa near 17
Ketones are acidic with (pKa &#8776; 20)
I think you have given the reverse order .

pka of alpha H in aldehyde < pka of alpha H in ketone.
In ketones there are two alkyl groups around C=O and alkyl groups are electron donating in nature.This makes C=O as less hungry for electrons and has less tendency to accept electrons and release H+ ions .Hence its pKa is more than aldehyde that means it alpha H is less acidic than alpha hydrogen of aldehyde .


acyl chloride > anhydride > ester > amide
Which can be accounted for on the basis of electron donating properties of the substituent attached. The greater the electron donating properties of the substituent, slower is the rate of release of alpha H .