Which hydrogen is more Acidic? pKa?

[RSD2014]

What is the pKa of the Hydrogen on Aldehyde?

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[loveoforganic]

Sp2 carbanion with no resonance stabilization vs an alkoxide

 

[UndergradGuy7]

Never seen an aldehyde h get depronated so go with the alcohol more acidic.

 

[UCB05]

I don't know the exact pka value, but you should know that the carbonyl hydrogen is very nonacidic. Loss of that proton leaves a carbanion with no resonance stabilization, so just about everything else involved in any relevant acid/base mechanism is more acidic than that hydrogen.

 

[chromosome21]

I've heard of alpha hydrogen acidity of an aldehyde, but never seen a hydrogen getting pulled off of an aldehyde. Even if we assume it to be true and pull off the hydrogen, there will be a negative charge on an SP2 carbon. Which is less stable than a negative charge on an electronegative atom like an oxygen. So it could be guessed that alcohol hydrogen could be more acidic than an aldehyde hydrogen.

It's important to know, though, that the carbonyl compound alpha hydrogens are less acidic than alcohol and water.

 

[orgohacks]

I've heard of alpha hydrogen acidity of an aldehyde, but never seen a hydrogen getting pulled off of an aldehyde. Even if we assume it to be true and pull off the hydrogen, there will be a negative charge on an SP2 carbon. Which is less stable than a negative charge on an electronegative atom like an oxygen. So it could be guessed that alcohol hydrogen could be more acidic than an aldehyde hydrogen.

It's important to know, though, that the carbonyl compound alpha hydrogens are less acidic than alcohol and water.

True, the alpha protons of the aldehyde are more acidic than the C-H proton of the aldehyde.

In fact I don't know that I've ever seen an aldehyde proton deprotonated in the manner shown on the diagram. The reason is that most bases (also being nucleophiles) preferentially add to the aldehyde (1,2-addition) instead of deprotonating it.

 

[RCT PC CRN]

Never seen Aldehyde H coming off so I guess pKa would be like 5 million 😀

-OH it is.

 

[loveoforganic]

It's because, like orgohacks said, you generally have a more acidic alpha proton. I imagine if you got rid of that alpha proton by replacing with methyl substituents or some such, a base like tBuLi would take off the aldehyde proton.

 

[Avery07]

In ranking proton acidity, you go by the following rules (in order of least to greatest):

1. Atom (pick the most electronegative)
2. Resonance
3. Induction (nearby halogens are good as they pull electrons to them, carbon groups are bad as they contribute electron density)
4. Orbital (sp > sp2 > sp3)

You only move down the table if there is a tie at the previous group.

Using these rules, the H on the alcohol immediately wins as being the most acidic due to O being more electronegative than C.