The Most Acidic Hydrogen

[rah08e]

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I know that the most acidic hydrogen in a diketone such as pentane-2,4-dione is the alpha hydrogen between both carbonyls. Which hydrogen would be most acidic in a Beta-Hydroxy-Aldehyde? Is it hydrogen between the carbonyl and the -OH or is it the hydrogen next to the C=O (part of the aldehyde)?

Thanks!

 

[SeattleRDH]

The alpha hydrogen.

 

[SeattleRDH]

Actually, wouldn't it be the hydroxy hydrogen? Definitely not the aldehyde though.

 

[rah08e]

Actually, wouldn't it be the hydroxy hydrogen? Definitely not the aldehyde though.

yeah, I'm not 100% sure... leaning toward the alpha; is the hydroxy hydrogen the one attached to the "O" in "OH"?

 

[SeattleRDH]

I looked it up. The alpha carbon has a pKa of about 17. The hydroxy hydrogen (attached to the oxygen) has a pKa of 16. Pretty close.

 

[rockclock]

I looked it up. The alpha carbon has a pKa of about 17. The hydroxy hydrogen (attached to the oxygen) has a pKa of 16. Pretty close.

The alpha carbon on an aldehyde alone on alkyl chain would have a pKa of 17, but adding the alcohol drops that a little bit. Intuitively, I'd say that the alpha hydrogen is more acidic than the alcohol. I tried out 3-hydroxybutanal and 3-hydroxypentanal on chemicalize.org and the calculations agree with that, for what it's worth. In both cases, the methylene group deprotonates before the alcohol.

 

[UBCStudent128]

the most acidic hydrogen that they would ask in DAT would probably be an acid halide...
But I'm sure they'd give you a big complex molecule and ask you to identify which of the 5 hydrogen are most acidic.

There can be many ways to tackle those kind of problems but the way I like to use most often is to pretend that the hydrogen is deprotonated. After that, see which anion would be most stable.

Most stable = weakest base = strongest conjugate acid.

For example, an alpha hydrogen of a carbonyl would be more stable than a deprotonated butane due to resonance.

EDIT: there are many alpha hydrogens so it's a bad idea to generalize which one is more acidic. Don't memorize, try to think and understand why a hydrogen would be more acidic than another hydrogen.

 

[SeattleRDH]

I think i only got a 20 on o chem. I'm so glad to be done with the DAT!