[madscientist]

I understand that acidic hydrogens in carbonyl compounds are those attached to alpha-carbons, but are these the only acidic hydrogens?

i.e. in the structures below, are the indicated acidic hydrogens (red) the only ones? and is it right to say that the structure on the left has no acidic hydrogens?

[Dan]

Second one looks good, although it's worth pointing out that the H alpha to three carbonyls will be the most acidic.

For the first one, I would say it had two relatively acidic H (both on the same C).

[Custos]

For the first one, I would say it had two relatively acidic H (both on the same C).

Dan is right and it's worth emphasising that, as he said, these protons are relatively acidic. You will often hear students talking about "the acidic protons" alpha to a carbonyl. They are by no means acidic like even the weakest carboxylic acid. They are "acidic" only relative to very strong bases.

As Dan put it you should always describe these as the relatively acidic hydrogens.

[movies]

To add to that, in reality every proton is acidic, it's just most important to be able to identify the most acidic proton in a compound because that is the one that will usually react first.

[english]

Look at the conjugate base of the carbonyl.  Stability wise, which site do you think is most acidic?

[Custos]

To add to that, in reality every proton is acidic....

Hmmm. Interesting philosophical question... is every proton actually acidic? If that is true (and it may well be), for every given proton there needs to be a base capable of abstracting it, right? I mean in order for a proton to be classified as "acidic" it would need to be at least theoretically capable of participating in an acid/base reaction. What about, say, the aromatic protons on an electron-rich substituted benzene ring? Would the pKa be so high that even superbases like (n-butyllithium/potassium tert-butoxide mixture) fall short?

[movies]

Oh you can definitely remove those aromatic protons.  The sp² hybridization of the carbon drops the pKa a lot.  The pKa of alkanes are generally considered to be the highest.  I have seen estimates of the pKa of methane as high as 60.

I guess I think of it this way: every compound that contains a atom-H bond can be cleaved heterolytically to make atom(anion) and H+, so that's acidity.  Even if there isn't a base involved and it's just a charge separation it still fufills the definition of a Bronstedt acid.

[madscientist]

Thanks for all your help peoples! much appreciated. Scooby snacks all round