[Retricide]

I'm struggling to understand why the carbonyl oxygen, in molecules containing a carbonyl and an oxygen, preferentially reacts with H+ over the alcohol group in reactions like acetal formation.

In solution, I thought alcohol was a stronger base than a carbonyl oxygen because the CA of an alcohol is ~-2.7 and the CA of a carbonyl compund is ~-7. This would seem to suggest alcohol is the stronger base so I'd think it would react with an acidic hydrogen before the carbonyl oxygen would.

However, in this reaction, the carbonyl oxygen reacts with the acidic hydrogen instead of the alcohol:

Why is this? Does the alcohol react with the acidic hydrogen but doesn't yield an isolatable product (i.e. it's in rapid equilibrium with the starting material)? Or is the carbonyl oxygen just more nucleophilic for some reason?

Basically, I don't understand why acidic hydrogens always react with the carbonyl oxygen in molecules containing a carbonyl and an alcohol. Any help is much appreciated!

Thanks!

[TheUnassuming]

Perhaps a better way to look at it, is simply what will happen if protonated. 
If you protonate the primary alcohol in that compound, what reactions do you think will happen?

 

[Retricide]

The deprotonated ion would substitute the protonated alcohol...and then the alcohol could reform by having the water substitute back in?

So basically protonating the alcohol creates a product in equilibrium with the unprotonated reactant - whereas protonating the carbonyl creates a more electrophilic site at the carbonyl-carbon which allows a different set of reactions to proceed?

[TheUnassuming]

Not sure what you mean by deprotonated ion.  But the second part is more or less dead on.  There isn't much that can happen with the protonated alcohol.  Protonation of the aldehyde does do something very productive though!  It gives you a hot electrophile that can be attacked!

[orgopete]

Regarding the pKa values, I agree. i do not agree with the mechanism. Perhaps that will help. If you add HCl to water or methanol, a lot of heat is evolved. Presumably this is due to protonation of the oxygen and forming the ions. Therefore, as suggested, methoxonium ion is the acid and methanol is the base. Chloride is simply a bystander ion. Of the ions present, a small amount of the time, the carbonyl group becomes protonated (or perhaps only collides with a proton). Once this happens, the protonated carbonyl is more electrophilic and can accept electrons from another oxygen.

One might also reason that electrons from the ring oxygen might also collide with the carbonyl group. That would increase the basicity of the carbonyl group. An intramolecular collision would be more favored by entropy than an intermolecular collision. Remember, an uncatalyzed reaction can also occur with water on simple aldehydes to give a hydrate, formaldehyde is completely hydrated (as I recall).