Question

Why is aldehyde hydrogen not acidic?

Answer 1

Because it would make the molecule far too unstable. It probably does have its own `"pKa"`, but it must be exceedingly high because we don't expect it to happen.

If we took acetaldehyde as an example:

The `"pKa"` of the methyl protons (also called `alpha` protons) is `17`. Let's see why those would be more acidic.

If you attempt to deprotonate acetaldehyde, clearly the second option is better.

• It doesn't exceed an octet for oxygen.
• The electrons are able to reasonably delocalize. The electronegativity of oxygen is greater than that of carbon, so oxygen is capably negatively charged.
• The equilibrium on the first one is skewed far to the left, whereas that on the second one is skewed enough to the right.

In fact, it is these `alpha` protons that get deprotonated from ketones, aldehydes, and similar compounds.