Why is aldehyde hydrogen not acidic?

1 Answer
Truong-Son N.
Jul 28, 2016

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.

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