Because of resonance stabilization of the conjugate base, an α-hydrogen in an aldehyde is weakly acidic with a pK near 17. Note, however, this is much more acidic than an alkane or ether hydrogen, which has pK near 50 approximately, and is even more acidic than a ketone α-hydrogen which has pK near 20. This acidification of the α-hydrogen in aldehyde is attributed to:
* the electron-withdrawing quality of the formyl center and
* the fact that the conjugate base, an enolate anion, delocalizes its negative charge.
The formyl proton itself does not readily undergo deprotonation.