alpha-hydrogen acidity

Most C-H bonds are not acidic at all — methane has a pKa near 50, meaning its hydrogens essentially never leave as protons. Yet the hydrogen on the carbon next to a carbonyl is dramatically more acidic: the alpha-hydrogen of a ketone or aldehyde has a pKa around 19-20. That is still a weak acid (water is 15.7, acetic acid is 4.76), but it is about a trillion-trillion-trillion times more acidic than an ordinary C-H. That gap is the whole story.

Why so much more acidic? It is not the C-H bond itself but the stability of what is left behind. When you remove the alpha-proton, the negative charge does not sit stranded on carbon; it spreads onto the carbonyl oxygen through the adjacent pi-system. The resulting anion — the enolate — is resonance-stabilized, with the charge shared between the alpha-carbon and the oxygen. Oxygen is electronegative and happy to hold negative charge, so the anion is far more stable than a bare carbanion, and a more stable conjugate base means a stronger acid.

Put a carbon between TWO carbonyls and the effect doubles. A 1,3-diketone, a beta-keto ester, or malonic ester has a doubly-activated CH2 with a pKa around 9-13 — acidic enough to be fully deprotonated by mild bases like sodium ethoxide. This is the key that unlocks the malonic ester and acetoacetic ester syntheses: those very acidic protons can be removed cleanly, the carbon alkylated, and the helper carbonyl removed later by decarboxylation.