# Why are the secondary alpha hydrogens more acidic than primary alpha hydrogens (keto-enol tautomerization)?

I learned in organic chemistry that secondary/tertiary carbanions are less stable than primary carbanions and therefore primary hydrogens are MORE acidic than secondary/tertiary hydrogens.

But lets take 3-methyl-2-butanone as an example. It seems that the more substituted alpha carbon is more acidic than the nearby primary carbon. In other words, it seems that the keto-enol tautomerization prefers to form a C=C with the secondary carbon over the primary carbon.

Why would a base prefer to deprotonate the secondary carbon over the primary carbon to trigger keto-enol tautomerization over the theoretically more acidic primary carbon?

• The acidity really only applies to direct $\ce{C-H}$ acidity. Here, you are generating a more stable double bond. – Jan Oct 4 '16 at 0:26
• @Jan When hydrogen is removed by a base, there is a resonance form that contains a double bond between carbonyl carbon and alfa carbon. Wouldn't the stability of this resonance form over the terminal alkene make the central hydrogen more acidic? – Adroit Feb 26 '18 at 16:41
• @Adroit I think we are saying the same thing but in different ways. – Jan Mar 1 '18 at 4:11
• @Jan would you agree that the pKa (being a measure of H3O+ concentration at eqm) of the central hydrogen is lower than that of the terminal one ? – Adroit Mar 1 '18 at 5:55
• @Adroit Yes, the most acidic position of an aldehyde is the alpha proton. – Jan Mar 1 '18 at 7:58