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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?

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  • $\begingroup$ The acidity really only applies to direct $\ce{C-H}$ acidity. Here, you are generating a more stable double bond. $\endgroup$ – Jan Oct 4 '16 at 0:26
  • $\begingroup$ @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? $\endgroup$ – Adroit Feb 26 '18 at 16:41
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    $\begingroup$ @Adroit I think we are saying the same thing but in different ways. $\endgroup$ – Jan Mar 1 '18 at 4:11
  • $\begingroup$ @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 ? $\endgroup$ – Adroit Mar 1 '18 at 5:55
  • $\begingroup$ @Adroit Yes, the most acidic position of an aldehyde is the alpha proton. $\endgroup$ – Jan Mar 1 '18 at 7:58
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You're actually asking two questions. There is both kinetic and thermodynamic acidity. There are times when performing deprotonation at low temperature will yield the kinetically favored conjugate base, i.e., the proton that is most exposed. If you increase the temperature of this intermediate, you can get to the point where proton transfers happen in reaction vessel to create the more stable thermodynamically favored conjugate base. Usually, this is the more substituted one.

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