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Student%20Notes%20-%20Section%2017

Student%20Notes%20-%20Section%2017 - Section 17 Reaction at...

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Section 17: Reaction at the α -Position to a Carbonyl Group 255 Section 17: Reaction at α -Position to Carbonyl Group 17.1 Introduction 17.1.1 Tautomerization A carbonyl compound with a hydrogen atom attached to a carbon atom α - to the carbonyl group may exist in one of two tautomeric forms: the keto- tautomer, or the enol form. R H R' R" O R R" R' OH keto- form enol form These two structures are rapidly interconvertible (a defining quality of tautomers), but in the majority of cases, the keto- tautomer is the dominant component of the equilibrium for ketones, and even more dominant for the carboxylic acid derivatives. O OH : 1 O OH 1x10 -4 : 1 1x10 -7 Note that tautomeric forms differ in the placement of their atoms: they are not resonance contributors, but distinct equilibrium species. Keto-enol tautomerization may be catalyzed by both acids and bases.
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