L_5_09 - based upon the pKa’s of the respective acids ...

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318N – Krische, Lecture 5: Tu - 02/03/09, Aldehydes and Ketones · Ketones and aldehydes are in equilibrium with their enol “forms”. The interconversion of “keto” and “enol” forms is termed “tautomerism”. Under neutral conditions, most ketones and aldehydes primarily reside in their “keto” forms. In class, we considered the structural features of enols and noted that they resemble electron rich alkenes. · Ketones and aldehydes are weak acids (pKa of 20 and 16, respectively). The deprotonated form of a ketones or aldehydes is an “enolate”. A relatively weak base, for example NaOEt, will reversibly deprotonate carbonyl compounds. A strong base, such as LDA, will irreversibly deprotonate a ketone or aldehyde. · In class, we learned how to calculate the equilibrium constant in such acid/base reactions involving carbonyl compounds
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Unformatted text preview: based upon the pKa’s of the respective acids · Tautomerism, the interconversion of keto and enol forms, can be catalyzed using acid or base. The mechanism for both acid and base catalyzed enol formation was described. · For certain carbonyl compounds, the enol form is preferred. For example, 1,3-dicarbonyl compounds such as acetylacetone reside primarily in their enol forms. The origins of this preference were described. · Deuterium exchange allows us to indirectly observe the acid/base catalyzed keto-enol equilibrium. In class, we went over the mechanism for acid and base catalyzed deuteration of carbonyl compounds. · Finally, based on our understanding of enolization, we rationalized the mechanism for the racemization of ketones possessing chiral centers directly adjacent to the carbonyl moiety....
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