Keto-Enol Tautomerism Only a-hydrogens are acidic a-Hydrogens are acidic because the enolate ion that results from deprotonation is resonance stabilized with the electronegative oxygen of the carbonyl B-,/Hydrogens (and so on)are not acidic because the ion that results from deprotonation is not resonance stabilized Nonacidic H HH HH H :0: Base y C H HH H H H H Acidic Enolate ion
Only a-hydrogens are acidic ▪ a-Hydrogens are acidic because the enolate ion that results from deprotonation is resonance stabilized with the electronegative oxygen of the carbonyl ▪ b-, g-, d-Hydrogens (and so on) are not acidic because the ion that results from deprotonation is not resonance stabilized Keto-Enol Tautomerism
Keto-Enol Tautomerism Mechanism of acid-catalyzed (a)Acidic conditions enol formation -Protonated intermediate Keto tautomer can lose H+,either from the 1The carbonyl oxygen is protonated by an acid H-A, oxygen atom to regenerate giving a cation with two the keto tautomer or from resonance structures. the a carbon atom to yield an enol tautomer ②Loss of H+from the a position by reaction with A日 a base A gives the enol tautomer and regenerates HA catalyst. HA Enol tautomer
Mechanism of acid-catalyzed enol formation ▪ Protonated intermediate can lose H+ , either from the oxygen atom to regenerate the keto tautomer or from the a carbon atom to yield an enol tautomer Keto-Enol Tautomerism
Keto-Enol Tautomerism Mechanism of base- (b)Basic conditions 0 catalyzed enol formation Base removes the Keto tautomer The intermediate enolate acidic a hydrogen, ion,a resonance hybrid yielding an enolate ion with two resonance of two forms,can be structures. protonated either on carbon to generate the starting keto tautomer ②Protonation of the Enolate ion or on oxygen to give an enolate ion on oxygen gives the enol and 2 enol tautomer regenerates base catalyst. HO Enol tautomer
Mechanism of basecatalyzed enol formation ▪ The intermediate enolate ion, a resonance hybrid of two forms, can be protonated either on carbon to generate the starting keto tautomer or on oxygen to give an enol tautomer Keto-Enol Tautomerism
17-2 Reactivity of Enols:a-Substitution Reactions Enols are nucleophiles that react with electrophiles -There is a substantial build-up of electron density on the a carbon of the enol Electron-rich Enol tautomer Cngg Loaring All Pights Reserved
Enols are nucleophiles that react with electrophiles ▪ There is a substantial build-up of electron density on the a carbon of the enol 17-2 Reactivity of Enols: α-Substitution Reactions
Reactivity of Enols:a-Substitution Reactions General mechanism of carbonyl a-substitution reaction 。 Enol is formed with acid catalysis Electron pair from C=C Acid-catalyzed enol formation occurs Acid catalyst bond of enol attacks an by the usual mechanism. electrophile(E),forming new C-E bond and a An electron pair from the enol oxygen resonance stabilized attacks an electrophile(E),forming a new bond and leaving a cation intermediate intermediate that is stabilized by resonance between two forms. ·Loss of H+from oxygen yields the neutral alpha- Loss of a proton from oxygen yields substitution product and the neutral alpha-substitution product as a new C-O bond is formed. restores the C=O bond
General mechanism of carbonyl a-substitution reaction • Enol is formed with acid catalysis • Electron pair from C=C bond of enol attacks an electrophile (E+ ), forming new C-E bond and a resonance stabilized intermediate • Loss of H+ from oxygen yields the neutral alphasubstitution product and restores the C=O bond Reactivity of Enols: α-Substitution Reactions