Base-Catalyzed Tautomerism HO: C=C +-OH enol form keto form enolate ion (vinyl alcohol) Copyright2010 Pearson Prentice Hall,Inc. In the presence of strong bases,ketones and aldehydes act as weak proton acids. A proton on the a carbon is abstracted to form a resonance- stabilized enolate ion with the negative charge spread over a carbon atom and an oxygen atom. The equilibrium favors the keto form over the enolate ion. Chapter 22 6
Chapter 22 6 Base–Catalyzed Tautomerism ▪ In the presence of strong bases, ketones and aldehydes act as weak proton acids. ▪ A proton on the a carbon is abstracted to form a resonancestabilized enolate ion with the negative charge spread over a carbon atom and an oxygen atom. ▪ The equilibrium favors the keto form over the enolate ion
Acid-Catalyzed Tautomerism HO: HO: C=C+H,0 keto form protonated carbonyl enol form Copyright 2010 Pearson Prentice Hall,Inc. In acid,a proton is moved from the a-carbon to oxygen by first protonating oxygen and then removing a proton from the carbon. Chapter 22 7
Chapter 22 7 Acid-Catalyzed Tautomerism ▪ In acid, a proton is moved from the a-carbon to oxygen by first protonating oxygen and then removing a proton from the carbon
Racemization enolizable hydrogens CH, CH H+or-OH a carbons (R)configuration enol (achiral) (S)configuration Copyright2010 Pearson Prentice Hall,Inc. For aldehydes and ketones,the keto form is greatly favored at equilibrium. ■ If a chiral carbon has an enolizable hydrogen atom,a trace of acid or base allows that carbon to invert its configuration,with the enol serving as the intermediate.This is called racemization. Chapter 22 8
Chapter 22 8 Racemization ▪ For aldehydes and ketones, the keto form is greatly favored at equilibrium. ▪ If a chiral carbon has an enolizable hydrogen atom, a trace of acid or base allows that carbon to invert its configuration, with the enol serving as the intermediate. This is called racemization
Acidity of a Hydrogens pKa for a H of aldehyde or ketone ~20. Much more acidic than alkane or alkene (pKa 40)or alkyne(pKa 25). Less acidic than water (pKa 15.7)or alcohol (pKa 16-19). Only a small amount of enolate ion is present at equilibrium. Chapter 22 9
Chapter 22 9 Acidity of a Hydrogens ▪ pKa for a H of aldehyde or ketone ~20. ▪ Much more acidic than alkane or alkene (pKa > 40) or alkyne (pKa = 25). ▪ Less acidic than water (pKa = 15.7) or alcohol (pKa = 16–19). ▪ Only a small amount of enolate ion is present at equilibrium
Formation and Stability of Enolate lons O H :0 R 1R'+OR R-C- R- ROH pKa=16-18 ketone or aldehyde major pKa20 enolate ion Example :Q一CH,CH3 CHCH,OH pKa=15.9 cyclohexanone ethoxide ion cyclohexanone enolate pKa =19 (equilibrium lies to the left) Copyright2010 Pearson Prentice Hall,Inc. The equilibrium mixture contains only a small fraction of the deprotonated,enolate form. Chapter 22 10
Chapter 22 10 Formation and Stability of Enolate Ions ▪ The equilibrium mixture contains only a small fraction of the deprotonated, enolate form