710 CHAPTEr EIGHTEEN Enols and enolates Our experience to this point has been that C-H bonds are not very acidic. Com- pared with most hydrocarbons, however, aldehydes and ketones have relatively acidic protons on their a-carbon atoms. Equilibrium constants for enolate formation from sim- ple aldehydes and ketones are in the 10 to 10- range(pKa= 16-20) (CH3)CHCH H++(CH3)2C=CHKa=3×10-16 2-Methylpropanal CsHsCCH3、H+C6HC=CH2Ka=1.6×10 Acetophenone (pKa=158) Delocalization of the negative charge onto the electronegative oxygen is responsi ble for the enhanced acidity of aldehydes and ketones. With Kas in the 10 to 10 range, aldehydes and ketones are about as acidic as water and alcohols. Thus, hydrox ide ion and alkoxide ions are sufficiently strong bases to produce solutions containing ignificant concentrations of enolate ions at equilibrium Diketones, such as 2, 4-pentanedione, are even more acidic contains molecular models of CH3 CCH, CCH3eH'+CH3C=CHCCH3 K=10 the enolates of acetone and 24. with respect to the distribution In the presence of bases such as hydroxide, methoxide, and ethoxide, these B-diketones are converted completely to their enolate ions. Notice that it is the methylene group ranked by the two carbonyl groups that is deprotonated. Both carbonyl groups partici pate in stabilizing the enolate by delocalizing its negative charge HC CH HC HIC CH H H PROBLEM 18.7 Write the structure of the enolate ion derived from each of the following B-dicarbonyl compounds. Give the three most stable resonance forms of each enolate (a)2-Methyl-1, 3-cyclopentanedione (b)1-Phenyl-13-butanedione SAMPLE SOLUTION (a) First identify the proton that is removed by the base. It is on the carbon between the two carbonyl groups. Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsiteOur experience to this point has been that C±H bonds are not very acidic. Com￾pared with most hydrocarbons, however, aldehydes and ketones have relatively acidic protons on their -carbon atoms. Equilibrium constants for enolate formation from sim￾ple aldehydes and ketones are in the 1016 to 1020 range (pKa 16–20). Delocalization of the negative charge onto the electronegative oxygen is responsi￾ble for the enhanced acidity of aldehydes and ketones. With Ka’s in the 1016 to 1020 range, aldehydes and ketones are about as acidic as water and alcohols. Thus, hydrox￾ide ion and alkoxide ions are sufficiently strong bases to produce solutions containing significant concentrations of enolate ions at equilibrium. -Diketones, such as 2,4-pentanedione, are even more acidic: In the presence of bases such as hydroxide, methoxide, and ethoxide, these -diketones are converted completely to their enolate ions. Notice that it is the methylene group flanked by the two carbonyl groups that is deprotonated. Both carbonyl groups partici￾pate in stabilizing the enolate by delocalizing its negative charge. PROBLEM 18.7 Write the structure of the enolate ion derived from each of the following -dicarbonyl compounds. Give the three most stable resonance forms of each enolate. (a) 2-Methyl-1,3-cyclopentanedione (b) 1-Phenyl-1,3-butanedione (c) SAMPLE SOLUTION (a) First identify the proton that is removed by the base. It is on the carbon between the two carbonyl groups. CH O O H3C C C H C CH3 O O H3C C C H C CH3 O O H3C C O C H C CH3  O CHCCH3 O CH3C O  H Ka 109 (pKa 9) CH3CCH2CCH3  O O (CH3)2C CH O  H Ka 3  1016 (pKa 15.5) (CH3)2CHCH  O 2-Methylpropanal C6H5C CH2 O  H Ka 1.6  1016 (pKa 15.8) C6H5CCH3  O Acetophenone 710 CHAPTER EIGHTEEN Enols and Enolates Learning By Modeling contains molecular models of the enolates of acetone and 2,4- pentanedione. Compare the two with respect to the distribution of negative charge. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website