CHAPTEr EIGHTEEN Enols and enolates FIGURE 18.2(a) molecular model and (b) bond istances in the enol form of 103pm intramolecular hydrogen H bond is 166 pm 133pm 124pm CH 134pm 141 pr does not have its carbon-carbon double bond conjugated with the carbonyl group, is not as stable, and is present in negligible amounts at equilibrium PROBLEM 18.6 Write structural formulas corresponding to (a) the two most stable enol forms of CHa CCH2CH (b)The two most stable enol forms of 1-phenyl-1, 3-butanedione SAMPLE SOLUTION (a)Enolization of this 1, 3-dicarbonyl compound can involve either of the two carbonyl groups: CH3CCH2CH CHaC、∠CH CH3C Both enols have their carbon-carbon double bonds conjugated to a carbonyl group and can form an intramolecular hydrogen bond. they are of comparable stability. 18.6 BASE-CATALYZED ENOLIZATION: ENOLATE ANIONS The proton-transfer equilibrium that interconverts a carbonyl compound and its enol can be catalyzed by bases as well as by acids. Figure 18.3 illustrates the roles of hydroxide ion and water in a base-catalyzed enolization. As in acid-catalyzed enolization, protons are transferred sequentially rather than in a single step. First(step 1), the base abstracts Back Forward Main MenuToc Study Guide ToC Student o MHHE Websitedoes not have its carbon–carbon double bond conjugated with the carbonyl group, is not as stable, and is present in negligible amounts at equilibrium. PROBLEM 18.6 Write structural formulas corresponding to (a) The two most stable enol forms of (b) The two most stable enol forms of 1-phenyl-1,3-butanedione SAMPLE SOLUTION (a) Enolization of this 1,3-dicarbonyl compound can involve either of the two carbonyl groups: Both enols have their carbon–carbon double bonds conjugated to a carbonyl group and can form an intramolecular hydrogen bond. They are of comparable stability. 18.6 BASE-CATALYZED ENOLIZATION: ENOLATE ANIONS The proton-transfer equilibrium that interconverts a carbonyl compound and its enol can be catalyzed by bases as well as by acids. Figure 18.3 illustrates the roles of hydroxide ion and water in a base-catalyzed enolization. As in acid-catalyzed enolization, protons are transferred sequentially rather than in a single step. First (step 1), the base abstracts CH O H O CH3C C H CH O H O CH3C C H CH3CCH2CH O O CH3CCH2CH O X O X 708 CHAPTER EIGHTEEN Enols and Enolates (a) C C CH3 O H C O CH3 H O---H separation in intramolecular hydrogen bond is 166 pm 124 pm 103 pm 133 pm 134 pm 141 pm (b) FIGURE 18.2 (a) A molecular model and (b) bond distances in the enol form of 2,4-pentanedione. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website