CHAPTEr EIGHTEEN Enols and enolates FIGURE acid-ca enolization of Overall reaction aqueous solution RCH= CR ldehyde or ketone Step 1: A proton is transferred from the acid catalyst to the carbonyl oxygen H H RCH,CR RCH, CR Aldehyde Hydronium Conjugate acid of Step 2: A water molecule acts as a Bronsted base to remove a proton from the a carbon atom of the protonated aldehyde or ketone H RCH-CR RCH=CR′+H— H Conjugate acid of Water Hydronium CH2= CHOH K≈3×10-7 Acetaldehyde (keto form) CH3CCH3、CH2=CCH3K≈6×10 (keto form) In these and numerous other simple cases, the keto form is more stable than the enol by some 45-60 kJ/mol(11-14 kcal/mol). The chief reason for this difference is that a car- bon-oxygen double bond is stronger than a carbon-carbon double bond With unsymmetrical ketones, enolization may occur in either of two directions: CH,=,CH CH3, CH3 CH3C=CHCH3 I-Buten-2-ol 2-Butanone (keto form) (enol form) The ketone is by far the most abundant species present at equilibrium. Both enols are also present, but in very I concentrations Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsiteIn these and numerous other simple cases, the keto form is more stable than the enol by some 45–60 kJ/mol (11–14 kcal/mol). The chief reason for this difference is that a car￾bon–oxygen double bond is stronger than a carbon–carbon double bond. With unsymmetrical ketones, enolization may occur in either of two directions: The ketone is by far the most abundant species present at equilibrium. Both enols are also present, but in very small concentrations. 2-Butanone (keto form) CH3CCH2CH3 O 2-Buten-2-ol (enol form) CH3C CHCH3 OH 1-Buten-2-ol (enol form) CH2 CCH2CH3 OH Acetaldehyde (keto form) CH3CH O Vinyl alcohol (enol form) CH2 CHOH K 3  107 Acetone (keto form) CH3CCH3 O Propen-2-ol (enol form) CH2 CCH3 OH K 6  109 706 CHAPTER EIGHTEEN Enols and Enolates Overall reaction: Step 1: A proton is transferred from the acid catalyst to the carbonyl oxygen. RCH2CR Aldehyde or ketone Aldehyde or ketone Enol fast O X RCH2CR  H±O RCH2CR  O H3O RCHœCR OH W Enol RCHœCR  H±O O±H W   Hydronium ion Conjugate acid of carbonyl compound Water Step 2: A water molecule acts as a Brønsted base to remove a proton from the carbon atom of the protonated aldehyde or ketone. Hydronium ion  O±H X O X H H ± ± H H ± ± H H ± RCH±CR  O ± Conjugate acid of carbonyl compound Water O±H X H H ± ± BNA BNA slow BNA W H FIGURE 18.1 Mechanism of acid-catalyzed enolization of an aldehyde or ketone in aqueous solution. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website