18.6 Base-Catalyzed Enolization: Enolate Anions FIGURE 18. 3 Mechanism of Overall reaction the base-catalyzed enoliza- OH ketone in aqueous solution RCH2CR′ RCHECR Aldehyde or ketone Step 1: A proton is abstracted by hydroxide ion from the a carbon atom of the H H RCH-CR RCH一 Aldehyde Conjugate base of Water or ketone arbonyl compound Step 2: A water molecule acts as a Bronsted acid to transfer a proton to the oxygen of the enolate ion O—H RCH=CR′+:O RCH=CR’ Hydroxide carbonyl compound a proton from the a-carbon atom to yield an anion. This anion is a resonance-stabilized ecies. Its negative charge is shared by the a-carbon atom and the carbonyl oxygen. RCH-CR′←>RCH=CR Electron delocalization Protonation of this anion can occur either at the a carbon or at oxygen. Protonation of the a carbon simply returns the anion to the starting aldehyde or ketone Protonation of oxygen, as shown in step 2 of Figure 18.3, produces the enol The key intermediate in this process, the conjugate base of the carbonyl compound, Examine the enolate of is referred to as an enolate ion, since it is the conjugate base of an enol. The term"eno- tone on Learning By Model- late"is more descriptive of the electron distribution in this intermediate in that oxygen ing. How is the negative bears a greater share of the negative charge than does the a-carbon atom. harge distributed between oxygen and the a carbon The slow step in base-catalyzed enolization is formation of the enolate ion. The second step, proton transfer from water to the enolate oxygen, is very fast, as are almost all proton transfers from one oxygen atom to another. Back Forward Main MenuToc Study Guide ToC Student o MHHE Websitea proton from the -carbon atom to yield an anion. This anion is a resonance-stabilized species. Its negative charge is shared by the -carbon atom and the carbonyl oxygen. Protonation of this anion can occur either at the carbon or at oxygen. Protonation of the carbon simply returns the anion to the starting aldehyde or ketone. Protonation of oxygen, as shown in step 2 of Figure 18.3, produces the enol. The key intermediate in this process, the conjugate base of the carbonyl compound, is referred to as an enolate ion, since it is the conjugate base of an enol. The term “eno￾late” is more descriptive of the electron distribution in this intermediate in that oxygen bears a greater share of the negative charge than does the -carbon atom. The slow step in base-catalyzed enolization is formation of the enolate ion. The second step, proton transfer from water to the enolate oxygen, is very fast, as are almost all proton transfers from one oxygen atom to another. RCH CR O  RCH CR O  Electron delocalization in conjugate base of ketone 18.6 Base-Catalyzed Enolization: Enolate Anions 709  Overall reaction: Step 1: A proton is abstracted by hydroxide ion from the carbon atom of the carbonyl compound. RCH2CR Aldehyde or ketone Aldehyde or ketone Enol slow O X RCH±CR  O RCH±CR  O HO RCHœCR OH W Enol RCHœCR  O O±H W    Hydroxide ion Conjugate base of carbonyl compound Water Step 2: A water molecule acts as a Brønsted acid to transfer a proton to the oxygen of the enolate ion. Hydroxide ion O X O X H ± H H ± ± H RCHœCR  O ± Conjugate base of carbonyl compound Water O W H H ± ± BNA BNA fast BNA W H FIGURE 18.3 Mechanism of the base-catalyzed enoliza￾tion of an aldehyde or ketone in aqueous solution. Examine the enolate of ace￾tone on Learning By Model￾ing. How is the negative charge distributed between oxygen and the carbon? Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website