Enolate Ion
An enolate ion is a reactive intermediate in organic chemistry that forms when a base deprotonates a carbonyl compound, creating a negatively charged carbon atom. Enolate ions are important in organic synthesis as they can undergo various reactions, such as alkylation and condensation, to form new carbon-carbon bonds. These reactions are key in the production of many organic compounds.
3 Key excerpts on "Enolate Ion"
eBook - ePubBiochemistry
An Organic Chemistry Approach
- Michael B. Smith(Author)
- 2020(Publication Date)
- CRC Press(Publisher)
...Examination of the two resonance contributors for the enolate anion in Figure 6.1 shows that the negative charge is delocalized on oxygen in one resonance contributor and on carbon in the other. Enolate anions are carbanions, because the resonance-stabilized enolate anion has a larger concentration of electron density on the carbon relative to oxygen. The electron potential map shows the charge distribution for this enolate anion, where more red (higher electron density) is concentrated on the carbon than on the oxygen, although this depends on the counterion. 6.3 The Aldol Condensation In acyl addition reactions, nucleophiles react with aldehyde or ketones to form a new C—C bond in the alkoxide product. An enolate anion reacts with an aldehyde or ketone as a carbanion nucleophile, as illustrated in Figure 6.2. Formation of a new C—C bond generates an alkoxide, and the carbonyl of the ketone enolate anion remains so the product is the keto-alkoxide. In other words, the product is the alkoxide of 4-hydroxypentan-2-one, and enolate anions react with aldehydes and ketones via nucleophilic acyl addition. Hydrolysis converts the keto-alkoxide to hydroxy ketones or hydroxy aldehydes, in this case 4-hydroxypentan-2-one. Such β-hydroxy ketones or aldehydes are known as aldols, and this transformation has come to be called the aldol condensation. FIGURE 6.2 Enolate anions react as carbon nucleophiles with aldehydes or ketones: the aldol condensation. Direct asymmetric catalytic aldol reactions have been successfully performed using aldehydes and unmodified ketones together with commercially available chiral cyclic secondary amines as a base...
eBook - ePub- Graham Patrick(Author)
- 2004(Publication Date)
- Taylor & Francis(Publisher)
...The Enolate Ion acts as the nucleophile while the aldehyde or ketone acts as an electrophile. Since the Enolate Ion is formed from a carbonyl compound and can then react with a carbonyl compound, it is possible for an aldehyde or ketone to react with itself. We can illustrate this by looking at the reaction of acetaldehyde with sodium hydroxide (Figure 11). Under these conditions, two molecules of acetaldehyde are linked together to form a β-hydroxyaldehyde. In this reaction, two separate reactions are going on — the formation of an Enolate Ion from one molecule of acetaldehyde, and the addition of that enolate to a second molecule of acetaldehyde. The mechanism begins with the formation of the Enolate Ion as described in Section G5. It is important to realize that not all of the acetaldehyde is converted to the Enolate Ion and so we still have molecules of acetaldehyde present in the same solution as the Enolate Ions. Since acetaldehyde is susceptible to nucleophilic attack, the next stage in the mechanism is the nucleophilic attack of the Enolate Ion on acetaldehyde (Figure 12). The Enolate Ion has two nucleophilic centers — the carbon and the oxygen — but the preferred reaction is at the carbon atom. The first step is nucleophilic addition to the aldehyde to form a charged intermediate. The second step involves protonation of the charged oxygen. Since a dilute solution of sodium hydroxide is used in this reaction, water is available to supply the necessary proton. (Note that it would be wrong to show a free proton (H +) since the solution is alkaline.) If the above reaction is carried out with heating, then a different product is obtained (Figure 13). This arises from elimination of a molecule of water from the Aldol reaction product. There are two reasons why this can occur. First of all, the product still has an acidic proton (i.e. there is still a carbonyl group present and an α-hydrogen next to it). This proton is prone to attack from base...
eBook - ePub- Michael North(Author)
- 2017(Publication Date)
- CRC Press(Publisher)
...This cyclic chelate adopts a chair conformation, and the most stable product (i.e. the thermodynamically controlled product) is the one in which as many substituents as possible are located in equatorial positions on the six-membered ring. An example of such a reaction is shown in Scheme 9.26. The aldol reaction between the lithium enolate of methyl propionate and benzaldehyde will initially give a mixture of the two possible chelates 9.33 and 9.34. However, since the reaction is reversible, these two chelates will equilibrate and the most stable chelate 9.33 will predominate. Protonation of 9.33 then leads to the u -diastereomer of the product. In this section it has only been possible to introduce briefly the stereochemical aspects of the aldol reaction. This is one of the central reactions in organic chemistry and a large amount of work has been done on the stereochemistry of this reaction. The reader may realize that there are other factors that can affect the stereochemical outcome of the reaction; these include the geometry of the enolate (E or Z), the reaction temperature (since at low temperatures the chelates may not equilibrate, so the kinetically controlled product may be obtained) and the nature of the metal. In some cases, the metal will not be able to form a chelate, so the mechanism described in Scheme 9.26 cannot operate in these cases. More detail on the stereochemistry of the aldol reaction will be found in the further reading. 9.5.4 The Wittig reaction The Wittig reaction is the reaction between an aldehyde or ketone and a phosphorus stabilized carbanion (an ylid) to generate eventually an alkene and a phosphine oxide. In the general reaction (Scheme 9.27), both the carbonyl compound and the ylid are prochiral, and there are two diastereomers of the alkene product (the E - and Z -isomers)...
