Enolate ions form in equilibrium with their parent carbonyl compounds if a moderately strong base like sodium hydroxide is used. A very strong base, like sodium amide (NaNH 2), butyllithium (CH 3 CH 2 CH 2 CH 2 Li, or BuLi) or sodium hydride (NaH), would result in complete enolate formation.
Full Answer
Which base is used for enolate anion formation?
Some bases that have been used for enolate anion formation are: NaH (sodium hydride, pK a > 45), NaNH 2 (sodium amide, pK a = 34), and LiN [CH (CH 3) 2] 2 (lithium diisopropylamide, LDA, pK a 36). Ether solvents like tetrahydrofuran (THF) are commonly used for enolate anion formation.
Which of the following will result in complete enolate formation?
A very strong base, like sodium amide (NaNH 2), butyllithium (CH 3 CH 2 CH 2 CH 2 Li, or BuLi) or sodium hydride (NaH), would result in complete enolate formation.
What are enenolate ions?
Enolate ions are just another example of anionic carbon nucleophiles. The reason they get a page to themselves is that they are especially important, especially in biological chemistry.
What is the difference between hydroxide and enolate anions?
The enolate anion that forms would be more basic than the hydroxide we began with, and most of the time it would just snatch the proton back from the water again, making ketone and hydroxide again. Enolate ions form in equilibrium with their parent carbonyl compounds if a moderately strong base like sodium hydroxide is used.
Examples
If the formed enolate is stabilized by more than one carbonyl it is possible to use a weaker base such as sodium ethoxide.
The Ambident Character of Enolate Anions
Since the negative charge of an enolate anion is delocalized over the alpha-carbon and the oxygen, as shown earlier, electrophiles may bond to either atom. Reactants having two or more reactive sites are called ambident, so this term is properly applied to enolate anions.
Why are enolate ions important?
They are also important in the synthesis of organic compounds, such as in the pharmaceutical industry.
What is the reaction of an enolate nucleophile with another carbonyl compound called?
The reaction of an enolate nucleophile with another carbonyl compound is called an aldol reaction. A simple example of this reaction is shown here. This example involves the reaction of 2-propanone with its enolate.
What is the reaction of aldol?
Sometimes, aldol reactions are followed by a subsequent reaction, called an elimination reaction. That reaction formally produces a molecule of water. Early studies of this reaction would result in droplets of condensation on the glassware in which the reaction occured; hence, it is sometimes called a condensation reaction.
Why are base conditions not suitable for aldol condensation?
The aqueous base conditions used for the aldol condensation are not suitable because the enolate anions of simple carbonyl compounds are formed in very low concentration, and hydroxide or alkoxide bases induce competing S N 2 and E2 reactions of alkyl halides.
Is sodium hydride a heterogeneous compound?
Although the reaction of carbonyl compounds with sodium hydride is heterogeneous and slow, sodium enolates are formed with the loss of hydrogen, and no other organic compounds are produced. If the formed enolate is stabilized by more than one carbonyl it is possible to use a weaker base such as sodium ethoxide.
Can a base be used to make an enolate anions?
Certain other strong bases, such as alkyl lithium and Grignard reagents, cannot be used to make enolate anions because they rapidly and irreversibly add to carbonyl groups.