What is enol and enolate?
Enols can be viewed a alkenes with a strong electron donating substituent. Enolates are the conjugate bases or anions of enols (like alkoxides are the anions of alcohols) and can be prepared using a base.
How are Enols and enolates formed?
When the enolate is formed, it can abstract a proton at either oxygen or carbon, both being positions of partial negative charge. Protonation at oxygen gives the enol, which protonation of carbon yields back the keto form. Thus, the enolate is the conjugate base of both the keto and enol forms.
How enolate can be generated?
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.
What reagent is needed for selective enolate formation?
Enolates, formed by the abstraction of the α hydrogen atom by a strong base, are nucleophiles. Lithium diisopropylamide (LDA) or sodium hydride are required as bases. The site of proton abstraction is related to the acidity of the two possible α hydrogen atoms, which is in the order primary > secondary > tertiary.
What is the meaning of enolate?
Definition of enolate : a metallic derivative of an enol.
How do you stop self condensation?
Therefore, chemists have adopted many ways to prevent this from occurring when performing a crossed aldol reaction.
- The use of a more reactive electrophile, and a non-enolizable partner.
- Making enolate ion quantitatively.
- Silyl enol ether formation.
How is enolate formed in thermodynamics?
Treatment of an unsymmetrical ketone with a base could result in two enolate ions. The more stable enolate ion is called the thermodynamic enolate and the faster forming one the kinetic enolate. eg: Ketone 1, upon treatment with a base, could yield the enolate ions 2 and 3.
Is enolate a nucleophile?
Enolates are nucleophiles and ketones are electrophiles – therefore there is always the potential problem for self condensation. If this is desirable we need to use a base which does not completely deprotonate the carbonyl compound i.e. set up an equilibrium.
What is the role of LDA in Enolate formation?
Strong organic bases such as LDA (Lithium DiisopropylAmide) can be used to drive the ketone-enolate equilibrium completely to the enolate side. LDA is a strong base that is useful for this purpose. The steric bulk of its isopropyl groups makes LDA non- nucleophilic. Even so, it’s a strong base.
What is the difference between an enol and an enolate?
Both the enolate and enol are minor components in equilibrium with the ketone or aldehyde at netural pH. Since the K for enol formation is larger, there is much more enol than enolate(see the K values for acid dissociation vs. enol formation).
How are enols formed in an acidic solution?
The mechanism whereby enols are formed in acidic solution is a simple, two step process, as indicated below: q Step 1 is simply the protonation of the carbonyl oxygen to form the conjugate acid of the carbonyl compound. Remember that proton transfers from oxygen to oxygen are virtually always extremely fast.
Why are more hindered amides preferred for the formation of enol?
Instead, the more hindered amide base LDA is used preferentially. Base Catalyzed Formation of the Enol. When the enolate is formed, it can abstract a proton at either oxygen or carbon, both being positions of partial negative charge. Protonation at oxygen gives the enol, which protonation of carbon yields back the keto form.
What is the mechanism of acid catalyzed enolization?
Mechanism of Acid-Catalyzed Enolization. q Enol formation is called “enolization”. The mechanism whereby enols are formed in acidic solution is a simple, two step process, as indicated below: q Step 1 is simply the protonation of the carbonyl oxygen to form the conjugate acid of the carbonyl compound.