Enols are alkenes with alcoholic functional groups. Enolates are the conjugate bases while enamines are the nitrogen analogs of such enols. In other words, enols are the organic compounds with alkene and alcoholic functional groups. The conjugate bases of these enols are termed enolates whereas, enamines are organic compounds similar to enols but with alkene and amine functional groups.
Differences between Enols, Enolates, and Enamines
|Enols are alkenols having hydroxyl group attached to an alkene||Enolates are derived conjugate bases of enols||Enamines are alkenamines having amine group attached to an alkene|
|Enols are least reactive among enols, enolates and enamines||Enolates are more reactive than enols and enamines both||Enamines have intermediate reactivity than enol and enolate counterparts|
|Enols are prepared by shifting of hydrogen atom from alpha carbon to carbonyl oxygen||Enolates are prepared by the deprotonation of enolizable ketones, aldehydes, and esters||Enamines are synthesized by condensation of an aldehyde or ketone with a secondary amine|
|They contain a hydroxyl as functional group||They contain a negatively charged oxygen atom as functional group||They contain amine as functional group|
|Tautomerization is usually seen as keto-enol interconversion||Resonance of extra electron takes place between oxygen and carbon atoms||Tautomerization is usually seen as enamine-imine interconversion|
|Enol derivatives are keto structures||Enolates have Aza-enolates as nitrogen analogous compounds||Enamine derivatives are imines, oximes, and hydrazones, etc|
|For examples, Ethenol, Enol-phenyl pyruvate, cyclooct-2-enol, etc||For examples, Ethenolate, Cyclohex-1-enolate, Glutacon aldehyde enolate, etc||For examples, Ethenamine, Pent-4-enamine, Cyclopent-2-enamine, etc|
What are Enols?
The term ‘enol’ comprises of ‘ene’ as in aldehydes and ‘ol’ as in alcohols. They are organic compounds containing a hydroxyl (-OH) functional group in direct connection with an alkene (-C=C-) functional group.
Enols are vinyl alcohol derivatives containing (-C=C-OH) functionality. They are also known as alkenols and are mostly found as reaction intermediates in organic chemistry. They are the precursors of enolates and many other compounds.
The alkene functional group shows the electron-donating substituent effect and so does the hydroxyl group. This makes enols so much reactive than simple alkenes for attacking on electrophiles. Despite such reactivity, enols are still the least reactive among enols, enolates, and enamines.
What are Enolates?
Enolates are organic anions derived from organic compounds especially enols. They are usually found and used as reagents for the synthesis of organic compounds. Enolates being the most reactive among enols, enolates, and enamines, are difficult to isolate and preserve.
Enolates are formed by the combination of oxygen anion and alkene carbon. The deprotonation reaction on carbonyl compounds containing a hydrogen atom on α-carbon like ketones, some aldehydes, and esters forms enolates. this is the reason why methanal (formaldehyde) cannot form enolates.
Since enolates are conjugate bases of enols, and the conjugate bases and always far better nucleophiles than acids themselves, enolates are more reactive than enols.
Enolate ions are electronically related to the allyl anions so, as a matter of fact, they have characters of both alkoxide and carbanion. This shows why enolates are more reactive or less stable than enols. This increased reactivity of enolate anions as compared to enols (their precursors) make them able to react on a wide scales.
What are Enamines?
The term enamine is derived from ‘ene’ as in alkenes and ‘amine’ as amine functional groups. Enamines are the yield of the dehydration condensation reaction of an aldehyde or ketone with amine compounds. They are in fact the nitrogen analogs of enols.
Enamines exhibit enamine-imine tautomerization. This tautomerization is similar to their analogous, keto-enol tautomerization.
Enamines are good nucleophiles due to the nucleophilicity of nitrogen and alkene functional groups but they are also functional as good bases. Moreover, they are used in the production of enantioselective versions of Robinson annulation, the ring formation reactions for the formation of α,β-unsaturated ketones in cyclohexane fused ring system.
Preparation of Enols, Enolates, and Enamines
Enolization (Enol preparation reaction)
The process of preparing enols is called enolization. It can be acid or base catalyzed based on the reagents used. It usually involves the formation of enols by organic esters, ketones, and aldehydes having alpha hydrogen.
Enolization from a ketone is termed keto-enol tautomerization. The enol form is usually considered less stable than the keto form because of the high nucleophilicity of enols. This increased nucleophilicity is due to the alkene and hydroxyl group’s combined nucleophilicity. Despite this fact, enols can be made stable either thermodynamically or kinetically.
Synthesis of enolates
Generally, the enolate anions of simple carbonyl compounds origin are synthesized in very low concentrations. Aldehydes and ketones can be converted into their respective enolates by the addition of strong bases (pKa > 25).
Common bases that are used for the enolate synthesis are:
- Sodium hydride (NaH),
- Sodium amide (NaNH2)
- [LDA] Lithium diisopropylamide LiN[(CHCH3)2]
Some bases and compounds that cannot be used to make enolate anions because they efficiently and irreversibly react with carbonyl groups, stopping further enolate formation are:
- Alkyl lithium
- Grignard reagent
Solvents used for enolates synthesis are:
- THF (Tetra hydrofuran) dissolves most of the reactive bases except sodium hydride, and sodium amide.
Synthesis of enamines
Enamines are industrially important chemical compounds. They can easily be synthesized via acid catalyzed nucleophilic reaction of ketones or aldehydes with secondary amines. This reaction holds if the aldehydes or ketones have alpha hydrogen (α-H).
Primary amines are not used for this condensation because they prefer thermodynamically stable imines formation than enamines (tautomers). Synthesis of carbinolamine intermediate is usually taken as standard synthesis procedure.
Key Differences between Enols, Enolates, and Enamines
Which is more reactive enol or enolate?
Enolate is essentially more reactive than enol, its precursor because of its increased nucleophilicity. It can even react with weak electrophiles like alkyl halides and give alkylation at the alpha positions.
Why are enolates more useful than enols?
Enolates are thermodynamically more suitable to be formed. They are more useful than enols due to their vast reactivity range and resonance-stabilized charges.
What are enol and enolate ions?
Enols are akenes with an alcoholic functional group on the alpha positions (preferred). Enolates on the other hand are their derivatives that form due to a proton shift. This changes the alcoholic ‘ol’ functional group to resonance stabilized ‘ate’ and removes the alkene functional group as well.
What is the meaning of enolate?
The word enolate is derived from ‘enol’ and ‘ate’. they are the derivatives of enols that form due to a proton shift in intramolecular change.
Is enolate a carbanion?
Enolates are resonance stabilized structures that create a partial negative charge on carbon atom. This makes the main carbon atom of enolates a ‘carbanion’.
Is enolate a good nucleophile?
Enolates are far better nucleophiles than their (parent) enols.
Why is it difficult to use two reactants that can form enolates?
Enolates are difficult to make because once they are formed, they react with all other chemical species and form various distinct compounds.
Is the conjugate base of acetylacetone a carbanion?
Conjugate base of acetylacetone, an enolate is a carbanion but is strongly resonance stabilized.
Important pKa values
- The pKa of Imine is approximately 7
- The pKa of aldehydes and ketones with alpha hydrogen (α-H) is usually 16 to 20.