 | Chapter 18: Enols and Enolates | |
Enols, Enolates and Tautomerism Keywords to grasp : enols, enolates and tautomerism
Trying to put it as simply as possible, enols are compounds that have alcohol groups, -OH, substituted directly onto alkenes, C=C, hence "alkene-ols" or enols.
Enols can be viewed a alkenes with a strong electron donating substituent. Since alkenes themselves (review) typically react as nucleophiles, the -OH group makes them even more reactive than a simple alkene. | |
| Enolates are the conjugate bases or anions of enols (like alkoxides are the anions of alcohols) and can be prepared using a base. | |
JMOL images of a ketone and the corresponding enol and enolate are shown below for comparison:
QUESTION Both enols and enolates are potential nucleophiles, but enolates are better nucleophiles than enols.... Why ? ANSWER
| Acetone electrostatic potential | When comparing the two electrostatic potentials note the increased electron density on both the O (it's more red) and the C (less blue) in the enolate compared to the ketone. (The more red an area is, the higher the electron density and the more blue an area is, the lower the electron density) | Acetone enolate electrostatic potential |
In general enols are unstable compounds and they are in an equilibrium with a more favourable carbonyl group. This process is known as tautomerism and is catalysed by both acids and bases. Try to draw the mechanism for these processes before you look at the answers.
In the example shown, 2-propanone ( = acetone) and 2-propenol can be described as tautomers. This is really a specific type of constitutional isomerism.. If you look at the bond strengths of the C=C and O-H compared to C=O and C-H, you should be able to verify the greater stability of the carbonyl group. | |
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| However, some enols are reasonably stable (usually there is an extra stabilising effect): for example phenols and 1,3-dicarbonyl compound |
Enolates are in principle, capable of reacting as nucleophiles at either the C atom or at the O atom (see below).
This is shown by the drawing out the main two resonance contributors, see right. | |
QUESTION Which of these two would you rank as being the more important contributor ?ANSWER
In fact, most of the reactions we will cover are reactions of the carbanion contributor !
This shows the merit of considering "minor" resonance contributors since they can be the ones controlling reactivity.
QUESTION Which is the best nucleophile, a negatively charged C or a negatively charged O ? ANSWER
| © Dr. Ian Hunt, Department of Chemistry | |
FAQs
Enols have a hydroxyl group adjacent to a C=C double bond, whereas enolates have a negative charge on the oxygen atom of an enol and enamines have an amine group next to a C=C double bond.
What is the difference between enol and tautomer? ›
Keto implies that the tautomer contains a carbonyl bond while enol implies the presence of a double bond and a hydroxyl group. The keto-enol tautomerization equilibrium is dependent on stabilization factors of both the keto tautomer and the enol tautomer.
How to identify keto-enol tautomerism? ›
2. Keto-Enol Tautomerism. A clue to this aberrant chemical behavior is provided not by lycanthropy, but by the fact that many aldehydes and ketones are in equilibrium with a structural isomer known as the enol form (part alkene, part alcohol). This behavior is known as keto-enol tautomerism.
Why are enolates more stable than enols? ›
Due to their negative charges, enolates are better nucleophiles and more versatile than enols. The increased reactivity of enolates makes them capable of a wider range of reactions than enols. Also, α-hydrogen-containing compounds can be completely converted to an enolate by reaction with a strong base.
Is enolate a tautomer? ›
Under acidic conditions, the enol tautomer forms. Under basic conditions, the enolate tautomer forms. Both the enol and enolate are nucleophiles that can undergo subsequent reactions. The mechanism for both acidic and basic reaction conditions are shown below.
Which enol is the most stable? ›
The stabilizing effect of enolic form is the intramolecular hydrogen bond present in enols. This provides another source of increasing bonding and hence, increased stabilization. Thus, CH3COCH2COOC2H5 is more stable.
Is keto or enol more stable? ›
The keto tautomer is preferred because it is usually more stable than the enol tautomer by about 45–60 kJ/mol, which is mainly due to the C=O. double bond (-749 kJ/mol) being stronger than the C=C double bond (-611 kJ/mol).
What is an example of a tautomerism? ›
Ketone-enol, enamine-imine,lactam-lactim are some of the examples of tautomers. Meanwhile, some key features of Tautomerism are that this process provides more stability for the compound. In this phenomenon, there exists an exchange of a hydrogen atom between two other atoms while forming a covalent bond to either one.
What is the difference between keto and enol? ›
Keto vs Enol bases. Standard keto and rare enol forms of T [above] & G [below] differ by a spontaneous proton shift [an H nucleus] between the adjacent C and N molecules. The molecular formula does not change: the keto and enol forms are therefore described as tautomers of each other.
How does an enol turn into a ketone? ›
The highly basic negatively charged carbon captures a proton to form the free ketone. The following examples show how an enol whose hydroxyl group is at the end of a carbon chain tautomerizes into an aldehyde, whereas an enol whose hydroxyl group is in the middle of a carbon chain tautomerizes into a ketone.
In general, enols are less stable than their keto equivalents because of the favorability of the C=O. double bond over C=C double bond.
What is the difference between an enol and enolate quizlet? ›
An enolate is the conjugate base of an enol. 2.) Enol: A molecule which has a hydroxyl group (OH) group directly bonded to an alkene (C=C). The name is a contraction derived from alkene alcohol.
What is the difference between enol and enolate MCAT? ›
- The enol form consists of a carbon-carbon double bond and an alcohol. - Enols are important intermediates in many reactions of aldehydes and ketones. - The enolate carbanion results from the deprotonation of the α-carbon by a strong base.
Why are enolates more nucleophilic than enols? ›
Due to their negative charges, enolates are better and more versatile nucleophiles than enols.
What do you mean by enolates? ›
In organic chemistry, enolates are organic anions derived from the deprotonation of carbonyl (RR'C=O) compounds. Rarely isolated, they are widely used as reagents in the synthesis of organic compounds.
