A rapid equilibrium between keto and enol form is referred to as keto-enol tautomerism. This process is catalyzed by both acid and a base. In most compounds, the keto form is more stable. However, in some cases, the enol form is more stable. The stability of keto-enol tautomers depends on different factors. Like, hydrogen bonding steric hindrance, aromaticity, and bond energies.
Introduction of keto-enol tautomerism
“The two isomers differ from each other in the placement of proton on one atom to another are known as tautomers. The phenomenon is called tautomerism.”
The most common example of tautomerism is keto-enol tautomerism.
“The shifting of the alpha hydrogen of a carbonyl compound to the carbonyl oxygen is known as keto-enol tautomerism.”
The carbonyl-containing isomer is known as the keto form. While the other contains a hydroxyl group attached to the doubly bonded carbon is known as enol form. These two forms are in equilibrium with each other.
Acid-catalyzed keto-enol tautomerism
The study of the acid-catalyzed keto-enol tautomerism was carried out in London by Arthur Lapworth.
The keto-enol tautomerism follows the following mechanism in acidic conditions.
In the first step, carbonyl oxygen abstracts the proton to form an oxonium ion.
In the second step, the base abstract he alpha hydrogen. Which results in the formation of enol.
Base-catalyzed keto-enol tautomerism
The mechanism of keto-enol tautomerism under basic condition is as follow:
In the first step, the base abstract the alpha hydrogen. Which causes the movement of pi electrons towards the carbonyl oxygen. In this result, an enolate ion is formed.
In this step, the oxygen of the enolate ion is protonated to form enol.
Factors affecting the stability of keto-enol tautomers
The stability of keto-enol tautomers can differ in different compounds.
We can compare the stability of keto and enol forms of mono carbonyl compounds by comparing their bond energies. However, the sum of the bond energies for the keto form is 1502 kJ/mol. While the enol form has 1433 kJ/mol bond energies. This shows that the keto form of the mono carbonyl compound is more stable than the enol form.
The enol form of beta-dicarbonyl compounds is more stable as compared to the keto form. This is because of the presence of hydrogen bonding in enol form.
The keto form of the conjugated system is less stable as compared to the enol form. For example, the keto form of phenol is less stable by 57 kJ/mol. This is because of the presence of resonance stabilization.
If bulky groups are present in the compound, they cause steric hindrance. This steric effect is greater in the keto form. This is because of the less bond angle. That’s why the enol form is more stable as compared to the keto form.
Factors affecting keto-enol tautomerism
There are different factors that affect equilibrium in keto-enol tautomerism.
Stability of keto and enol forms
Stability of molecules also affects the position of equilibrium. If the keto form is more stable then equilibrium will shift toward the keto form whereas, if the enol form is more stable then equilibrium will shift toward the enol form. For example,
Phenol exists in enolic form. This is stabilized by conjugation. The enolic form is aromatic and is stabilized by the resonance energy of benzene. A negligible amount of keto form is also present. Thus equilibrium is shifted toward enol.
Solvent also affects the position of equilibrium in keto-enol tautomerism. Polar protic solvents like water, alcohol, acetic acids, etc increase the concentration of keto form whereas non-polar aprotic solvents like benzene, hexane, etc shift the equilibrium toward enol form. Similarly in the absence of solvent enol content increase. For example,
In the above example, enol content is negligible when water is used as a solvent. On the other hand when a non-polar solvent like hexane is used the enol content increase to 46%. This is because non-polar solvent shifts equilibrium toward enol.
Steric crowding also affects the equilibrium of keto-enol tautomerism. Steric hindrance decreases the stability of the molecule. The molecule with more steric hindrance is present in small quantities whereas the molecules with less steric hindrance will present in greater excess.
Temperature also has a significant effect on the position of equilibrium in keto-enol tautomerism. Low temperature favors keto form whereas at high-temperature enol form is favored. For example,
Examples of keto-enol tautomerism
How does keto-enol tautomerism occur?
It occurs due to the acidity of alpha hydrogens.
What is the condition for tautomerism?
The compounds containing polar molecules and acidic functional groups are necessary for the keto-enol tautomerism.
Why is keto tautomer more stable?
The keto tautomer is more stable than enol tautomer. This is because the carbon-oxygen bond is more stable as compared to the carbon-carbon double bond.
What are the types of tautomerism?
There are different types of tautomerism. Such as:
- Keto-enol tautomerism
- Phenol-keto tautomerism
- Nitroso-oxime tautomerism
- Imine-enamine tautomerism
Why is Acetylacetone highly stable in enol-form?
The keto form of acetylacetone is more stable as compared to the enol form. This is because of the presence of intramolecular hydrogen bonding.
- March’s Advanced Organic Chemistry 7th edition by Michael B. Smith (Professor of Chemistry)
- Organic Chemistry tenth edition by Francis A. Carey (University of Virginia) and Robert M. Giuliano (Villanova University)
- A Text Of Organic Chemistry by M. Younas ( University of Punjab)
- Enol (Wikipedia.org)