In this article, you will learn about keto enol tautomerization, including the meaning of “tautomerization,” trends in enol stability, and important mechanisms under acidic and basic conditions.
Topics Covered in Other Articles
- Aldol Condensations
- DNA Tautomerization
- Carbonyl Functional Group
- Alcohol Functional Group
- Alkene Functional Group
What is Keto Enol Tautomerization?
Keto enol tautomerization is the reaction of converting between the structures of a ketone and an enol molecule. Before we go through the mechanism of such a reaction, let’s first clarify the meaning of “tautomerization.”
Chemists use the term “tautomerization” to describe the process of two molecular structures that interconvert under a state of dynamic equilibrium. Often, this involves the movement of some chemical group between two sites on a molecule.
Put differently, let’s say you have a Molecule A with some functional group X. In Molecule A, group X attaches to site A. However, there exists another suitable location for group X to attach, called site B. Sometimes, group X moves from site A to site B, which changes Molecule A to Molecule B, its tautomer. Once at site B, group X can also move back to site A. The movement of group X between sites, converting the molecular structure, is called tautomerization.
When you have a whole bunch of Molecule A, such as a mole dissolved in solvent, it will constantly convert to Molecule B and back again. However, the relative proportions of Molecule A and Molecule B remain consistent, because eventually, their rates of conversion will cancel out. This is what chemists mean by “dynamic equilibrium.”
Often, Molecule A has more stability than Molecule B, meaning that Molecule A outnumbers B at equilibrium. This unequal state of equilibrium occurs commonly in tautomer pairs, including between ketones and enols. However, in keto enol tautomerization, different structural factors determine which molecule outnumbers the other.
Keto Enol Tautomer Structure
Generally, ketones are favored heavily over enols in many of the most common molecular structures. However, the structure of the ɑ-carbon (the carbon once-removed from the carbonyl) can alter the favorability of the ketone.
One such structural trend holds that ɑ-carbons with more non-hydrogens have more stability as enols. For instance, if the ɑ-carbon has a methyl structure, with three hydrogens, it has less stability forming an enol than a methylene ɑ-carbon, which has two hydrogens. Enols formed by methylene ɑ-carbons stabilize enols less than methine ɑ-carbons, which have one hydrogen and two hydrocarbon attachments.
However, there exist some molecules in which the enol form predominates over the ketone. One such example are 1,3-dicarbonyl molecules, which frequently form from certain aldol condensations. In these molecules, an enol can form, shuffling a hydrogen to the oxide ion. This new hydroxide structure stabilizes from the adjacent carbonyl providing a hydrogen bond. Ultimately, this enol-ketone combination forms this self-stabilizing structure which has the character of a six-membered ring.
Aromatic enols also tend to have stability, as aromaticity offers such stability that it outweighs that of ketones. Specifically, enols are favored when the resulting alkene completes an aromatic electron cycle. This explains why phenols tend to form stable structures, while 2,4-cyclodienones tend to remain rare.
Keto Enol Tautomerization Mechanism
Tautomerization under Acidic Conditions
The keto-enol tautomerism under acidic conditions essentially has protonation driving the conversion between ketones and enols.
To form an enol, an acid protonates a lone electron pair on the carbonyl. Then, the acid’s conjugate base deprotonates the ɑ-carbon. This drives a transfer of electrons, ultimately forming an enol.
To form a ketone, the acid protonates the alkene, forming a C-H bond between the ɑ-carbon and the proton. Then, the conjugate base deprotonates the hydroxide. A lone pair from the oxide anion then forms a pi bond with the carbocation, resulting in a ketone.
Tautomerization under Basic Conditions
The keto-enol tautomerism under basic conditions essentially has de-protonation driving the conversion between ketones and enols.
To form an enol, a generic Brønsted-Lowry base deprotonates the ɑ-carbon. This drives the pi electrons of the carbonyl to form an alkene pi bond with the deprotonated carbon. Then, the conjugate acid protonates the oxide anion, forming an enol.
To form a ketone, the base deprotonates the hydroxide, liberating a lone electron pair. These electrons then form a carbonyl, pushing a lone electron pair to the ɑ-carbon. Then, the conjugate acid protonates the ɑ-carbon, forming a ketone