If a ketone or aldehyde containing an alpha proton is present with an acid or base, the ketone or aldehyde exists in equilibrium between the keto form and the enol form.
In the presence of an acid or a base, a ketone or aldehyde that has an alpha proton (any proton joined to an alpha carbon) is in equilibrium with an enol, a compound that has a (-ene) bond and a hydroxyl (OH) group (-ol). When the equilibrium compound has a carbonyl (), it is said to be in its keto form, even if the compound is not a ketone. When the equilibrium compound has an enol, it is said to be in the enol form. In the equilibrium between a carbonyl and an enol, the keto form is the compound that has a carbonyl. The enol form is the compound that has an alcohol in the equilibrium between a carbonyl and an enol. The main difference between the keto and enol forms is the location of the double bond and one proton. The proton is on the alpha carbon in the keto form. In the enol form, the proton is on the oxygen atom in the enol, forming a hydroxyl () group.
A tautomer is one of two or more constitutional isomers that differ only in the placement of a single proton. In other words, the keto form is a tautomer of the enol and vice versa. Chemical equilibrium between a ketone or aldehyde and an enol is known as keto-enol tautomerism. The keto and enol forms are not the same chemical compound, and they are not resonance structures of each other. Rather, these two different compounds are in equilibrium with each other, and the equilibrium generally favors the direction of the keto form.
Location of Protons in Ketones versus Enols
A comparison of the molecular structures shows why equilibrium is generally much closer to the keto form than the enol form. The greater bond energy, in particular the strong carbonyl () bond, makes the keto form more stable. Aromaticity, however, shifts the equilibrium strongly toward the enol form because of the stability provided by conjugation and the ring’s resonance bonds. The keto form won’t have this resonance because the carbonyl group draws electrons away from the ring. Other factors, such as the effects of a solvent or substituents, can produce more subtle shifts in the equilibrium between the two forms.