What is Keto-Enol Tautomerism?

Organic chemistry (carbon-based chemistry) would be of little consequence were it not for special features called functional groups. This is in part because basic carbon-hydrogen chains and rings possess in themselves a low degree of reactivity. It is these groups that impart to organic chemistry its importance and its ability to sustain life. One such group is carbonyl (written ‐CO, or for more detailed visualization purposes, ‐C=O), consisting of only one carbon and one oxygen atom.

What Tautomerism Means

The atoms of a carbonyl group are not static - they can tautomerize or exchange a tiny hydrogen atom. The carbonyl form (keto) changes to an alkene-alcohol, or enol form. For acetone, the two structures, which interchange are,

• CH₃‐CO‐CH₃

• CH₃‐C(OH)‐CH₃

For chemical structures and their reactions referenced here, please see the image associated with this article.

Both Aldehydes and Ketones Participate

A ketone has its carbonyl group carbon atom linked to two other carbon atoms. An aldehyde, however, has its carbonyl carbon atom linked to one carbon atom and one hydrogen atom. For specific examples of each, see the image associated with this article. Although an aldehyde is not a ketone, for purposes of nomenclature, and since the nature of the reaction is the same, it is also said to undergo keto-enol tautomerism. Note that in all instances, a hydrogen atom, is transferred to or from oxygen.

Why is Tautomerism Important?

Since the exchange is back and forth, it might seem there is no particular importance to the phenomenon of keto-enol tautomerism - but there is! When an aldehyde or a ketone is in its keto form, the double bond between the carbonyl carbon and oxygen atoms is subject to addition, with the double bond converting into a single bond. The tautomerism strongly the method of attack and what specific addition products will result. For further information concerning attack mechanisms, consider this moderately comprehensive Michigan State University reference.

Which Form is More Stable

In general, it is the keto form that is more stable form, since the oxygen atom has the greatest electronegativity of the participant atoms. What "encourages" formation of the enol form is the presence of hydronium ion, provided in the form of a minute quantity of a mineral acid such as hydrochloric or sulfuric acid.