Keto–enol tautomerism refers to a chemical equilibrium between a keto form (a ketone or an aldehyde) and an enol (an alcohol) as they are tautomers of each other.

Keto–enol tautomerism is a chemical equilibrium between a keto form (a ketone or an aldehyde) and an enol (alcohol) in a monosaccharide. They are tautomers of each other as they exist in equilibrium and are readily interchangeable.

A compound containing a carbonyl group (C=O) is normally in rapid equilibrium with an enol tautomer, which contains a pair of doubly bonded carbon atoms adjacent to a hydroxyl (−OH) group, C=C-OH. In keto-enol tautomer, the enol form is the one with the alcohol and the keto form is the one with the ketone. The keto form is more stable, it is the predominant form.

In monosaccharides, the keto form takes the form of an aldose that is a straight-chain version of a monosaccharide. The C5 hydroxyl group cyclizes when it attacks the C1 carbonyl. The product formed is the enol form of the monosaccharide, a hemiacetal ring.

Practice Questions

Sample Test C/P Section Passage 8 Question 39

Key Points

• Monosaccharides exist in a chemical equilibrium as two isomers in keto-enol tautomerism. The tautomerism is because they are interchangeable in equilibrium and readily change.

• The keto form of a monosaccharide is a straight-chain aldose (keto) that upon an intramolecular bond forming between the C1 hydroxyl and C5 carbonyl forms a ring which is the hemiacetal (enol) ring.

Key Terms

Keto–enol tautomerism: Refers to a chemical equilibrium between a keto form (a ketone or an aldehyde) and an enol (an alcohol).

Keto: Refers to a ketone or an aldehyde.

Enol: Refers to an alcohol.

Tautomer: Constitutional isomers of two compounds that readily interconvert between one another

Ketone: An organic compound containing a carbonyl group =C=O bonded to two hydrocarbon groups, made by oxidizing secondary alcohols.

Monosaccharide: Any of the class of sugars (e.g., glucose) that cannot be hydrolyzed to give a simpler sugar.