Tautomere

Tautomere is a historical term that has been used to describe one member of a pair of interconvertible forms in tautomerism. In modern chemistry, the standard word is tautomer, and the two forms are usually referred to simply as tautomers. The tautomere concept emphasizes the dynamic relationship between these isomers rather than a distinct, isolated species.

Mechanism and characteristics: Tautomeres arise from rapid intramolecular or solvent-catalyzed proton transfers accompanied by relocation of a double bond or adjacent atoms. The equilibrium between tautomeres depends on factors such as solvent, temperature, pH, and substituents. Some systems show a strong preference for one form, while others exhibit substantial populations of both forms at equilibrium.

Common examples: The archetypal keto–enol tautomerism involves ketone and enol forms, as seen in compounds like acetylacetone. Other well-known pairs include imine–enamine tautomerism and lactam–lactim tautomerism. The distribution between tautomeres can influence reactivity, color, UV absorption, and hydrogen-bonding patterns.

Significance and applications: Tautomere (tautomer) distribution affects reaction pathways, stability, and binding properties in organic synthesis and biochemistry. In medicinal chemistry, tautomeric balance can alter pharmacokinetics, target interaction, and metabolic fate. Understanding tautomerism helps in interpreting spectra and predicting behavior under different conditions.

Detection and study: Tauto­mer ratios are investigated by NMR spectroscopy, infrared spectroscopy, and UV–visible spectroscopy, often aided by computational chemistry. Experimental conditions such as solvent polarity and pH are routinely varied to assess shifts in tautomer populations.

History: The term tautomere appears mainly in historical literature and in some languages; in contemporary English, tautomer is the preferred term. The concept of tautomerism itself was established in the late 19th to early 20th centuries. See also tautomerism, keto–enol tautomerism.