5.4 Tautomers

Some molecules are capable of changing their internal shape. Imidazole, which we will remember from the heterocycles table, has two nitrogen atoms, one of them double-bonded to a neighboring carbon and one bearing a hydrogen atom. The molecule can flip its internal bonds so that the hydrogen-bearing atom has the double bond, and the hydrogen proton can then jump over to the other nitrogen atom. For plain unsubstituted imidazole, this doesn't matter, since the two states, called tautomers, are the same compound anyway, but as we will see later, this capability of imidazole makes it extremely useful in biological systems.

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Another kind of tautomer is the enol form of ketones. As you may have guessed, enol is a contraction of -ene as in alkene and -ol as in alcohol. A ketone such as acetone CH₃COCH₃ can reversibly convert to its enol form CH₃C(OH)=CH₂, and the ketone and enol tautomers of a ketone compound exist in some kind of equilibrium, usually strongly favoring the ketone form.

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