Tautomerization

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Tautomerization

Also known as dynamic isomerization, tautomerization is the process of one tautomer converting itself into another. Tautomers are structural isomers that exist in equilibrium with one another. Isomers are nothing more than two or more compounds that have identical molecular formulas ("iso" means "same"), but different chemical structures and physical properties.

Tautomerization usually involves the movement of a hydrogen atom between a different location on the molecule, resulting in two or more molecular structures. These structures are called tautomers, which exist in dynamic equilibrium with each other. In fact, unlike isomers, they are readily and directly interconvertible, meaning that with minor adjustments they can easily change from one isomeric form to the other.

Tautomers are interconvertible because of how easily their hydrogen atoms move around. These atoms are highly mobile, and can shift from place to place within the molecule. Often, this migration brings about rearrangement of a double bond as well.

Perhaps the most common tautomers are those between the keto (CH₃-CO-CH₂-COOC₂H₅) and enol (CH₃-C(OH)=CH-COOC₂H₅) forms of ethyl acetoacetate. Because acetoacetate contains these tautomers, it has the characteristics of both an unsaturated alcohol (from the enol) and a ketone (from the keto). Other kinds of tautomers are the three-carbon type and the amido-amidol type. There are also certain tautomers called desmotropes that can be separated from each other, unlike regular tautomers.

Just as there are different sorts of tautomers, there are different kinds of tautomerization. However, only two of these occur in laboratories with any regularity. The first is keto-enol tautomerization, after the enol-keto pair. This kind of tautomerization involves a shift of electrons and a hydrogen atom at the same time. The other type is known as ring-chain tautomerization. This phenomenon occurs with glucose when the aldehyde group in a sugar chain molecule reacts with a hydroxy group in the same molecule. The reaction gives the tautomer a cyclic shape. Lactam-lactim tautomerization, which chemists long suspected but which required the advent of sophisticated spectroscopy to prove, also occurs in a cyclic system.