Types of Tautomerism

The phenomenon in which a single chemical compound exists in a couple or more structures that are inconvertible and are non-similar in terms of the relative position of 1 atomic nucleus that is usually hydrogen is known as Tautomerism. The 2 structures are known as Tautomers. Such compounds are non-similar only in the number of protons and electrons. Their existence even takes place in dynamic equilibrium.

The process of Tautomerism takes place in the presence of the catalyst.

Acidic Catalyst: In this, at first the protonation occurs initially then the delocalization of the cation will take place.

Basic Catalyst: For the basic catalysts the initial step is deprotonation. In this step instead of cation delocalization, the occurring of anion delocalization happens. In the end, the protonation of the different positions of the anion.

Ketone-enol, enamine-imine, lactam-lactim, and so on are examples of tautomerism.

Types of tautomerism

During the period of the 1880s scientist Emil Erlenmeyer constituted a rule for tautomerism. The scientist is among the first ones to have studied the concept of keto-enol tautomerism. In this rule, the group of hydroxyl in all the alcohols is directly attached to the double-bonded carbon atom. The occurrence of this happened because of the stability of the keto firm.

There exist various types of tautomerism. Among them, the prominent one happens to be keto-enol tautomerism. In the type of keto-enol tautomerism one structure is in enol form and the other structure in the form of the ketone. Enolization is the process of conversion from ketone to enol.

Prototrophy:

This type of tautomerism occurs as a result of the acid-base behaviour of the compound. In this type of tautomerism two forms differ in the position of a proton. The structure will have the same number of charges and the same empirical formula.

Annular Tautomerism:

In the annular tautomerism, the proton occupies a couple or more positions in the heterocyclic system. Because of the delocalization of the proton in tautomerism, it can be called ring-chain tautomers if the open structure gets converted to the ring structure. Glucose is one of the examples of ring-chain tautomers.

Valence Tautomerism:

Valence tautomerism is a kind of tautomerism in which the continuous forming and breaking of the single bonds and double bonds take place without the migration of atoms or groups. This is a rapid process and is not similar to the other forms of tautomerism.

Unsymmetrical Ketones of the tautomeric form:

There is only one form of tautomer in the symmetric form. But, there can be two forms of the unsymmetrical form of tautomerism.

Tautomerism reaction mechanism in keto-enol tautomerism.

This is a 2 step process in the acid of an aqueous solution. The alpha-carbon atom is the carbon atom that is closest to the functional group. Thus, for this kind of mechanism to take place, there should be one atom of hydrogen with an alpha carbon atom. Thus, it can also be known as an alpha hydrogen atom.

The addition of this hydrogen atom is done in a parallel way to the carbonyl group of the antibonding pi-orbital. The bond will undergo hyperconjugation with the bond of C-H and it reduces the density of electrons at the alpha atom of carbon. In this, the atom of alpha hydrogen will get more acidic than before. There will be slowness in the process of tautomerism if the position of the alpha is not so. Adamatanone is a prime example of this slow process.

Conclusion:

Tautomerism is a single compound theory in structures that are more than one. It can be understood by the study that these structures are not similar to each other. It should also be noted that Tautomerism are not similar in the number of electrons and protons. It was Emil Erlenmeyer 1800 who was among the first ones to propose the theory of Tautomerism. It can be understood as per the rule of Emil Erlenmeyer that all the alcohols are directly attached to the double-bonded carbon atom.