Tautomerism

What is Tautomerism?

Tautomerism is a phenomenon where a single chemical compound tends to exist in two or more interconvertible structures that are different in terms of the relative position of one atomic nucleus which is generally the hydrogen. The two structures are called tautomers and these type of isomer compounds usually differ only in the number of electrons and protons. They also exist in dynamic equilibrium.

When a reaction occurs between these compounds there is only transfer of protons. Tautomerism is also termed as desmotropism.

Table of Content

Tautomerism basically happens in the presence of a catalyst.

  • Acid-catalyst: Here firstly the protonation occurs, cation will be delocalized. Then, deprotonation will occur in the adjacent position of the cation.
  • For base catalysts, deprotonation is the first step. Here, instead of cation delocalization, anion delocalization occurs and finally protonation to the different position of the anion.

Tautomerism Example

If we consider the simple definition of tautomerism then it is described as a type of isomerism wherein the isomers interchange into or between one another very easily in order to exist together in equilibrium. During the reaction, there is proton transfer occurs in an intramolecular fashion. Consider an example of tautomerism given below,

Ketone-enol

Ketone-enol, enamine-imine,lactam-lactim, etc are some of the examples of tautomers

Meanwhile, some of the key features of tautomerism are that this process gives more stability for the compound. In this phenomenon, there is an exchange of a hydrogen atom between two other atoms while forming a covalent bond to either one. Tautomerism is a reversible process.

Structural Requirement of Tautomerism

  • Compounds contain polar molecules and weakly acidic group functional groups.
  • It involves the change in position of an atom.
  • It has no effects on bond length or such features.
  • Generally, it occurs in planar or non-planar molecules.

Also Read: Structural Isomerism

Tautomerism Types

In the 1880’s a scientist named Emil Erlenmeyer developed rule for the tautomerism. He is one of the first one to have studied about the keto-enol tautomerism. This rule states that hydroxyl group in all alcohols is attached with double-bonded carbon atom directly, and forms ketones or aldehydes. This occurred due to the more stability of the keto form.

There are different types of tautomerism but among them, keto-enol tautomerism is the most important one. In this form, one structure is a ketone and the other is in enol form. Both of the tautomeric forms are interconvertible to each other by use of acid or base catalysts. This process of conversion of the ketone to enol is known as enolization.

Prototropy

It is a type of tautomerism that occurs due to the acid-base behaviour of the compound. Here two forms differ only in the position of a proton. This structure will have the same empirical formula and the number of charges.

Annular Tautomerism

If a proton occupies two or more position of a heterocyclic system, then such process is called annular tautomerism. In tautomerism, due to the delocalization of proton, if an open structure is changed to a ring structure, then such tautomer is called as ring-chain tautomers. Glucose is an example of ring-chain tautomers.

Valence Tautomerism

Valence tautomerism is a type of tautomerism where there is continuous formation and breaking of single and double bonds in the compound without any migration of groups or atoms. It is different from the previous type of tautomerism, and it is a rapid process.

In this tautomerism, there is a change in geometrical structure but no change for canonical resonance structure or mesomers.

Tautomerism in Non-Carbonyl Compounds

Most of the non-carbonyl systems are available as a mixture of tautomers. Some of the examples are given below;

Tautomerism in Non-Carbonyl Compounds

Tautomerism can have a huge impact on the chemical properties of the compound.

Tautomeric Form of Unsymmetrical Ketones

In symmetric form, there is only one form of tautomer. But for an unsymmetrical form, there can be two. Let’s have a look to the example given

Tautomeric Form of Unsymmetrical Ketones

Tautomerism Reaction Mechanism

Let’s discuss the acid catalyzation of keto-enol tautomerization. It is a two-step process in an aqueous solution of acid. The carbon atom closest to the functional group is called the alpha carbon atom. So, for this mechanism to happen at least one hydrogen atom should be with the alpha carbon atom. It can also be called an alpha hydrogen atom.

This hydrogen atom is added parallelly to the anti-bonding pi-orbital of the carbonyl group. This bond will undergo hyperconjugation with the C-H bond and reduces the electron density at the alpha carbon atom, where the alpha hydrogen atom will become more acidic than before. If the position of alpha hydrogen is not so, the process of tautomerism will be very slow. Adamatanone is an example of this slow process.

In this process, we should follow Markovnikov’s rule for addition. Firstly, in the mechanism, there is a hydronium ion (H3O+) is present which is an electrophile, so the electrons exposed near the C=C bond will be donated. If the no. of hydrogen atom present in the compound is more, the addition of protons also increases.

Tautomerism Reaction

The reaction mechanism is given below;

Tautomerism Reaction Mechanism

Important Questions On Tautomerism

  1. What do you mean by the term tautomer?
  2. Explain the process of tautomerization.
  3. What are the structural requirements of a compound to possess tautomerization?
  4. Explain the mechanism of tautomerism with examples step by step procedure.
  5. Which are the different types of tautomerism? How they are classified? Explain.
  6. What is meant by annular tautomerism?
  7. How a ring- chain tautomerism is obtained?
  8. Protonation and deprotonation are the two-essential processes in tautomerism. Justify.
  9. What is valence tautomerism?
  10. Explain the tautomerism related to acid-base behaviour of the molecule.
  11. Write some examples for tautomerism, ring- chain tautomerism, annular tautomerism.
  12. How tautomerism is depending on the catalyzation process? Which is the type of such process? Explain briefly.
  13. Write the importance of alpha carbon atom in tautomerism.
  14. List the applications of tautomerism in various fields.
  15. Explain the stability of keto-enol form of tautomer?

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