Tautomerism

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What is Tautomerism?

Tautomerism is a phenomenon where a single chemical compound tends to exist in either two or more structures of interconvertible type, which are different considering the relative position of one atomic nucleus i.e. hydrogen, in general. The two structures are known as tautomers. These types of isomer compounds only differ usually in the number of electrons and protons. They also exist in the dynamic equilibrium.

If a reaction takes place between these compounds, there is only the transfer of protons. Tautomerism is otherwise termed as desmotropism.

Tautomerism usually happens in the presence of a catalyst.

  • Acid-Catalyst: Here, firstly, the protonation takes place, cation will be delocalized. Then, the deprotonation will take place in the adjacent position of the cation.

  • Whereas, for base catalysts, deprotonation is the primary step. Here, rather than cation delocalization, anion delocalization takes place and finally protonation to a different position of the anion.

Tautomerism Example

As a simple definition of Tautomerism, it is expressed as a type of isomerism, where the isomers interchange into/between one another easily to exist together in equilibrium. At the time of reaction, there occurs a proton transfer in an intramolecular fashion. Consider a tautomerism example given below.

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

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

Tautomerism Types

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

There are different types of Tautomerism. Among those, keto-enol Tautomerism is the most important one. Here, one structure is in enol, and the other is in ketone form. Both tautomeric forms are interconvertible to each other by the use of acid or base catalysts. This conversion process of the ketone into enol is called enolization.

The types of Tautomerism include:

  • Prototropy

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

  • Annular Tautomerism

If a proton occupies either two or more positions of a heterocyclic system, then such a process is known as annular Tautomerism. In Tautomerism, because of the delocalization of protons, if an open structure is altered to a ring structure, then such a tautomer is known as ring-chain tautomers. An example of ring-chain tautomers is Glucose.

  • Valence Tautomerism

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

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

Tautomerism in Non-Carbonyl Compounds

Many non-carbonyl systems are available as mixtures of tautomers. A few examples are given below.

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

Tautomeric Form of Unsymmetrical Ketones

There is only one form of tautomer in symmetric form. But there can be two for an unsymmetrical form. Let us have a look at the example given.

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Tautomerism Reaction Mechanism

Let us discuss the keto-enol tautomerization acid catalyzation. It is a two-step method in an aqueous solution of acid. The carbon atom, which is closest to the functional group, is known as the alpha carbon atom. So, for this mechanism to take place, one hydrogen atom at least should be with the alpha carbon atom. It can also be known as an alpha hydrogen atom.

The resultant hydrogen atom is added to the antibonding pi-orbital of the carbonyl group parallelly. Then, this bond will undergo hyperconjugation with the C-H bond and decrease the electron density at the alpha carbon atom, where the alpha hydrogen atom will become more acidic compared to before. If the position of alpha hydrogen has not happened, the tautomerism process will be very slow. An example of this slow process is Adamatanone.

We should follow Markovnikov's rule in addition to this process. Firstly, in the mechanism, there exists a hydronium ion (H3O+), an electrophile, so the electrons exposed near the C=C bond will be donated. If the number of hydrogen atoms present in the compound is more, thereby, the addition of protons also increases.

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The reaction mechanism can be given below.

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FAQ (Frequently Asked Questions)

1. How does Acid Show Tautomerism?

Ans: Tautomerism is the special kind of functional isomerism in which migration OH H occurs in dynamic equilibrium condition.

Acid Mechanism :

Step 1 - Acid will give H+ ion, and the Oxygen of ketone will donate its lone pair to H+ and carries a +ve charge on itself (this is a fast process)

CH3—C=0—CH3 + H+ → CH3—C=OH—CH3

Step 2 - Because Oxygen carries a +ve charge, it will attract an electron from Carbon, and a double bond between C=O will break, and it will become OH, and simultaneously Carbon will form a double bond with the neighbouring Carbon. (it's a slow and rate-determining process)

CH3—C=OH—CH3 - H+ → CH2=C-OH—CH3

Key Point - Tautomerism can be achieved in acidic and basic mediums (the major product is different for both cases) as well.

Basic Medium -  A double bond is formed between Carbon and its functional group, which has a lower number of alkyl groups.

Acidic Medium - A double bond is formed between the Carbon and its functional groups, which has more number of alkyl groups.

For example, consider the below reactions given.

CH3—CO—CH2—CH3 + OH-(basic) → CH2=C(OH)—CH2—CH3

CH3—CO—CH2—CH3 + H+(acidic) → CH3-C(OH)=CH—CH3

2. What is the difference between Tautomerization and Resonance?

Ans: Tautomerism is a real phenomenon, whereas resonance happens hypothetically. There is a movement of atoms (alpha hydrogen) involved in Tautomerism. But in resonance, the electron movement of the p orbital happens. Thus, there's no movement of atoms in the latter. While Tautomerism is represented by the double arrows (same as equilibrium), resonance is represented by one double-headed arrow.

Tautomerism happens in solutions; it means the alpha hydrogen gets removed, and another hydrogen from the water gets attached to the oxygen atom. But resonance is not restricted like Tautomerism, but only happens in solutions.