Tautamerism of carbonyl compounds:

Because of the acidity of α-hydrogens, many carbonyl containing compounds undergo a proton-transfer equilibrium called tautomerism. Tautomers are readily interconverted constitutional isomers, usually distinguished by a different location for an atom or a group.

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 hydrogen. The two structures are called tautomers, and these types 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 a transfer of protons. Tautomerism is also termed as desmotropism.

Tautomerism basically happens in the presence of a catalyst.

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

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Tautomerism Example

If we consider the simple definition of tautomerism, 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, proton transfer occurs in an intramolecular fashion. Consider an example of tautomerism given below.

example of tautomerism

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

Besides, some of the key features of tautomerism are that this process gives more stability to the compound. In this phenomenon, there is an exchange of a hydrogen atom between two other atoms while forming a covalent bond with 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 the position of an atom.
• It has no effects on bond length or such features.
• Generally, it occurs in planar or non-planar molecules.

Tautomerism Types

In the 1880s, a scientist named Emil Erlenmeyer developed a rule for tautomerism. He is one of the first ones to have studied about keto-enol tautomerism. This rule states that the hydroxyl group in all alcohols is attached to a double-bonded carbon atom directly and forms ketones or aldehydes. This occurred due to the 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 an enol form. Both of the tautomeric forms are interconvertible to each other by the 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, the 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 positions of a heterocyclic system, then such a process is called annular tautomerism. In tautomerism, due to the delocalisation of the proton, if an open structure is changed to a ring structure, then such a tautomer is called a ring-chain tautomer. Glucose is an example of ring-chain tautomers.

Valence Tautomerism

Valence tautomerism is a type of tautomerism where there is a 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 in canonical resonance structure or mesomers.

Tautomerism in Non-Carbonyl Compounds

Most of the non-carbonyl systems are available as a mixture of tautomers. Some 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 at 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, a hydronium ion (H₃O⁺⁾ is present, which is 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, the addition of protons also increases.

The reaction mechanism is given below: