Tautomerism - Meaning, Definition, Types, Examples and Important Questions - JEE PDF Download

What is Tautomerism?

Tautomerism refers to the occurrence of a single chemical compound existing in multiple structures that can interconvert. These structures vary in the relative position of a specific atomic nucleus, often hydrogen. Known as tautomers, these distinct forms of the compound typically differ only in the number of electrons and protons. Moreover, they maintain a dynamic equilibrium, constantly transitioning between each other.
When these compounds engage in reactions, the transfer of protons is the primary mechanism at play. Tautomerism is also commonly referred to as desmotropism, emphasizing the interchangeable nature of these compound structures.

Tautomerism Example

Tautomerism can be defined as a form of isomerism characterized by the easy interconversion of isomers, allowing them to coexist in equilibrium. In this phenomenon, proton transfer takes place within the same molecule during a reaction. Let's explore an example of tautomerism below.

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, the scientist Emil Erlenmeyer made significant contributions to the study of tautomerism, particularly in relation to keto-enol tautomerism. Erlenmeyer's rule, formulated during this period, states that in alcohols, the hydroxyl group is directly attached to a carbon atom involved in a double bond, resulting in the formation of ketones or aldehydes. This rule arose from the enhanced stability of the keto form.
Tautomerism encompasses various forms, but one of the most significant ones is keto-enol tautomerism. This phenomenon involves two distinct structures: a ketone and an enol form. These tautomeric forms can readily interconvert with each other, facilitated by acid or base catalysts. The conversion process of the ketone into the enol form is commonly referred to 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 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.

Tautomerism Reaction Mechanism

Let's delve into the mechanism of acid-catalyzed keto-enol tautomerization, which occurs in a two-step process within an aqueous acid solution. The carbon atom adjacent to the functional group, known as the alpha carbon atom, plays a crucial role in this mechanism. For the tautomerization to take place, there must be at least one hydrogen atom attached to the alpha carbon, also known as an alpha-hydrogen atom.
During the mechanism, the alpha-hydrogen atom undergoes parallel addition to the anti-bonding pi-orbital of the carbonyl group. This bonding results in hyperconjugation between the C-H bond and the formed bond, leading to a reduction in electron density at the alpha carbon atom. Consequently, the alpha hydrogen atom becomes more acidic compared to its initial state. Notably, if the position of the alpha hydrogen is unfavorable, the tautomerism process will occur at a significantly slower rate. A notable example of this sluggish process is observed in adamantanone.
In this process, it is essential to adhere to Markovnikov's rule for addition. The mechanism involves the presence of a hydronium ion (H₃O⁺), which acts as an electrophile, attracting the electrons near the C=C bond. Additionally, the number of hydrogen atoms in the compound directly affects the extent of proton addition.

The reaction mechanism is given below: