It is a phenomenon in which the molecules tend to possess the same molecular formula but a different spatial arrangement of the atoms. This is all the different arrangements that are due to the molecules that are rotating as a whole unit or are rotating around the particular bonds. The substances which tend to undergo Isomerism are called Isomers. Isomerism is further divided into two types, Structural Isomerism and Stereoisomerism. In structural isomerism, the atoms of the molecules are arranged in different order but have the same molecular formula. There are many types of structural isomerism like Chain isomerism, positional isomerism, functional isomerism, Metamerism and 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. Other names that are widely used for tautomerism are kryptomerism, allotropism or merotropy. Tautomerism is the most widely accepted term in the field of chemistry. There are several types of tautomerism, keto-enol tautomerism being the most important. In this type, one tautomer exists as a ketone while the other tautomer exists as an enol. Acetone and phenol are the two most common examples.
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.
Characteristics of Tautomerism-
There is a movement of atoms involved which are alpha hydrogen atoms.
The compounds can be separated and isolated as they are definite compounds.
Tautomeric forms have two different structures.
The compounds of tautomers are in equilibrium with each other.
Tautomerism does not have an effect on the bond length.
Tautomerism does not decrease the energy of the molecules and hence stabilises the molecule.
It occurs in planar as well as non- planar molecules.
Examples of Tautomerism
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.
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.
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:
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.
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 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.
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.
Tautomerism Reaction Mechanism
Let us discuss the keto-enol tautomerization acid catalysis. 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.
The reaction mechanism can be given below.