All About Tautomerism

The structural isomers of chemical compounds that mutually convert are called tautomers. The chemical reaction by which the interconversions are performed is known as tautomerisation. This conversion generally results from the arrangements of hydrogen atoms in the compound. This phenomenon of interconversion or mutual conversion of tautomerisation is known as tautomerism and is also known as geotropism. Tautomerism is relevant to the amino acids and nucleic acids’ behaviour, these two are the rudimentary building blocks.

Tautomerisation

Tautomerisation is the chemical reaction by which the interconversions are performed. Tautomerism in the event where a chemical compound occurs in two or more two mutually convertible structures. These interconvertible structures are different in the case of the relative position of the hydrogen atom. There is the only difference in the number of protons and electrons in these types of isomers. In dynamic equilibrium, tautomers exist. Only transfer of protons occurs when between these compounds a reaction occurs. Generally, in the presence of a catalysed tautomerisation or tautomerism occurs.

Examples of Tautomerism:

Keto-Enol Tautomerism

Keto-Enol tautomerism is the type of tautomerism that takes place in compounds called carbonyl compounds which have alpha hydrogen within them. In keto-enol tautomerism alkenes, the double bond is produced due to the breaking of the carbonyl bond. The keto-enol tautomerism is the chemical reaction that is equilibrium in nature between the keto group and the enol group of a compound. They are said to be each other’s tautomers. Keto-enol tautomerisation is base or acid-catalysed. Among the keto form and enol form, it is examined that generally, the keto form of the compound is thermodynamically more stable than enol, but sometimes enol form is also more stable than the keto form. On the factors like hydrogen bonding, aromaticity, substitution, solvent, and conjugation the stability depends. 

Imine-enamine Tautomerism

If imine has an -hydrogen then equilibrium will be built between an imine and enamine tautomerism aqueous solution. It is a similar process as it is being seen in keto-enol tautomers. To ket-enol tautomerism imine-enamine tautomerism is a nitrogen analog. The hydrogen atom in both the tautomerization interchanges its position between the nitrogen or oxygen with the second atom of carbon. The behaviour of an enamine tautomer and an enol is almost similar. Enamine at the Carbon is nucleophilic. The functional group of organic compounds imines is C=N and the functional group of organic compounds emanates contains an amine group that is adjacent to the C=C. The main difference between the imine and enamine group is imine molecule consists of a C=N bond but enamines molecules consist of a C-N bond.

Tautomerism Reaction Mechanism

Acid catalyzation of keto-enol tautomerism

In an acid’s aqueous solution it is a process of two steps. The alpha (α) the carbon atom is closest to the functional group. One atom of hydrogen must be present with the alpha carbon atom for this mechanism to take place also known as an alpha hydrogen atom. The hydrogen atom in the carbonyl group gets summed up at the right angle with the antibonding pi-orbital. At the α- Carbon atom, this bond of a hydrogen atom with pi-orbital goes through hyperconjugation with the carbon-hydrogen (C-H) bond and the electron density of the alpha carbon atom gets reduced. Here the acidic property of the alpha hydrogen atom will increase more. The process of tautomerism becomes very slow when the alpha hydrogen’s position is not proper. An example of tautomerism’s slow process is Adamatanone. In this mechanism, for addition, Markonikov’s rule must be followed. In the mechanism, firstly there is an electrophile named hydronium ion. The addition of protons increases as there are more hydrogen atoms in the compound. 

What is isomerism?

Isomerism is defined as an event in which the chemical compounds have a similar chemical formula but differ in individual structure i.e. arrangement of the atoms in a structure of the chemical compound and its properties. So, the compounds that have the properties of isomerism are called an isomer. In the year 1830, a Swedish chemist named Jacob Berzelius introduced the term “isomer”, its meaning is “equal part”. There are two types of isomerism such as structural isomerism and stereoisomerism. Structural isomerism is again sub-divided into 6 types, these are – Chain isomerism, Functional isomerism, Positional isomerism, Tautomerism, Metamerism, and Ring-chain isomerism. Stereo isomerism is again divided into two parts, such as Geometrical isomerism and optical isomerism. 

Conclusion

It is to conclude that tautomerisation is the chemical reaction by which the mutual conversions of the position of electrons and protons are performed. A compound’s tautomers can easily interchange and in equilibrium, they exist together. Examples of tautomerism are keto-enol tautomerism and imine-enamine Tautomerism.