Tautomerism Reaction Mechanism

This configuration change of the compound is a process called tautomerization. They are certain types of structural isomers. Structural isomers are all molecules that have the same chemical formula. What makes tautomers worthy of classification is that they easily interconvert between different morphologies. Many structural isomers do not easily convert to the corresponding isomers. 

Mechanism of Tautomerism 

Firstly, if we consider acid-catalysed tautomerisation that is of the keto form, the hydronium ion protonated and finally, the carbonyl oxygen atom, we have to remove the water and the α – hydrogen atom so that the enol can be obtained. All of these reactions are very much reversible, therefore, a conversion is done by the catalyzation of acid to obtain the enol and then converted into the keto form. It must be kept in mind that the entire process occurs through the reversion of every step of the process. In the first step of base-catalysed tautomerization of the keto form, the hydroxide ion removes the αhydrogen atom to give the enolate anion. Now, water starts to react with the enolate anion and the enol is produced. If the bond-like structures for the reaction are to be simplified, the step that forms the enolate can be seen to showcase one resonance form and the corresponding step will show the alternate form of resonance. All the reactions are reversible in nature. 

Markovnikov’s Rule 

Markovnikov’s rule is considered to be an empirical rule and it is to predict the regioselectivity of electrophilic addition reactions of alkynes and alkenes. The rule states; that in hydrohalogenation of an unsymmetrical alkene, the hydrogen atom in the hydrogen halide will form a bond with the doubly bonded carbon atom in the alkene, bearing the greater number of hydrogen atoms.

Enolization

Enolization is a molecular process by which a carbonyl compound is converted to form an enol. When this molecular process is done by using a basic catalyst, it is called basic catalyst enolization. 

Characteristics of Tautomerism

1. A movement of atoms can be witnessed in tautomerism and these atoms are alpha hydrogen atoms. 
2. Because the compounds are definite compounds, they can be separated and isolated as well.
3. There are two structures for tautomeric forms. 
4. No effect is experienced by the bond length due to tautomerism. 
5. All the compounds of the tautomer are in a state of equilibrium with each other. 
6. Tautomerism is not responsible for decreasing the energy of the molecules. Therefore, the molecules present are in a stable state. 
7. Tautomerism can be witnessed in both planar and non-planar molecules. 

Types of Tautomerism 

There are many types of tautomerism. They are as follows:

1. Prototropy- This type of tautomerism is caused by the acid-base behaviour of the compound and in this type of tautomerism, the two forms are only different in the proton’s position. This particular structure has the same empirical formula and the same number of charges. 
2. Annular Tautomerism- if a situation occurs when a proton is seen to be occupying two or more two positions of a heterocyclic system, it will be known as annular tautomerism. If an open structure is altered to a ring-like structure due to the delocalization of the protons, then it will be known as a ring-chain tautomer. Glucose is a great example of a ring-chain tautomer. 
3. Valence Tautomerism- This is a type of tautomerism where there is a continuous process of formation and both single and double bonds are continuing to break in the compound without any atoms or groups migrating. This is a very rapid process and is much different from other types of tautomerism. 

Conclusion

A phenomenon where a single chemical compound is found to be existing in two or more structures and that structure is interconvertible. Here, it must be remembered that this situation is much different if we consider the relative position of only one atomic nucleus like hydrogen. In this case, the molecular formula remains the same but the spatial arrangement of the atoms is altered convincingly.