To understand the structural requirements of tautomerism, we need to first understand what is meant by the phenomenon of tautomerism. Tautomerism can be defined as the process in which a particular chemical compound exists in several interconvertible structures. These structures are only different due to the difference in α hydrogen atoms’ position. The two or more structures within tautomerism are known as tautomers. In simple words, these are isomeric compounds that only differ only in the number of protons and electrons. These tautomeric compounds exist in dynamic equilibrium. 

Structural requirements of Tautomerism

Tautomerism of compounds occurs only when the key structural requirements for the compound have been fulfilled. The structural requirements for Tautomerism have been outlined as follows.

• The compounds that have weakly acidic functional groups, as well as polar molecules, undergo tautomerism. In this context, it should be mentioned that a polar molecule refers to a chemical species in which the electron distribution within the atoms that are bonded covalently is not even. Polarity refers to how different the molecule’s electrical poles are. If the species are highly different then the molecules are highly polar. 
• The phenomenon of Tautomerism has no particular effect on bond length or other similar features. 
• The process of Tautomerism includes a change in atom position. The position change mostly occurs in the case of α hydrogen atoms
• Tautomerism is seen to happen in the case of both planar and non-planar molecules. 
• Lastly, tautomerism is essentially a chemical reaction that takes place when a catalyst is present. A catalyst can be of two types which have been outlined in the following. 

Basic catalyst: Here firstly deprotonation occurs. This is followed by delocalization of anion and delocalization of cation. Lastly, protonation takes place in a separate position within the anion. 

Acid Catalyst: in presence of an acid catalyst, protonation of the acid catalyst occurs and the cation also gets delocalized. Lastly, deprotonation occurs in the adjacent cation positions 

Different types of Tautomerism

A scientist named Emil Erlenmeyer developed the Tautomerism rule in the 1880s. He can be regarded as one of the first scientists who studied keto-enol tautomerism. There are several kinds of Tautomerism. However, keto-enol tautomerism is the most vital one. In this particular tautomeric compound, one structure is ketone whereas the other compound is enol form. Each Tautomeric form is interconvertible to one another through the use of a base or acid catalyst. The process through which ketone is converted to enol is called enolization. The different types of tautomerism have been outlined in the following. 

Annular Tautomerism

If two or even more positions are occupied by a proton within a heterocyclic system then the process is known as annular tautomerism. Moreover, in Tautomerism as a result of proton delocalization if an open structure is converted into a ring structure then the tautomer is known as ring tautomerism. Glucose can be regarded as a ring chain tautomer. 

Prototropy

It is a particular type of tautomerism that occurs due to the compound’s acid-base behavior. Here the two forms that are present differ only in the position of the proton. The structure of the protons will have the same charge number as well as a similar empirical formula. 

Valence Tautomerism

Valence Tautomerism can be regarded as another important type of tautomerism. In the valence tautomerism, there is continuous breaking and formation of double and single bonds. However, it should be mentioned that the migration of atoms or groups does not occur during the breaking of the single or double bonds. The process of valence tautomerism is quite rapid and it is a different type of tautomerism compared to the previous ones. Here no change in mesomers or canonical resonance structure can be observed. However, change within the geometrical structure can be noticed. 

Conclusion

The overall article has been written on the core topic of structural requirements of Tautomerism. Tautomerism is a core concept of organic chemistry and to understand the various aspects of the phenomenon of Tautomerism, firstly we need to understand the different structural requirements of tautomerism. Throughout the article, the core topic has been discussed through discussion of the specific requirement. After this, the different types of tautomerism that are normally seen have also been provided.