All About Tautomerism and Its Examples

To understand Tautomerism we should first know about isomerism. It is the process in which the molecules of the compound have the same molecular formula. However, the atoms’ spatial arrangement is quite different. The reason is that all the arrangements are different as a result of the rotation of the whole unit or rotation as a particular bond.  The substances in which isomerism occurs are known as isomers. Isomerism is of two types namely Stereoisomerism and Structural Isomerism. Specifically, in the case of Structural isomerism, the different atoms present within the molecules are arranged in a different order. However, their molecular formula is the same. Structural isomerism are of several types such as positional isomerism, Chain isomerism, Metamerism, tautomerism, and functional isomerism. Here we will mostly discuss Tautomerism.

Tautomerism definition

Tautomerism is defined as the phenomenon in which an individual chemical compound exists in several structures that are interconvertible. The structures are different concerning one particular atomic nucleus. This atomic nucleus is generally hydrogen. The two separate comparable structures are called tautomers. These isomeric compounds mostly differ in the number of protons as well as electrons and exist in a form of dynamic equilibrium. When these tautomers undergo a reaction, then only proton transfer is seen to occur between them. The process of tautomerism occurs when there is a catalyst present. Now catalysts can be of two types which have been outlined as following: 

• Basic Catalyst: In presence of a basic catalyst, firstly deprotonation takes place. After that delocalization of anion and delocalization of cation takes place and lastly, protonation occurs at several anion positions.

• Acid Catalyst: In presence of acid catalyst firstly protonation occurs and cation gets delocalized. After that deprotonation occurs in the adjacent cation position.

Examples of Tautomerism

Let us suppose the basic definition of Tautomerism which was discussed earlier. From this definition, it is clear that tautomerism is essentially a form of isomerism. However, in this form of isomerism, the isomers readily interchange in between or into each other to existing together in equilibrium. At the time of reaction, proton transfer takes place in a particular intramolecular form. In this context the tautomerism between keto form and enol form is vital. Other examples of tautomers involve lactam-lactim, enamine-imine, and so on. However, an important feature of Tautomerism is that the process gives stability to the overall compound. Within the process, hydrogen exchange takes place within two atoms and covalent bonds are formed between each of them. The process of tautomerism is reversible. The example of keto–enol tautomerism will be further discussed in the following.

Keto enol tautomerism

Due to the α hydrogen’s acidity, several molecules that contain carbonyl achieve equilibrium due to proton transfer. This is known as Tautomerism. In this context Keto – enol tautomerism is quite important. The equilibrium in Keto – enol tautomerism is dependent on the factors of stabilisation of both the enol tautomer and the keto tautomer. In the case of a simple carbonyl compound, the equilibrium mostly prefers the keto tautomer. For instance, acetone is greater than 99.99% keto tautomer. The keto tautomer is mostly seen to be favoured as compared to the enol tautomer which is seen to be more stable by nearly 45 – 60kJ / mol. This is mostly due to the fact the double bond between C and O (-749kJ/mol) is stronger than the double between C and C (- 611kJ/mol). Moreover, in the case of ketones, two alkyl groups are present that donate electron density within the carbonyl carbon atom. Hence, they are comparatively more stable and it is less likely that the enol tautomer will be formed compared to aldehyde. There is a 1000 times chance that propanal will be present within enol tautomer instead of acetone. ‘

Conclusion

The phenomenon in which an individual chemical compound exists in several structures that are interconvertible are known as Tautomers. The overall article has been written to provide some key concepts concerning tautomerism. The concept of tautomerism is highly important in the subject of organic chemistry. The core topic has been discussed in detail through the discussion of tautomerism definition, examples concerning tautomerism, and the particular example of keto-enol tautomerism.