What is Tautomerization? Its mechanism

Tautomers & Tautomerization:

Two structural isomers which differ in relative positions of their atoms exist in dynamic equilibrium and are readily inconvertible are called Tautomers. Tautomers are basically structural isomers. The phenomenon of tautomers formation is called Tautomerization. Tautomerization involves the transfer of electrons and protons from one molecule to another molecule.

“Two different structures of same molecular formula obtained due to oscillatory movement oh Hydrogen (H) between two polyvalent atoms is called tautomerization.”

1,3 – Hydrogen migration result in tautomerism.

Tautomerism is also called Desmotropism and Dynamic isomerism. If there is transferring of a proton from one molecule to another then it is called Proton Tautomerization. Tautomers are the same molecules having the same structural formula and different connectivity. It means that these are constitutional isomers that are interconvertible at rapid equilibrium. The common pairs of the tautomers are as under:

Tautomeric Relationships:

Keto-Enol tautomerization:

Keto-enol tautomerization is the equilibrium process in which two constitutional isomers are involved. To study keto-enol tautomerization it is necessary to study the Enols and enolates. Enols are basically the alkene alcohol having -OH substituted to alkenes. Enols are also the unstable isomers of aldehydes and ketones. On the other hand, Enolate is the conjugate base of enols or anion of enols which can be prepared in the presence of a base.

Conversion of enol form to keto form or vice versa is called Enolization. These show the lewis acid-base structure. Enolate is a conjugate base because it accepts a proton from enols (alkene alcohol). “According to G. N Lewis in 1923: Acid is a substance that can accept a pair of electrons and base is a substance that can donate a pair of electrons and conjugate acid-base is opposite to the lewis acid-base”. The diagram explained below shows the conversion of enol into enolate in the presence of a base.

Difference between keto and enol forms:

Now, it is important to make a difference between keto form and enol form:

Keto form is more stable than the enol form because it is noted that the Keto form contains a C=O bond and the Enol form contains a C=C bond. The bond order of C=O is greater than that of C=C. So, the conclusion arises that the stability of the keto form or enol form depends upon the bond order and bond strength of C=O.

Hence Proved, KETO FORM > ENOL FORM

Keto-enol tautomerization is catalyzed and carried out by the acid and base. So, these reactions are referred to as Acid-Catalyzed reactions and Base-catalyzed reactions. Tautomerization of any form may require two general processes i.e. protonation & deprotonation but the order of these steps depends upon whether the reaction takes place in acid or base.

Acid-catalyzed Process:

In the acid-catalyzed process, enol form is protonated at the alpha carbon. The resulting cation is resonance stabilized. That compound is very stable that contains delocalization of electrons (RESONANCE). When enol form is protonated the lone pair on the -OH^– ion is shifted to the C-H bond due to delocalization forming reaction intermediate and in the last step, there is deprotonation of enol group to form keto group. For acid-catalyzed processes, protonation is the first necessary step.

Base-catalyzed Process:

In base-catalyzed Tautomerization, the base donates a proton to the oxygen atom. Then enolate ion is obtained which is the conjugate base of enol form. The next step is slower in which enolate ion is protonated and keto form is obtained. For base-catalyzed tautomerization, deprotonation is the first and essential step.

Condition for Tautomerism:

Let’s take an example for a tautomeric structure X-Y=Z so that,

• X= A compound which should always possess Hydrogen. But it cannot be itself hydrogen
• Y= It may be any atom that has the ability to make a double bond. No condition is applied.
• Z= It is the atom that may be carbon, oxygen, nitrogen, and sulfur. (C/O/N/S)
• If a compound occurs which posses a double bond it we will consider that carbon as X which is sp₃ hybridized.
• We cannot take that carbon having a double bond in it because a double bond next to a double bond always results in an unstable species.
• Always aromatic compounds, conjugated compounds, cyclic and planar compounds show tautomerism

Mechanism of tautomerism:

We will discuss earlier the Basic mechanism of Tautomerism. Now to discuss the general mechanism of tautomerism we will discuss that there should be at least one hydrogen attached to the alpha carbon called alpha hydrogen. The hydrogen atom is attached parallelly to the anti-bonding pi-orbital of the carbonyl group. This bond will undergo hyperconjugation with the C-H bond and reduces the charge density on the alpha carbon where alpha hydrogen became more acidic. Change in the position of hydrogen may cause hyperconjugation.

If the position of hydrogen is not changed then there will be no hyperconjugation which means the reaction is too slow. Adamatanone is the best example of a slow process. Slow Process is the process that follows Markonikove’s rule. Firstly in this process, Hydronium ion (H₃O^₊) acts as an electrophile that attacks the C=C bond, and hence there is protonation on it which causes a change in the position of hydrogen causing tautomerism and converting keto form to enol form.

Types of Tautomerism:

1880, scientist Emil Erlnmyer develops the rules of tautomerization and also study keto-enol tautomerization. He also studies enolization and said that for tautomerization it is necessary to have alpha hydrogen attached to the alpha carbon. Types of Tautomerization are given below:

Protropy:

It is the type of tautomerism that occurs due to acid-base behavior. These two forms differ due to the position of proton/hydrogen. The structures obtained from prototrophy may possess the same empirical formula and same charges.

Annular tautomerism:

This tautomerism may occur when there are two positions of hydrogen forming a heterocyclic compound. This is called Annular tautomerism. But if these protons are delocalized and the compound is converted to a ring and the open-chain structure is changed to the closed-chain ring structure. And tautomers form are called ring-chain tautomers. Glucose is an excellent example of Ring-chain tautomers.

Valance Tautomers:

Valance tautomerism is the type of tautomerism that is formed when then there is breakage and formation of the single or double bond without any migration of atoms or protons. It is a rapid process.