Valence Tautomerism

What is Valence Tautomerism? Valence tautomerism is a kind of tautomerism where there is the formation of single or double bonds without the delocalization of atoms or groups. This process involves a continuous reorganization of the bonding electrons. Valence tautomerism involves the interconversion of isomers without any specific rearrangement or any kind of proton transfer. 

Examples of Valence Tautomers

• There are various examples of valence tautomers such as Benzene oxide and Orepin

• Imido and nitrene ligands are considered valence tautomers where imido ligands are the electron-deficient species and the nitrene ligands are the electron-rich ones 

• Bullvalene also exhibits valence tautomerism and so is seen in open and closed heterocyclic compounds such as azides and tetrazoles, mesoionic munchnone, and acylamino ketene

Characteristics of Tautomerism 

• All those compounds that are polar in nature and have weakly acidic functional groups exhibit tautomerism

• Tautomerism involves changes in the transfer of atoms or groups

• Tautomerism does not have any profound impact on the bond lengths

• Tautomerism is a property that is exhibited in planar and non-planar molecules

• Also, tautomerism should be considered as a chemical reaction that occurs in the presence of a catalyst

• There are two types of catalysts:

  • Acid Catalyst- Here, the protonation takes place and the positively charged ion becomes delocalized and deprotonation takes place in the adjacent place of this ion

  • Basic catalyst- Here, first the deprotonation occurs, then negatively charged ion delocalization occurs and then protonation occurs

Tautomerism Reaction Mechanism

The alpha hydrogens are acidic in nature and because of this; many carbonyl compounds undergo Tautomerization or proton transfer equilibrium. By keto, it means that the tautomer has a carbonyl bond and by enol, it means that there is a double bond and a hydroxyl group. Let us discuss the features of keto-enol tautomerism which is the mechanism of Tautomerization.

Features of Keto Enol Tautomerism 

• Keto enol tautomerism equilibrium requires that the keto and enol tautomers both are stabilized

• Under any normal circumstances, for simple carbonyl compounds, the equilibrium generally favours the keto tautomers

• The equilibrium favours the keto tautomers the most and even more than enol tautomers because the carbon-oxygen double bond is stronger than the carbon-carbon double bond

• The presence of two alkyl groups donating electrons into the carbonyl atom makes ketones more stable and less apt to form enol tautomers than aldehydes

• Propanal is more likely to be in its enol form than its acetone

Valence Tautomers are more than just Electroactive Ligands 

Valence tautomers generally combine redox-active ligands and metal ions having two or more than two possible oxidation states, showing nearly degenerated oxidation states with the same electronic structures. The charge distribution in such compounds is extremely sensitive to environmental changes and so, any external change like temperature and photons may lead to intramolecular redox-active units and subsequently lead to changeable interconversion between two degenerate electronic states. 

This phenomenon was discovered in the 1980s and so, many valence tautomers are based on Quinone or its similar type ligands with a series of transition metal ions like Ni, Mn, Co, etc. however several other factors that can modulate the observation of valence Tautomerism have been found in the past many years.

Conclusion 

Tautomers are distinct chemical species with different atomic connectivity, molecular geometries, and spectroscopic properties. Tautomerism is extremely important for the existence of fundamental building blocks of life namely- amino acids and nucleic acids.