Problem 53 - A Complete Resource Book in Chemistry for JEE Main

Question

Keto \(-\) enol tautomerism is observed in : (a) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHO}\) (b) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OCH}_{3}\) (c) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCH}_{2} \mathrm{COOH}\) (d) \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{COC}_{6} \mathrm{H}_{5}\)

Step-by-Step Solution

Answer

Keto-enol tautomerism is observed in (c) \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCH}_{2} \mathrm{COOH} \).

Step 1: Introduction to Keto-Enol Tautomerism

Keto-enol tautomerism is a chemical equilibrium between two structural isomers: a ketone and an enol. It generally occurs when a ketone contains a hydrogen atom on the carbon adjacent to the carbonyl group. The hydrogen can shift to form a hydroxyl group, converting the ketone into an enol. This is most common in specific ketone-containing compounds that allow the formation of a stable enol form.

Step 2: Analyze Each Compound for Tautomerism

Let's analyze each compound for the possibility of keto-enol tautomerism:(a) \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHO} \) (Benzaldehyde) lacks an alpha-hydrogen next to the carbonyl carbon, so it does not undergo tautomerism.(b) \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OCH}_{3} \) is an ether and lacks a ketone group, so no tautomerism occurs.(c) \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCH}_{2} \mathrm{COOH} \) (Acetoacetic acid) contains a keto group with an alpha-hydrogen, allowing keto-enol tautomerism.(d) \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COC}_{6} \mathrm{H}_{5} \) (Benzophenone) does not have an alpha-hydrogen, so tautomerism is not possible.

Step 3: Conclusion on Keto-Enol Tautomerism

Based on the analysis, only option (c), \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCH}_{2} \mathrm{COOH} \), has the structural prerequisites to undergo keto-enol tautomerism due to the presence of an alpha hydrogen adjacent to the keto group. This allows the rearrangement to the enol form.

Key Concepts

Structural Isomers

Structural isomers are molecules with the same molecular formula but different structural arrangements. In the context of keto-enol tautomerism, we see structural isomerism as a dynamic equilibrium between the keto form (a ketone) and the enol form (an alcohol). Each has distinct structural features:

• The keto form features a carbonyl group (C=O) linked to other carbon atoms, generally in a more stable and lower energy state.
• The enol form consists of a double bond between two carbon atoms (C=C) and a hydroxyl group (–OH) attached to one of them.

These isomers are interconvertible via a shift in the position of a hydrogen atom and the movement of a double bond. Though isomers, their functional groups and properties vary enough that they can have different chemical reactivities. For example, enols are generally more reactive than their ketone counterparts, often participating in nucleophilic reactions where the hydroxyl group plays a central role.

Chemical Equilibrium

Chemical equilibrium refers to a state where the concentrations of products and reactants in a reversible reaction do not change over time because the rate of the forward reaction equals the rate of the reverse reaction. In keto-enol tautomerism, the equilibrium exists between keto and enol isomers. This involves a dynamic balance where:

• Keto forms are typically more predominant due to their stability.
• Enol forms can become significant in certain chemical environments or reactions, especially those that stabilize the enol structure (such as hydrogen bonding).

This equilibrium is vital because tiny changes in conditions, such as pH or temperature, can shift the balance more toward one form or the other. This concept plays a crucial role in many biochemical and synthetic applications where selective stabilization and reactivity of either the keto or enol form is desired.

Alpha Hydrogen

Alpha hydrogen refers to the hydrogen atoms that are attached to the carbon atom adjacent to a functional group, such as a carbonyl carbon in a ketone. The significance of alpha hydrogens lies in their role in tautomerism and reaction mechanisms.

• In keto-enol tautomerism, the presence of an alpha hydrogen is essential for the transformation from keto to enol form.
• Alpha hydrogens are more acidic compared to other hydrogen atoms and can be abstracted easily by bases, facilitating the enolization process.

Without the presence of alpha hydrogens, particularly in the alpha position to a carbonyl, the compound cannot undergo keto-enol tautomerism. This explains why some compounds do not show this equilibrium; for instance, benzaldehyde or benzophenone lack the alpha hydrogens necessary for the tautomerization process.