Problem 191 - A Complete Resource Book in Chemistry for JEE Main

Question

Identify the compound that exhibits tautomerism. [2011] (a) Lactic acid (b) 2-Pentanone (c) Phenol (d) 2-Butene

Step-by-Step Solution

Answer

(c) Phenol

Step 1: Understanding Tautomerism

Tautomerism is a concept where a chemical compound exists in two (or more) quickly interchanging isomeric forms. These isomers, called tautomers, differ mainly by the position of a hydrogen atom and involve a shift of an electron pair, typically occurring between an sp² and sp³ hybridized carbon.

Step 2: Analyzing the Options

Evaluate each option to determine if it can exhibit tautomerism. Lactic acid, containing a carboxylic acid group, does not typically undergo tautomerism. 2-Pentanone, a ketone, can exhibit keto-enol tautomerism in certain cases. Phenol can exhibit keto-enol tautomerism, particularly due to resonance. 2-Butene is an alkene and does not show tautomerism.

Step 3: Considering Phenol

Phenol (C_6H_5OH) has a hydroxyl group attached to an aromatic ring, making keto-enol tautomerism possible, particularly in derivatives and compounds where tautomerization would lead to increased stability.

Step 4: Determining the Best Answer

Of the given options, 2-Pentanone is the most traditional example of tautomerism involving keto-enol forms. Phenol, however, can also exhibit active tautomerism in systems where aromatic stabilization is present.

Key Concepts

Keto-enol tautomerism

Keto-enol tautomerism is a specific type of tautomerism where a ketone isomerizes to an enol form and vice versa. This transformation involves the migration of a hydrogen atom and the shift of a double bond. These tautomers, the keto and the enol forms, coexist in equilibrium. In many cases, the equilibrium favors the keto form because it is typically more stable due to the strength of the carbonyl double bond compared to the enol double bond. Nonetheless, certain conditions—such as the presence of acid or base catalysts—can shift the balance toward the enol form.

**Example of Keto-enol Tautomerism**
In 2-pentanone, which is a ketone, keto-enol tautomerism is possible. The carbonyl carbon (C=O) acts as the site for this transformation, allowing a hydrogen from an adjacent carbon to migrate to the oxygen of the carbonyl, thus forming the enol, with a C=C double bond adjacent to an OH group. Though less stable compared to its keto counterpart, the enol form plays a vital role in chemical reactivity and is key in various organic reactions.

Isomeric forms

Isomeric forms are different structural arrangements of the same molecular formula. In the context of tautomerism, we focus on structural isomers that rapidly interconvert through changes in atomic connectivity. In keto-enol tautomerism, the critical change is between the keto and enol forms.

**Understanding Isomeric Behavior**
- **Keto Form**: Features a carbonyl group (C=O) and is generally more stable. - **Enol Form**: Features an alkene group (C=C) and a hydroxyl group (OH).

• Within tautomers, the position of hydrogen and the double bond is crucial.
• This relationship connects these forms and gives rise to their rapid equilibrium.

Certain compounds, like phenol, can also display tautomerism. Phenol primarily exhibits resonance stabilization of its aromatic ring, but derivatives may show a considerable tautomeric shift when the system benefits from increased aromatic stability. This dynamic interplay adds another layer of complexity to the already diverse world of isomers.

Hydrogen atom position shift

The hydrogen atom position shift is a fundamental component of tautomerism. It involves the movement of a hydrogen atom from one atom to another within a molecule, accompanied by a rearrangement of π (pi) electrons. In keto-enol tautomerism, the hydrogen migrates from a carbon adjacent to the carbonyl group in the keto form to the oxygen, creating an enol.

**Significance in Tautomerism**

• The ability to shift hydrogen atoms allows molecules to stabilize temporarily unstable intermediates.
• This flexibility can influence the molecule's reactivity, making specific forms more or less reactive depending on the environment.

The shift is swift and usually reversible, leading to the establishment of equilibrium between the tautomeric forms. The environment, pH, and other chemical interactions can greatly influence which form predominates by altering the ease of hydrogen migration and electron pair shifts.