Problem 59 - A Complete Resource Book in Chemistry for JEE Main

Question

The ionization constant of phenol is higher than that of ethanol because (a) phenoxide ion is a stronger base than ethoxide ion (b) phenoxide ion is stabilized through delocalization electron (c) phenoxide ion is less stable than ethoxide ion (d) phenoxide ion is bulkier than ethoxide ion

Step-by-Step Solution

Answer

(b) phenoxide ion is stabilized through delocalization electron.

Step 1: Understand Ionization Constant

The ionization constant is a measure of the strength of an acid in solution. It indicates the tendency of an acid to dissociate into a proton and an anion. A higher ionization constant implies a stronger tendency for dissociation.

Step 2: Analyze the Phenoxide Ion

The phenoxide ion is the anion formed when phenol loses a hydrogen ion. The negative charge on the phenoxide ion can be delocalized over the aromatic ring through resonance, enhancing its stability.

Step 3: Analyze the Ethoxide Ion

The ethoxide ion is the anion formed when ethanol loses a hydrogen ion. The negative charge on the ethoxide ion is localized on the oxygen, lacking resonance stabilization.

Step 4: Compare Stability of Ions

Since the phenoxide ion is stabilized by resonance due to electron delocalization over the aromatic ring, it is more stable than the ethoxide ion, which lacks such stabilization.

Step 5: Determine Effect on Ionization

A more stable conjugate base, such as the phenoxide ion, results in a stronger acid, which in turn has a higher ionization constant. Therefore, phenol has a higher ionization constant than ethanol.

Step 6: Choose the Correct Option

The statement that explains why the ionization constant of phenol is higher than that of ethanol is: "phenoxide ion is stabilized through delocalization electron". Thus, the correct option is (b).

Key Concepts

Ionization Constant

The ionization constant, often represented by the symbol \( K_a \), is a crucial concept in acid-base chemistry. It provides insight into how easily an acid releases protons (\( H^+ \)) in a solution.
The greater the ionization constant, the more tendency the acid has to donate protons, making it a stronger acid.

• If the ionization constant is high, the acid strongly dissociates into its ions.
• A low ionization constant means limited dissociation, indicating a weaker acid.

The ionization constant is essential for comparing the strength of different acids. For example, understanding why phenol has a higher ionization constant than ethanol involves analyzing their ability to release protons. Phenol's ionization constant is higher because its conjugate base (phenoxide ion) is more stable. This stability often hinges on the structural aspects that allow charge distribution, a concept central to determining acid strength.

Phenoxide Ion

The phenoxide ion is the conjugate base formed when phenol, a weak acid, releases a hydrogen ion. This process leaves a negative charge behind.
Phenol is differentiated from ethanol through the phenoxide ion's ability to stabilize this negative charge.
The key to understanding the stability of the phenoxide ion lies in "resonance," where the negative charge is delocalized across the aromatic ring structure.

• Resonance allows for multiple structures, distributing the negative charge over a larger area.
• This distribution prevents the concentration of charge on one atom, reducing energy and increasing stability.

This stabilization via delocalization is what makes the phenoxide ion more stable compared to the ethoxide ion, as it allows phenol to have a higher ionization constant. The more stable the conjugate base, the stronger the parent acid, explaining why phenol is stronger than ethanol.

Ethoxide Ion

The ethoxide ion arises when ethanol loses a proton, and it forms the anion \( \text{CH}_3\text{CH}_2\text{O}^- \). This process gives rise to a negative charge localized on the oxygen atom.
Unlike the phenoxide ion, the ethoxide ion lacks the ability to stabilize this negative charge through resonance. As a result:

• The charge remains concentrated on a single atom - the oxygen.
• This lack of distribution limits its stability.

The absence of delocalization in ethoxide renders it less stable as a conjugate base compared to phenoxide. As stability of a conjugate base is directly linked to the strength of its acid, ethanol becomes a weaker acid than phenol, resulting in a lower ionization constant.