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: Understanding Ionization Constant

The ionization constant of an acid depends on the stability of the conjugate base formed after ionization. A more stable conjugate base results in a higher ionization constant for the acid.

Step 2: Conjugate Base of Phenol

Phenol ionizes to form the phenoxide ion. The stability of the phenoxide ion is enhanced through resonance, as the negative charge can be delocalized over the aromatic ring.

Step 3: Conjugate Base of Ethanol

Ethanol ionizes to form the ethoxide ion. The negative charge on the ethoxide ion is localized on the oxygen atom and cannot be delocalized due to the absence of a conjugated system.

Step 4: Comparing Stability

Since the phenoxide ion can stabilize its negative charge through resonance, it is more stable compared to the ethoxide ion, which lacks this ability. More stable conjugate bases correspond to stronger acids.

Key Concepts

Ionization Constant

The ionization constant is fundamentally a measure of an acid's strength and its tendency to donate a proton (H⁺) in a solution. This is often denoted as Ka. When an acid dissociates in water, it forms its conjugate base and releases a proton. The ionization constant essentially gives us an idea of how far this reaction proceeds.

• An acid with a higher ionization constant (Ka) is considered stronger because it dissociates more completely in solution.
• The key relation here is that a more stable conjugate base tends to correspond to a higher Ka, which means a stronger acid.

In the case of phenol and ethanol, the ionization constant of phenol is higher. This means phenol is a stronger acid compared to ethanol. The reason, as we'll see, lies in the stability of their conjugate bases.

Conjugate Base Stability

The stability of a conjugate base is crucial in determining the acidity of a compound. A conjugate base is what you get when an acid donates a proton.
Conjugate bases are more stable when they can easily disperse or distribute the negative charge that results when a proton is lost. This usually means:

• The presence of electronegative atoms that can retain the negative charge.
• The availability of resonance structures, which can spread the charge over a larger area.

For phenol, the conjugate base is the phenoxide ion. This ion is more stable due to the electron delocalization over the aromatic ring, which allows the negative charge to spread out. On the other hand, with ethanol, the ethoxide ion's charge is firmly localized on the oxygen atom, without resonance stabilization, making it less stable.

Resonance Stabilization

Resonance stabilization is key to understanding why some acids are stronger than others. Resonance occurs when electrons can be shared or delocalized across multiple atoms in a structure. In the case of phenoxide ion, after phenol loses a hydrogen ion, the remaining negative charge can resonate throughout the aromatic ring. This delocalization involves sharing the negative charge with several atoms, reducing energy and increasing stability.

• Resonance allows the phenoxide ion to have multiple contributing structures, where the negative charge is partially held by different atoms in the ring.
• This makes the phenol a stronger acid than ethanol, by virtue of forming a more stable conjugate base.

Ethanol lacks such a conjugated system to stabilize the ethoxide ion, hence the negative charge remains localized mainly on the oxygen atom. Without resonance, the ethoxide ion is less stable compared to the phenoxide ion.