Question

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

Short answer

Phenoxide ion is stabilized through delocalization (option c).

Step-by-step solution

Understanding Ionization Constant

The ionization constant refers to the measure of how easily a molecule can lose a proton to form an ion. For phenol and ethanol, the ionization constants depend on the stability of the ions formed after losing a proton.

Evaluating the Phenoxide Ion

Phenol, after losing a proton, forms phenoxide ion. This ion is stabilized through resonance. The negative charge on the oxygen atom can be delocalized over the aromatic ring, equilibrating its structure.

Evaluating the Ethoxide Ion

Ethanol, after losing a proton, forms ethoxide ion. Ethoxide ions do not have resonance stabilization due to the absence of conjugation or an aromatic ring. Thus, ethoxide ions are less stable compared to phenoxide ions.

Comparing Stability

Phenoxide ion is more stable than ethoxide ion due to delocalization of the negative charge across the aromatic ring in phenoxide, whereas ethoxide ions lack this stabilizing delocalization.

Conclusion Based on Stability

The stability of the ion affects the ionization constant. Phenoxide ion being more stable due to delocalization, results in a higher ionization constant for phenol compared to ethanol.

Key concepts

Phenoxide Ion

The phenoxide ion plays a crucial role in understanding why phenol has a higher ionization constant than ethanol. When phenol loses a proton, it transforms into the phenoxide ion, which carries a negative charge on its oxygen atom.
What makes the phenoxide ion particularly interesting is its capability for stabilization. Phenoxide ions differ from many other ions because they can distribute this negative charge across their aromatic ring. This is possible because of a property called resonance. The phenoxide ion, with its extra stability, contributes significantly to the [[ionization constant]] of phenol being higher when compared to ethanol. The ability of the phenoxide ion to spread out its negative charge makes the formed ion less reactive, which means phenol is more likely to lose its proton than ethanol is.
This property makes the ionization of phenol relatively easier than ethanol, emphasizing the importance of resonance in chemical stability.

Resonance Stabilization

Resonance stabilization is a key concept in understanding why certain molecules form more easily ionized ions. In the case of phenoxide ions, resonance stabilization is responsible for their increased stability. This phenomenon occurs when electrons, like those in the phenoxide ion, are delocalized. In simpler terms, it means that the electrons can "move around" or "spread out" over different parts of the molecule.
In the phenoxide ion, after phenol loses a proton, the negative charge on oxygen can be shared across the entire aromatic ring.

• Delocalization lowers the potential energy of the molecule.
• It results in a more stable structure.
• Resonance creates multiple forms of a molecule contributing to its overall structure.

This delocalization is absent in structures like the ethoxide ion, which lacks an aromatic ring for electron sharing. Thus, resonance stabilization significantly enhances the stability of the phenoxide ion compared to the ethoxide ion.

Phenol vs Ethanol

When comparing phenol and ethanol, their chemical structures play a pivotal role in how they react when their protons are lost. Phenol is an aromatic compound, which means it contains a ring-like structure called a benzene ring. This structure allows phenol to stabilize its phenoxide ion through resonance. On the other hand, ethanol has a simplified carbon chain and lacks this aromatic quality.
Without the ability for resonance stabilization, ethanol forms the ethoxide ion, which remains less stable as the negative charge cannot be delocalized. Here’s why phenol stands out:

• Aromatic ring allows resonance, stabilizing charge.
• Higher ionization constant due to stabilization.
• Greater tendency to lose proton than ethanol.

This comparison highlights the importance of molecular structure and resonance stabilization in understanding why phenol has a higher ionization constant than ethanol.