upgrade

🥼organic chemistry review

Acidity of Phenols

Written by the Fiveable Content Team • Last updated September 2025
Written by the Fiveable Content Team • Last updated September 2025

Definition

The acidity of phenols refers to their ability to donate protons (H+) to other substances, making them weakly acidic compounds. This property is crucial in understanding the reactivity and behavior of phenols in the context of the reactions discussed in section 17.10 Reactions of Phenols.

5 Must Know Facts For Your Next Test

  1. Phenols are considered weakly acidic compounds due to the stabilization of the phenoxide ion through resonance.
  2. The acidity of phenols is influenced by the presence and position of substituents on the benzene ring, with electron-withdrawing groups increasing acidity and electron-donating groups decreasing acidity.
  3. The pKa value of phenols typically ranges from 9 to 10, making them more acidic than alcohols (pKa ~16) but less acidic than carboxylic acids (pKa ~4-5).
  4. The acidity of phenols is crucial in understanding their reactivity, particularly in nucleophilic aromatic substitution reactions and acid-base reactions.
  5. The ability of phenols to form hydrogen bonds with other molecules also contributes to their acidity and reactivity.

Review Questions

  • Explain how the acidity of phenols is influenced by the presence and position of substituents on the benzene ring.
    • The acidity of phenols is influenced by the presence and position of substituents on the benzene ring due to the effects on the stability of the phenoxide ion formed upon deprotonation. Electron-withdrawing groups, such as halogens or nitro groups, increase the acidity of phenols by stabilizing the phenoxide ion through inductive effects and resonance. Conversely, electron-donating groups, like alkyl or alkoxy groups, decrease the acidity of phenols by destabilizing the phenoxide ion. The position of the substituents on the ring also plays a role, with ortho and para substituents generally having a greater impact on acidity compared to meta substituents.
  • Describe the relationship between the pKa value of phenols and their reactivity in acid-base and nucleophilic aromatic substitution reactions.
    • The pKa value of phenols is directly related to their reactivity in acid-base and nucleophilic aromatic substitution reactions. Phenols with lower pKa values (higher acidity) are more likely to participate in acid-base reactions, where they can act as proton donors. These more acidic phenols are also more reactive in nucleophilic aromatic substitution reactions, as the phenoxide ion formed upon deprotonation is a better nucleophile. Conversely, phenols with higher pKa values (lower acidity) are less likely to participate in these reactions, as they are less prone to deprotonation and the formation of the reactive phenoxide ion. Understanding the acidity of phenols, as reflected in their pKa values, is crucial for predicting and explaining their reactivity in various organic chemistry reactions.
  • Analyze how the resonance stabilization of the phenoxide ion contributes to the acidity of phenols and their overall reactivity.
    • The resonance stabilization of the phenoxide ion is a key factor contributing to the acidity of phenols. When a phenol is deprotonated, the negative charge on the oxygen atom can be delocalized throughout the benzene ring through resonance, resulting in a more stable phenoxide ion. This resonance stabilization lowers the energy required to remove the proton from the phenol, making it a more acidic compound compared to alcohols. The ability of the phenoxide ion to be resonance-stabilized is directly related to the acidity of phenols and their reactivity in various organic reactions, such as nucleophilic aromatic substitution. The more resonance-stabilized the phenoxide ion, the more likely the phenol will participate in acid-base and other reactions where the phenoxide ion acts as a nucleophile or base.