Phenoxide Anion

Definition

The phenoxide anion is a negatively charged species formed when a phenol (an aromatic alcohol) loses a hydrogen atom from the hydroxyl group. This anionic species is an important intermediate in various organic reactions and has unique properties that distinguish it from other organic anions.

5 Must Know Facts For Your Next Test

1. The phenoxide anion is a resonance-stabilized species, with the negative charge delocalized across the aromatic ring.
2. Phenoxide anions are more nucleophilic than alkoxide anions (RO-) due to the resonance stabilization and the ability of the aromatic ring to accommodate the negative charge.
3. Phenoxide anions are important intermediates in various organic reactions, such as nucleophilic aromatic substitution (SNAr) and electrophilic aromatic substitution (SEAr).
4. The pKa of phenols is typically around 10, indicating that they are weakly acidic. Deprotonation of phenols leads to the formation of phenoxide anions.
5. Phenoxide anions can participate in hydrogen bonding, which can affect their reactivity and solubility in different solvents.

Review Questions

• Explain the significance of the resonance stabilization in the phenoxide anion and how it affects the nucleophilicity of this species.
  • The phenoxide anion is a resonance-stabilized species, where the negative charge is delocalized across the aromatic ring. This resonance stabilization reduces the overall energy of the system, making the phenoxide anion more stable compared to other organic anions. As a result, the phenoxide anion is more nucleophilic, meaning it has a greater ability to donate electrons and attack electrophilic centers in chemical reactions. The resonance stabilization also makes the phenoxide anion less basic, as the negative charge is not as concentrated on the oxygen atom.
• Describe the role of the phenoxide anion as an intermediate in nucleophilic aromatic substitution (SNAr) reactions.
  • In nucleophilic aromatic substitution (SNAr) reactions, the phenoxide anion can act as a key intermediate. The reaction typically involves the displacement of a good leaving group (e.g., halide, nitro group) from an aromatic ring by a nucleophile. The phenoxide anion is formed as an intermediate when the nucleophile attacks the aromatic ring, leading to the formation of a Meisenheimer complex. This complex then undergoes further rearrangement and elimination steps to give the final substituted aromatic product. The resonance stabilization of the phenoxide anion helps to stabilize the Meisenheimer complex, facilitating the overall substitution reaction.
• Analyze how the acidity of phenols and the basicity of the phenoxide anion can influence their reactivity and solubility in different solvents.
  • The acidity of phenols, as indicated by their pKa values (typically around 10), means that they can be deprotonated to form the phenoxide anion. The phenoxide anion is more basic than the parent phenol, and this difference in basicity can affect the reactivity and solubility of these species in various solvents. In more polar, protic solvents (e.g., water, alcohols), the phenoxide anion can participate in hydrogen bonding, which can enhance its solubility. Conversely, in less polar, aprotic solvents, the phenoxide anion may be less soluble due to the lack of solvation. The reactivity of the phenoxide anion as a nucleophile can also be influenced by the solvent, as the degree of solvation can affect its nucleophilicity. Understanding the acid-base properties and solvent effects on phenols and phenoxide anions is crucial for predicting and controlling their behavior in organic reactions.

Related terms

Phenol:

Phenol is an aromatic compound with a hydroxyl group (-OH) attached directly to a benzene ring. Phenols are weakly acidic and can form phenoxide anions.

Nucleophilicity:

Nucleophilicity refers to the ability of a species to donate electrons and attack electrophilic (electron-deficient) centers in chemical reactions.

Resonance Stabilization:

Resonance stabilization is a phenomenon where the delocalization of electrons in a molecule or ion leads to a more stable structure, reducing the overall energy of the system.