Antioxidant properties of phenols

Area of Science:

• Physical Chemistry
• Organic Chemistry
• Antioxidant Mechanisms

Background:

• Phenols (ArOH) are known antioxidants that inhibit oxidation by donating a hydrogen atom to peroxyl radicals (ROO*).
• This process, the proton-coupled electron transfer (PCET) mechanism, is sensitive to the surrounding chemical environment.

Purpose of the Study:

• To review the antioxidant properties of phenols, focusing on the critical role of solvent effects.
• To elucidate how solvent properties influence the kinetics and mechanisms of phenol-radical interactions.

Main Methods:

• Review of existing literature on phenol antioxidant activity and solvent effects.
• Analysis of kinetic solvent effects based on solute acidity (α(2)(H)) and solvent basicity (β(2)(H)).
• Discussion of reaction mechanisms, including proton-coupled electron transfer (PCET) and sequential proton loss electron transfer (SPLET).

Main Results:

• Hydrogen-bond accepting solvents significantly reduce phenol antioxidant activity by forming unreactive hydrogen-bonded complexes (ArOH---S).
• Hydroxyl solvents (alcohols) exhibit a dual effect: inhibiting reactions via hydrogen bonding but promoting faster reactions through phenoxide anion (ArO-) formation.
• The overall antioxidant efficacy is determined by the balance between these competing solvent interactions and the degree of phenol ionization.

Conclusions:

• Solvent polarity and hydrogen-bonding capacity are crucial determinants of phenol antioxidant effectiveness.
• Understanding solvent effects is essential for optimizing phenol-based antioxidant strategies in various applications.
• The interplay between PCET and SPLET mechanisms, modulated by solvent properties, dictates the overall antioxidant performance.