Reaction of phenol with singlet oxygen

Area of Science:

• Environmental Chemistry
• Photochemistry
• Physical Chemistry

Background:

• Photo-degradation is crucial for removing environmental pollutants.
• Phenol is a biodegradable-resistant organic pollutant.
• Understanding phenol's reaction mechanisms is vital for effective remediation.

Purpose of the Study:

• To investigate the reaction mechanism and kinetics of singlet oxygen with phenol in aqueous solution.
• To identify reaction intermediates and products using experimental and theoretical methods.
• To provide data for quantifying phenol degradation in wastewater treatment.

Main Methods:

• Customized LED-photoreactors and high-performance liquid chromatography (HPLC) for experiments.
• Electron paramagnetic resonance (EPR) imaging to identify oxidation intermediates.
• Density functional theory (DFT) calculations (M062X level) to model reaction pathways.

Main Results:

• Two competitive reaction pathways identified: 1,4-cycloaddition and ortho ene-type.
• Para-benzoquinone identified as the primary experimental product via 1,4-endoperoxide rearrangement.
• Para-semibenzoquinone anion (PSBQ) detected for the first time, supporting the proposed mechanism.
• Overall reaction rates determined: kr-solvation = 1.21 × 10^4 M⁻¹s⁻¹ and kr = 1.14 × 10^4 M⁻¹s⁻¹.

Conclusions:

• Singlet oxygen is the sole active species in phenol photosensitization.
• The reaction proceeds through distinct cycloaddition and ene pathways, with para-benzoquinone as the main product.
• The quantified reaction rates are applicable for phenol degradation monitoring in wastewater treatment.