Theoretical and Experimental Approach for the Study of 2,4-Dichlorophenoxyacetic acid Photodegradation: C-O versus C-Cl Bond Dissociation Energies in the Gas Phase and Aqueous Medium

A theoretical, gradient-corrected Hartree-Fock-density functional theory (HF-DFT) approach was applied to the determination of the bond dissociation energy (BDE) for the photodegradation processes of the 2,4-dichlorophenoxyacetic acid (2,4-D) herbicide in the gas phase and in aqueous medium. According to the results of these calculations, the phenoxy C-O homolytic BDE value was found to be approximately two times lower than the corresponding C-Cl (2) and C-Cl (4) BDE ones, in both gas phase and under continuous solvation by water. An experimental study of the 2,4-D photodegradation reaction kinetics and photoproducts was also performed in water. At lower concentration, the formation of only two photoproducts (2-Cl- and 4-Cl-phenoxyacetic acid) was observed, whereas at higher concentration, three photoproducts were formed, with 2,4-dichlorophenol as the third one. These photoproducts appeared competitively within about 40-80 min, and were eventually photodecomposed. The experimental data are in good agreement with the results of our HF-DFT calculations.