Critical evaluation of rate coefficients for hydroxyl radical reactions with antibiotics: A review

Abstract

Rate coefficients (k_•OH) of hydroxyl radical + antibiotic molecule reactions (c.a. 200 k_•OH values for 70 molecules) were collected from published studies. Data measured in different laboratories, occasionally by different methods, were compared. Excluding the rate coefficients with values above the diffusion controlled limit and adjusting rate coefficients determined by competition kinetics, the data were averaged and the most probable values were selected. Rate coefficients for compounds with aromatic rings (e.g., amoxicillin) or conjugated double bonds (e.g., tylosin) are in the (6 – 8) × 10⁹ mol⁻¹ dm³ s⁻¹ range. The basic reaction is radical addition. The value may be smaller when strong electron withdrawing substituent, like –NO₂ is attached to the ring, and N-atoms in the aromatic ring also reduce the rate coefficient. The rate coefficients for molecules without aromatic rings are smaller with values in the (3 – 6) × 10⁹ mol⁻¹ dm³ s⁻¹ range, where hydrogen abstraction reactions dominate. Carefully executed pulse radiolysis and steady-state experiments (with correct reference values) give similar rate coefficients.

Keywords:

• rate coefficient
• hydroxyl radical
• degradation
• antibiotics
• structure dependence