Montelukast photodegradation: Elucidation of Ф-order kinetics, determination of quantum yields and application to actinometry.

A recently developed F–order semi–emperical integrated rate–law for photoreversible AB(2F) photoreversible reactions has been successfully applied to investigate Montelukast sodium (Monte) photodegradation kinetics in ethanol. The model equations also served to propose a new stepwise kinetic elucidation method valid for any AB(2F) system and its application to the determination of Monte forward (ΦA→Blirr) and reverse (ΦB→Alirr ) quantum yields at various irradiation wavelengths. It has been found that ΦA→Blirr undergoes a 15–fold increase with wavelength between 220 and 360 nm, with the spectral section 250 – 360 nm representing Monte effective photodegradation causative range. The reverse quantum yield values were generally between 12 and 54 % lower than those recorded for ΦA→Blirr, with the trans–isomer (Monte) converts almost completely to its cis–counterpart at high irradiation wavelengths. Furthermore, the potential use of Monte as an actinometer has been investigated and an actinometric method was proposed. This study demonstrated the usefulness of Monte for monochromatic light actinometry for the dynamic range 258 – 380nm.