Reactivity of N-hydroxyphthalimide and N-hydroxysuccinimide in reaction with permanganate

Abstract

Kinetics of oxidation of N-hydroxyphthalimide (NHPI) and N‑hydroxysuccinimide (NHSI) by potassium permanganate was investigated. Reactions of NHSI and NHPI with KMnO 4 were studied at 298 K in an acetonitrile inert to oxidation, in which the organic and inorganic components of our system dissolve well. Merk reagents were used. The acidic medium was created by adding acetic acid. The kinetics of the reactions were studied by measuring the consumption of permanganate ions (MnO 4 – ), determining the change in the optical density (D) of a band with a wavelength of 540 nm in quartz cuvettes 10 mm thick. The investigation was carried out in the concentration region where the Lambert-Beer law was fulfilled. The kinetic curves show the drop in optical density (D) throughout the process. At the end of the reaction, it decreases to almost zero. The discoloration of the initial violet solutions suggests the transition of the MnO 4 – ion to Mn 2+ . It is worth noting that the kinetic curve (1–60 min) is properly (R 2 = 0,996) described in the coordinates of the first order log D – t. The obtained data indicate that the oxidation of N-hydroxysuccinimide and N‑hydroxyphthalimide with potassium permanganate in pure acetonitrile is very slow. The acceleration of reaction rate by acetic acidhas been shown, the rate constant of the oxidation reaction linearly depends on the square root of the concentration of acetic acid in the reaction medium. That was explained by the dominant role of protonated form of H 2 MnO 4 + in these processes. The РМ6 quantum-chemical method was used to estimate electronic (the difference of energies of the highest occupied molecular orbitals) and thermodynamics (the difference of enthalpies of isodesmic reactions of hydroxyimide with a permanganate) factors. The obtained values of both factors explain the lower reactivity of N‑hydroxysuccinimide, which is observed in kinetic experiments. The enthalpies of the first stage of oxidation – the H-atom abstraction from the O-H bond of NHPI and NHSI molecules with MnO 4 ¯ , HMnO 4 , H 2 MnO 4 + were calculated. The influence of the degree of protonation of MnO 4 ¯ on the enthalpy of this reaction has been shown. The highest reactivity in the H-atom abstraction from the NHPI and NHSI molecules has the most protonated form – H 2 MnO 4 + . This explains the accelerating role of acid in the processes of oxidation of N‑hydroxyimides by permanganate. Keywords: N-hydroxyimide, permanganate, kinetics, oxidation, quantum-chemical method.