Changing the humic acids antioxidant properties at their modifications by benzoyl chloride in the inversion phase transfer catalysis
Humic acid (HA) is the most common in nature class of organic compounds that make up the soil, peat, lignite and sapropel. The high humic acids reactivity due to the presence comprising a large number of macromolecules of functional groups: carboxyl, hydroxyl, carbonyl, nitrogen and sulfur. For this reason, HA capable of entering into a redox reaction, complexation, etc. To improve the efficiency of natural humic acid is particularly promising them directed chemical modification to introduce new functional groups or changing the amount available as part of the macromolecule groups to obtain the desired functions. Products of humic acids chemical modification can be used as redox polymers in engineering (for example, remediation of soils contaminated with heavy metals and radioactive) and biological (antioxidants, pro-oxidants) directions. The aim of this work was to obtain chemical modification products of low-temperature humic acids fraction from brown coal and determination of their antioxidant properties. We obtain the modified humic acid (HA  mod) by acylation of the low-temperature humic acids (HA) fraction under phase-transfer catalysis conditions in the presence of 4-dimethylaminopyridine-N-oxide. Proposed reaction scheme involves reacting benzoyl chloride with a catalyst (4-dimethylaminopyridine-N-oxide) in the organic phase, the transfer of the formed N-benzoyloxy-pyridinium chloride in the aqueous phase, and finally reaction of the N-benzoyloxy-pyridinium chloride ion and humate water. To determine the antioxidant activity obtained HA  mod and changes of antioxidant properties compared with the original HA, we studied the process initiated by azobisisobutyronitrile (AIBN) radical-chain liquid-phase oxidation of cumene (Cm) in the presence of HA and HA  mod in dimethyl sulphoxide (DMSO). Adding humic substances (HA and HA  mod) to the oxidized mixture Cm - AIBN - DMSO lowers the amount of oxygen absorbed by the studied system. With increasing concentration of humic substances (HA and HA  mod) in the oxidized mixture observed decrease in the oxygen uptake rate of oxidized system. Introduction HA in an oxidizing mixture leads to a more intense reduction in the rate of oxygen absorption system than in the case of HA  mod. Chemical modification of the HA, aimed at changing the structure of macromolecules of natural phenolic fragments, should help shift the redox balance, thereby changing the antioxidant properties of HA. Described in this paper, a decrease of antioxidant activity HA  mod indicates the binding of the OH groups in the macromolecule due to their interaction with benzoyl chloride.