Abstract
The aim of the study was to investigate the processes of formation and structural group composition of the products formed during the reaction of D-glucose with p-toluidine in anhydrous ethanol using electronic and vibrational spectroscopy methods. The evolution of the profiles of the electronic spectra of working solutions shows the formation of a chromophore system: clear peaks at 380 and 440 nm are already present at a reaction time of 60 min. The appearance of an additional maximum at 520 nm (90, 120 min) likely corresponds to the formation of chromophores in condensed structures, as confirmed by a decrease in the values of the E 4 /Е 6 indices over time. The dynamics of the transformation of structural elements is also clearly seen in the analysis of IR spectra: as the reactions proceed, the CH aliphatic component of the structure decreases, while the aromatisation and functionalisation of products by chromophore groups increases. For products obtained between 90 and 120 min, a clear band at 1656 cm -1 , attributed to stretching vibrations of multiple bonds in conjugated C=C – C=O systems and azomethine fragments, indicates the of condensation reactions. In the spectra of insoluble solid products fractionated by water, the intensity of the bands at 1656 and 1190 cm -1 is significantly reduced; this is possibly due to the transfer of some of the coloured low molecular weight products to the aqueous phase. A comparison of the IR Fourier transform spectra of the final solid products with the spectra of natural and synthetic humic-like substances indicates their similar structural group composition. The intensity of the bands at 1620, 1508 and 815 cm -1 indicates a significant contribution of the aromatic component to the structure of the products of water-insoluble fractions; the water-soluble fractions mainly include aliphatic oxygen-containing structures resulting from the oxidative destruction of the initial carbohydrate. The synthesised products seem promising as convenient plant growth regulators with controlled release of the active substance.
Highlights
Synthetic analogues of natural compounds are important products of modern chemistry and chemical technology
The direct determination of the structure of humic substances is greatly complicated by the compositional heterofunctionality and large number of combinations of structural units [10, 11]
The dynamics of the transformation of structural elements can be clearly seen in the frequency analysis (Fig. 1): – as the target processes proceed, a clear peak appears at 1190 cm-1, whose intensity increases with time; – for products obtained at 90 and 120 min incubation, a fixed band at 1656 cm-1 is attributable to the stretching vibrations of multiple bonds in conjugated system C=C‒C=O, indicating the condensation reaction [2]
Summary
Synthetic analogues of natural compounds are important products of modern chemistry and chemical technology. Humic substances are known to display a wide range of known properties (antioxidant, antiviral, surface-active), and can be used as plant growth regulators [3, 5] These factors, as well as new works in the field of developed surface materials and nanostructures [6, 7] suggest the potential for further developments in the search for new reaction systems for synthesis. The solubility of carbohydrates in ethanol-water solutions decreases significantly with an increase in the proportion of alcohol, which can shift the amino-carbonyl interactions in such systems into the partially heterogeneous category In this case, process dynamics, which are determined by the desorption rate of products from the surface of the carbohydrate, for the most part do not depend on the nucleophilicity of the amine [8], expanding the possibilities of the classical Maillard reaction with the participation of aromatic amines [9]. In connection with the stated purpose of the present study, processes of formation and structural group composition of the products of the interaction of D-glucose with p-toluidine in anhydrous ethanol were examined
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