Deep eutectic melt of betaine and trichloroacetic acid; its anomalous thermal behavior and green promotion effect in selective synthesis of benzimidazoles

Abstract

Trichloroacetic acid and betaine form glassy liquids on melting together in some molar ratios, of which the melt with mole fraction of trichloroacetic acid about 0.83 displayed the lowest freezing point, so was detected as their deep eutectic mixture. It showed two reversible DSC effects on cooling from 70 °C to −70 °C; a normal exotherm on freezing followed by an abnormal endotherm on melting. Electric conductance of the melt against temperature exhibited two regions of deviations from a perfect exponential curve. These regions correspond to the temperatures of the two reversible phase transitions detected by DSC analysis. As another evidence in support of its freezing on heating, the kinematic viscosity of the melt is increased within the temperature range of 40–50 °C. Moreover, the FT-IR spectrum and TGA analysis of the eutectic mixture point to its exceptional nature in comparison to other mixtures of the two components. The melt proved to be an efficient promoter for selective synthesis of benzimidazoles from o-phenylenediamine and arylaldehydes at room temperature. No N-substituted derivatives of the products were resolved from the reaction mixtures.