Influence of the alkyl chain length on the physicochemical properties and biological activity in a homologous series of dichlorprop-based herbicidal ionic liquids

Abstract

This study focused on the synthesis and subsequent characterization of a homologous series of herbicidal ionic liquids (HILs) that incorporate a 2-(2,4-dichlorophenoxy)propionate (dichlorprop) anion and 1-alkyl-1-methylpiperidinium cation with a varied alkyl length from ethyl to octadecyl. Novel derivatives of dichlorprop were synthesized in high yields (≥90%) via a simple and eco-friendly two-step procedure. Subsequently, the obtained ionic liquids were subjected to analyses of their physicochemical properties (density, viscosity, refractive index, thermal decomposition profiles, octanol-water partition coefficient and solubility) and an evaluation of their herbicidal activity under greenhouse conditions. Generally, all the products possess great thermal stability and hydrophobicity; however, their affinities to various organic solvents were affected by the length of the alkyl substituent. Moreover, the densities and refractive indices of the ionic liquids containing middle-length alkyl chains (from hexyl to decyl) decreased with alkyl chain elongation. All the synthesized ionic liquids maintained biological activity against selected dicotyledonous weeds, including common lambsquarters, cornflower and oilseed rape. Among the tested ionic liquids, the products with dodecyl and octadecyl groups had the highest herbicidal efficacy against all plants. The octanol-water partition coefficients of these new herbicides indicated their low potential for bioaccumulation in the soil and for leaching into groundwater, which minimizes their adverse impacts on the environment.