Solvation descriptors for pesticides from the solubility of solids: Diuron as an example

Abstract

Solubilities of diuron in 19 organic solvents are reported. These are combined with the solubility in water and the vapour pressure of diuron to give 19 values of water-solvent partitions, P, and 19 values of gas-solvent partitions, L. Coefficients in the general solvation equations (i) and (ii) are known for all 38 of these systems. In equations (i) and (ii), the independent variables are solute descriptors as follows: R2 is an excess molar refraction, π2/(H) is the dipolarity/polarizability, Σα2/(H) and Σβ2/(H) are the overall hydrogen-bond acidity and basicity, Vx is the McGowan characteristic volume, and log L16 is a descriptor where L16 is the solute L coefficient on hexadecane at 298K. logSP = c + rR2 + sπ2(H) + aΣα2(H) + bΣβ2(H) + vVx (i) log SP = c + rR2 + sπ2(H) + aΣα2(H) + bΣβ2(H) + log L16 (ii). We estimate R2 as 1.28 and calculate Vx as 1.5992, and then solve the total set of 38 equations to yield π2/(H) = 1.60, Σα2/(H) = 0.57, Σβ2/(H) = 0.70 and logL16 = 8.06 log units. These descriptors reproduce the 38 observed log P and log L values with a standard deviation of only 0.12 log units, and a test set of 13 independent log P and logL values with a standard deviation of 0.10 log units. Once the solvation descriptors for diuron are known, a range of physicochemical properties can be predicted; some examples are given. Similar analyses are reported for monuron and other substituted 3-phenyl-1,1-dimethylureas. (C) 2000 Society of Chemical Industry.