Estimation of selected physicochemical properties for methylated naphthalene compounds

Liquid aqueous solubility (Sw,L), octanol/water partition coefficients (Kow), liquid vapor pressure (Pv,L), and Henry’s law constants (Hc) were estimated for 20 methylated naphthalenes ranging from monomethyl to tetramethylnaphthalenes. Chromatographic methods were used for the estimation. Chromatographic retention measurements were conducted for 11 reference compounds and regressions were fit between the retention indices and the physicochemical properties. HPLC octadecylsilyl column with acetonitrile/water eluent was used for the estimation of Sw,L and Kow. Two GC columns, HP5-MS and a more hydrophobic HP-1, were tested for the estimation of Pv,L. Measured retention indices for the methylated naphthalenes were entered to the regression equations to calculate the physicochemical properties for these compounds. Literature values, where available, were used to validate the calculated values. The method accurately estimated the physicochemical properties. Estimated Sw,L and Pv,L decreased with the number of methyl groups. Kow increased with the number of methyl groups. There was no obvious relation between Hc and the number of methyl groups. Log Sw,L ranged from 0.885 for 1,2,5,6-tetramethylnaphthalene to 2.269 for 1-methylnaphthalene (mmol/m3). Log Kow varied from 3.89 for 1-methylnaphthalene to 4.95 for 1,2,5,6-tetramethylnaphthalene. Log Pv,L ranged from −0.983 for 1,2,5,6-tetramethylnaphthalene to 0.789 for 2-methylnaphthalene (Pa). Log Hc varied from 1.03 for 1,4,5-trimethylnaphthalene to 1.73 for 2,6-dimethylnaphthalene (Pa m3/mol). There were no apparent effects of GC column hydrophobicity on the accuracy of the results. Estimation of Sw,L and Kow based on GC retention indices was not as accurate as with HPLC. Comparison of the estimated values with values predicted by EPIWIN indicated that EPIWIN is useful in giving order-of-magnitude prediction of physicochemical properties.