The relative toxicity of 32 methyl- and/or chloro-substituted anilines was assessed in the 48-h sublethal, static Tetrahymena pyriformis population growth assay. Quantitative structure-activity relationships (QSARs) were examined. Regression analysis of toxicity (log IGC50−1) versus the 1-octanol/water partition coefficient (log KOW) showed excellent correlation as revealed by the equation log IGC50−1 = 0.698 (log KOW) −1.190; n= 23, r-square= 0.931, s= 0.136, f= 282.47, Pr > f= 0.0001. This equation is considered to represent the polar narcosis mode of action. Nine derivatives were not included in the model. The parent compound aniline and the 4-chloro derivative were observed to be more toxic than predicted. The seven di- and trimethyl derivatives were observed to be less toxic than predicted. The latter seven derivatives could be included in the model with the addition of the summation of the Hammett sigma constants (∑σ) as a second descriptor, log IGC50−1 = 0.428 (log KOW) + 0.733 (∑σ) −0.828; n= 30, r-square= 0.919, s= 0.169, f= 153.60, Pr > f= 0.0001. Abiotic loss was not considered to be a factor in the observed toxicities.
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