Correlation analyses for bimolecular nucleophilic substitution reactions of chloroacetanilide herbicides and their structural analogs with environmentally relevant nucleophiles.

Katrice A Lippa, A Lynn Roberts

Index: Environ. Toxicol. Chem. 24(10) , 2401-9, (2005)

Full Text: HTML

Abstract

Second-order rate constants (kNuc) for aqueous-phase bimolecular nucleophilic substitution (SN2) reactions of a range of anionic nucleophiles with alachlor, propachlor, and two analogs of propachlor (a thioacetanilide and a beta-anilide) were fit to the Swain-Scott and Edwards models. Correlations of literature kNuc values for analogous reactions of methyl chloride and methyl benzenesulfonate were included for comparison. The Swain-Scott correlation yielded poor to fair results for chloroacetanilides and their analogs, with adjusted (adj) r2 values ranging from 0.67 to 0.89, which are comparable to correlations for CH3Cl and CH3OSO2Ph (r2 (adj) = 0.80 and 0.85, respectively). Both the one- and two-parameter Edwards models yielded improved correlations for the majority of substrates. A pronounced dependence on the substrate polarizability (alpha) factor generally was observed for the Edwards model, with a negligible dependence on the substrate basicity (beta) factor. Substrate polarizability factors for the one-parameter Edwards model were significantly larger for alachlor, propachlor, and the beta-anilide analog of propachlor than for methyl chloride and methyl benzenesulfonate. This indicates that the chloroacetanilides are activated toward SN2 reactions with highly reactive nucleophiles (e.g., HS-, PhS-, and Sn(2-)) relative to other saturated carbon substrates. At best, the Swain-Scott and Edwards models only furnish order-of-magnitude predictions of kNuc for the substrates investigated.