Kinetics and Mechanism of the Nucleophilic Displacement Reactions of Chloroacetanilide Herbicides:  Investigation of α-Substituent Effects

• Published in: Journal of agricultural and food chemistry 2004-05, Vol.52 (10), p.3010-3021
• Main Authors: Lippa, Katrice A, Demel, Sandra, Lau, Irvin H, Roberts, A. Lynn
• Format: Article
• Language: English
• Subjects: Acetamides - chemistry; Acetanilides - chemistry; alachlor; Anions; aqueous solutions; Biological and medical sciences; Chemical Phenomena; chemical reactions; chemical structure; Chemistry, Physical; chloroacetanilide herbicides; Fundamental and applied biological sciences. Psychology; Kinetics; metolachlor; Osmolar Concentration; Parasitic plants. Weeds; Phytopathology. Animal pests. Plant and forest protection; propachlor; reaction kinetics; reaction mechanisms; Solutions; structure-activity relationships; Sulfur Compounds - chemistry; Water; Weeds
• ISSN: 0021-8561; 1520-5118
• DOI: 10.1021/jf030290d

The ease with which α-chloroacetanilide herbicides undergo displacement reactions with strong nucleophiles, and their recalcitrance toward weak ones, is intimately related to their herbicidal properties and environmental chemistry. In this study, we investigate the kinetics and mechanisms of nucleophilic substitution reactions of propachlor and alachlor in aqueous solution. The role played by the α-amide group was examined by including several structurally related analogs of propachlor possessing modified α substituents. The overall second-order nature of the reaction, the negative ΔS ⧧ values, the weak influence of ionic strength on reactivity, and structure−reactivity trends together support an intermolecular SN2 mechanism rather than an intramolecular reaction for α-chloroacetanilides as well as the α-chlorothioacetanilide analog of propachlor. In contrast, the α-methylene analog exhibits kinetics and a salt effect consistent with anchimeric assistance by the aniline nitrogen. Electronic interactions with the α-anilide substituent, rather than neighboring group participation, can be inferred to govern the reactivity of α-chloroacetanilides toward nucleophiles. Keywords: Chloroacetanilides; chloroacetamides; nucleophilic aliphatic substitution; activation parameters; propachlor; alachlor; metolachlor; intramolecular substitution; anchimeric assistance; neighboring group participation; SN2