Kinetics and mechanism of the nucleophilic displacement reactions of chloroacetanilide herbicides: investigation of alpha-substituent effects.
The ease with which alpha-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 alpha-amide group was examined by including several structurally related analogs of propachlor possessing modified alpha substituents. The overall second-order nature of the reaction, the negative DeltaS(double dagger) values, the weak influence of ionic strength on reactivity, and structure-reactivity trends together support an intermolecular S(N)2 mechanism rather than an intramolecular reaction for alpha-chloroacetanilides as well as the alpha-chlorothioacetanilide analog of propachlor. In contrast, the alpha-methylene analog exhibits kinetics and a salt effect consistent with anchimeric assistance by the aniline nitrogen. Electronic interactions with the alpha-anilide substituent, rather than neighboring group participation, can be inferred to govern the reactivity of alpha-chloroacetanilides toward nucleophiles.
Lippa
Katrice A
KA
Department of Geography and Environmental Engineering, 313 Ames Hall, Johns Hopkins University, 3400 N. Charles Street, Baltimore, Maryland 21218-2686, USA.
Demel
Sandra
S
Lau
Irvin H
IH
Roberts
A Lynn
AL
D000081:Acetamides / Q000737:chemistry*; D000083:Acetanilides / Q000737:chemistry; D000838:Anions; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D007700:Kinetics; D009994:Osmolar Concentration; D012996:Solutions; D013457:Sulfur Compounds / Q000737:chemistry; D014867:Water
D000081:Acetamides; D000083:Acetanilides; D000838:Anions; D012996:Solutions; D013457:Sulfur Compounds; C011088:propachlor; D014867:Water; C000188:alachlor; C078188:2-chloro-N-(ethoxymethyl)-N-(2-methyl-6-(trifluoromethyl)phenyl)acetamide