Article

Kinetics and mechanism of the nucleophilic displacement reactions of chloroacetanilide herbicides: investigation of alpha-substituent effects.

Department of Geography and Environmental Engineering, 313 Ames Hall, Johns Hopkins University, 3400 N. Charles Street, Baltimore, Maryland 21218-2686, USA.
Journal of Agricultural and Food Chemistry (impact factor: 2.82). 06/2004; 52(10):3010-21. DOI:10.1021/jf030290d pp.3010-21
Source: PubMed

ABSTRACT 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.

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Keywords

alpha-amide group
 
alpha-anilide substituent
 
alpha-chloroacetanilide herbicides
 
alpha-chlorothioacetanilide analog
 
alpha-methylene analog exhibits kinetics
 
anchimeric assistance
 
aniline nitrogen
 
aqueous solution
 
displacement reactions
 
environmental chemistry
 
group participation
 
herbicidal properties
 
intermolecular S(N)2 mechanism
 
ionic strength
 
nucleophilic substitution reactions
 
salt effect consistent
 
second-order nature
 
strong nucleophiles
 
weak influence
 
weak ones
 

Katrice A Lippa