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| Dechlorination reaction and mechanism of chloroacetanilide herbicides by L-cysteine |
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Abstract Dynamics, thermodynamic and mechanism of the reaction between chloroacetanilide herbicides and L-cysteine were investigated. L-cysteine could significantly promote the dissipation of four chloroacetanilide herbicides, following second-order kinetics. The degradation rates of the herbicides were linearly correlated with electrophilic index of chloroacetanilide herbicides, giving the order: alachlor (kL-cysteine=7.65×10-3mol/(L×s)) > acetochlor (kL-cysteine=7.23× 10-3mol/(L×s)) > butachlor (kL-cysteine=6.01×10-3mol/(L×s)) > S-metolachlor (kL-cysteine=2.15×10-3mol/(L×s)). This order was consistent with those of chloroacetanilide herbicide degradation in soil or by high efficient bacteria. Mass spectra identification of the products revealed that the chlorine was replaced by L-cysteine. Thus the dechlorination of these herbicides by L-cysteine followed bimolecular nucleophilic substitution reaction. Analysis of reaction thermodynamics indicated that the reaction was enthalpy-controlled and that differences in herbicide activity stemmed primarily from entropic factors(ΔS). There was good linear correlation between degradation rates of herbicides and respective ΔS values, suggesting that branched-chain structure and chain length of N-alkoxyalkyl had significant effect on reaction activity of these herbicides.
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Received: 24 February 2012
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