Abstract
The nucleophilic attack of hydrogen sulfide (HS−) on six different chloroacetanilide herbicides was evaluated theoretically using the dispersion-corrected hybrid functional wB97XD and the 6-311++G(2d,2p) Pople basis sets. The six evaluated substrates were propachlor (A), alachlor (B), metolachlor (C), tioacetanilide (D), β-anilide (E), and methylene (F). Three possible mechanisms were considered: (a) bimolecular nucleophilic substitution (SN2) reaction mechanism, (b) oxygen assistance, and (c) nitrogen assistance. Mechanisms based on O- and N-assistance were discarded due to a very high activation barrier in comparison with the corresponding SN2 mechanism, with the exception of compound F. The N-assistance mechanism for compound F had a free activation energy of 23.52 kcal/mol, which was close to the value for the corresponding SN2 mechanism (23.94 kcal/mol), as these two mechanisms could occur in parallel reactions with almost 50% of each one. In compounds A to D, an important electron-withdrawing effect of the C=O and C=S groups was seen, and consequently, the activation free energies in these SN2 reactions were smaller, with a value of approximately 18 kcal/mol. Instead, compounds E and F, which have a CH2 group in the β-position, presented a higher activation free energy (≈22 kcal/mol). Good agreement was found between experimental and theoretical values for all cases, and a reaction force analysis was performed on the intrinsic reaction coordinate profile in order to gain more details about the reaction mechanism. Finally, from the natural bond orbital (NBO) analysis, it was possible to evaluate the electronic reorganization through the reaction pathway where all the transition states were early in nature in the reaction coordinate (δBav < 50%); the transition states corresponding to compounds A to D turned out to be more synchronous than those for compounds E and F.
Highlights
Among the diversity of chemical compounds, pesticides constitute a heterogeneous category used for the control of pests, plant diseases, and to eradicate the unwanted weeds
The evolution on the electronic density through the reaction pathway plays an important role in the reaction mechanism, in order to gain information about the changes in the atom charges involved in the reaction mechanism for the different stationary points
Compounds A–D, which have the presence of a neighboring carbonyl group (C=O) and C=S, possess almost the same barrier (∆G‡ ≈ 19 kcal/mol)
Summary
Among the diversity of chemical compounds, pesticides constitute a heterogeneous category used for the control of pests, plant diseases, and to eradicate the unwanted weeds. The carcinogenic mechanism of chloroacetanilide compounds is still unclear, despite some experiments showing evidence that the carcinogenic properties relate to the herbicides’ ability to react with DNA through nucleophilic reactions [3,4] Another important characteristic of chloroacetanilide herbicides is their resistance to natural degradation in various environments. In all cases where the oxygen anchimeric assistance was considered, higher values on the reaction barriers were found; this possibility can be discarded for those particular substrates. For the case of compound E, a small positive entropy value (4.97) is reported in the experimental work, which suggests a unimolecular process involving an anchimeric assistance through the nitrogen atom, as depicted in Scheme 3, where a possible four-membered ring can be generated as an intermediate. In more detail, all the stationary points considered in this work, we describe the changes in geometric parameters involved in these reactions
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
