Theoretical Investigation Of The Formation Mechanisms Of Nitriles And (N-Chloro) Amides From Acetaldehyde And Its Substituted Aldehydes During Chloramination

As an alternative disinfectant to free chlorine,monochloramine can not only reduce the occurrence of waterborne infectious diseases through effectively killing pathogenic microorganisms in water,but also greatly reduce the formation of regulated disinfection by-products（DBPs）,therefore,it is favored by water utility.However,it has been found that monochloramine can lead to the formation of more toxic nitrogenous disinfection by-products（N-DBPs）.Aldehydes are the only precursors that do not contain nitrogen atom but can generate N-DBPs such as nitriles and amides under chloramination.Thus,this aldehyde pathway has attracted much attention,however,the current understanding of its mechanisms is still limited.Moreover,so far there have been research about formaldehyde,acetaldehyde and its chlorides,but less regarding other aldehydes.Quantum chemistry can simulate each step of the elementary reaction process from a microscopic perspective,and is widely used in mechanism research.Therefore,in this dissertation,based on the available experimental values of acetaldehyde and its chlorides,the formation mechanisms of nitriles,amides and N-chloro-amides from them under chloramination were studied by using the quantum chemical calculation method.On this basis,aldehyde precursors were extended,the effects of different substituents at theα-H position of acetaldehyde on the formation of the above three N-DBPs were systematically investigated.The detailed results are given as follows:1.Validation of computational methods for the reaction of acetaldehyde with monochloramineThe reaction of acetaldehyde with monochloramine was selected to validate computational methods.The M06-2X functional,two basis sets 6-311G（d,p）and 6-311+G（d,p）,and two implicit solvent models CPCM and SMD were used to optimize geometry,and the coupled cluster CCSD（T）method along with two basis sets def2-TZVP and cc-pVTZ was selected for single point calculations.The results indicate that the activation free energy（ΔG^≠）of acetaldehyde reacting with monochloramine calculated with single point calculations at the CCSD（T）/cc-pVTZ level based on the structures optimized with M06-2X/6-311G（d,p）along with the CPCM solvent model is15.2 kcal/mol with the estimated rate constants k_est of～48 M^-1 s^-1,which agrees well with the experimental rate constant k_exp of 24.3 M^-1 s^-1.Hence,CCSD（T）/cc-pVTZ//M06-2X/6-311G（d,p）combined with CPCM solvent model was selected to explore the subsequent part of the formation mechanism of nitriles and（N-chloro）amides from acetaldehyde and its chlorides.For the substituted acetaldehyde part,since some substituted groups are relatively large and there is generally the same trend forΔG^≠values calculated from the different method for the same reaction,only M06-2X/6-311G（d,p）method combined with CPCM solvent model was used and no single point calculations were further performed to simplify the calculation.2.Formation mechanisms of nitriles and（N-chloro）amides from acetaldehyde and its chloridesAcetaldehyde and its chlorides（mono-and dichloroacetaldehyde）were selected as precursors to systematically explore the formation mechanisms of nitriles,amides and N-chloro-amides under chloramination.The results indicate that the first step of all the above three N-DBPs formation is the nucleophilic addition reaction between acetaldehyde with monochloramine to generate N-chloro-ethanolamine withΔG^≠values of～10-15 kcal/mol.For nitriles formation,N-chloro-ethanolamine subsequently undergoes three processes including isomerization,dehydration,and HCl elimination withΔG^≠of～7-8,22-28 and 14-15 kcal/mol,respectively.It can be seen that the dehydration reaction is the rate-limiting step for the formation of nitriles,which is consistent with the experimental results.The amides are formed by N-chloro-ethanolamine through HCl elimination and hydrogen atom transfer reactions,in which HCl elimination is the rate-limiting step withΔG^≠of～21 kcal/mol.For N-chloro-amides,different from the experimentally reported concerted mechanism of the direct oxidation of N-chloro-ethanolamine by monochloramine,a stepwise mechanism was found to be a more likely pathway,in which N-chloro-ethanolamine undergoes HCl elimination and chlorination reaction withΔG^≠values of～21-22 and 18-23 kcal/mol,respectively.For acetaldehyde precursor,the chlorination reaction is the rate-limiting step,while for mono-and dichloroacetaldehyde precursors,the HCl elimination reaction is the rate-limiting step.It can be found that iminol is a key intermediate for amides and N-chloro-amides formation.In addition,by comparingΔG^≠of the rate-limiting step(ΔG_rls^≠)for the formation of the above N-DBPs,it can be easily found that theΔG_rls^≠ value of acetonitrile is lower than that of N-chloro-acetamide,while theΔG_rls^≠value of dichloroacetonitrile is higher than that of N,2,2-trichloroacetamide.This can explain the experimental phenomenon that the concentration of acetonitrile is greater than that of N-chloro-acetamide,while the concentration of dichloroacetonitrile is less than that of N,2,2-trichloroacetamide.3.Effects of different substituents at theα-H position of acetaldehyde on the formation of nitriles and（N-chloro）amidesThirteen kinds of substituted acetaldehyde,α-H position of acetaldehyde substituted by five kinds of electron-donating groups（EDGs）,six kinds of electron-withdrawing groups（EWGs）,and two kinds of conjugated groups（CGs）,were chosen to explore the substituent effects on the rate-limiting steps of above three N-DBPs formation,i.e.,dehydration,HCl elimination of N-chloro-ethanolamine and chlorination of iminol intermediate.The results indicate that theΔG^≠values of the dehydration reaction for N-chloro-ethanolamine substituted with EDGs are～19.8-20.9kcal/mol,lower than those of the unsubstituted values of 21 kcal/mol,indicating that the EDGs are more favourable for the nitriles formation.For the HCl elimination reaction,both EDGs and EWGs lead to theΔG^≠values equal to or higher than the unsubstituted values,indicating that they are not conducive to the amides formation.For the chlorination reaction of iminol intermediate,EWGs reduce theΔG^≠values,facilitating the N-chloro-amides formation.Moreover,theΔG^≠values of dehydration and chlorination are～19-20 kcal/mol for the substituted acetaldehyde by CGs,slightly lower than those of non-substituted values of 22 kcal/mol,indicating that CGs are favourable for the formation of both nitriles and N-chloro-amides.Based on the above results,it can be inferred that the acetaldehydes substituted by EDGs and CGs are more conducive to the formation of the corresponding nitriles,and the precursors of propionitrile,n-butanitrile,hydroxyacetonitrile,3-butenonitrile,and phenylacetonitrile,which were included in List of"Catalogue of Hazardous Chemicals"of 2015 edition,belong to the above aldehydes.Therefore,potential aldehyde precursors of the above similar highly toxic nitriles are particularly concerning.In this dissertation,the formation mechanisms of nitriles and（N-chloro）amides from aldehydes under chloramination were revealed,and the effects of different substituents at theα-H position of acetaldehyde on the formation of nitriles and（N-chloro）amides were discovered.The results are helpful for further understanding the aldehyde pathway mechanisms,and provide a theoretical guidance for identifying potential precursors of toxic products and developing water treatment processes to prevent and control the formation of toxic by-products.