Theoretical Study On The Molecular Mechanism Of Degradation And Transformation Of Hydroxyhalobenzoquinone By N-MeBHA And N-PhAHA Hydroxamic Acid Under Weak Alkaline Condition

Halogenated benzoquinones（HBQs）are a type of common persistent organic pollutants（POPs）with easy accumulation,high residue,and difficult degradation,which can cause a large number of dangerous effects on humans,including acute hepatotoxicity and carcinogenicity.Nowadays,as an emerging class of drinking water disinfection by-products,a variety of HBQs have been detected in drinking water.HBQs are easily hydrolysed under neutral or alkaline conditions to generate hydroxyl haloquinones（OH-HBQs）.Although the toxicity of the OH-HBQs has been somewhat reduced,it is still greater than that of conventional disinfection byproducts.In recent years,the control,reduction,and degradation of HBQs have received extensive attention,but little research has been carried out on their hydrolysis product OH-HBQs.A series of hydroxamic acids have been shown experimentally to promote the degradation of HBQs.Inspired by this,we systematically investigate the degradation and transformation reaction of OH-HBQs by N-methylphenylhydroxamic acid（N-MeBHA）and N-phenylmethylhydroxamic acid（N-PhAHA）at the molecular level using quantum chemistry calculations,so as to further enrich and develop the relevant experimental studies and improve the knowledge of the transformation and degradation of HBQs and their hydrolysis products by hydroxamic acids.The specific studies are as follows.（1）The reaction mechanism of N-MeBHA with the hydrolysis products of common HBQs was systematically investigated using density functional theory.The reaction pathways for the degradation and transformation of various types of OH-HBQs by N-MeBHA and the rules of free radical rearrangement reactions were elucidated.For comparison,the degradation and transformation of 2,5-dichloro-1,4-benzoquinone（2,5-DCBQ）by N-MeBHA,which has been studied extensively experimentally,was also systematically investigated.The feasibility of the proposed pathway was evaluated by calculating the Gibbs free energy barrier（ΔG*）,the Gibbs free energy change（ΔG）,the enthalpy change（ΔH）,and the bond dissociation enthalpy（BDE）.The influences of the types and positions of halogen atoms on the reaction mechanism were examined.The results show that N-MeBHA can be used to degrade OH-HBQs via substitution or Michael addition reactions,of which only 3-hydroxy-chloro-1,4-benzoquinone（3-OH-CBQ）is favourable for Michael addition reactions.The generated nucleophilic reaction intermediates are homolytically cleaved through the N-O bond to produce FRO·and·N（CH₃）-COAr radicals.In addition to being able to undergo hydrogen abstraction reactions with N-MeBHA,both radicals can also recombine to produce the C-N bonding products when the C atom adjacent to the nucleophilic site is attached to the H atom.For the different halogenated hydrolysis products,the reaction rules for the hydroxychlorobenzoquinone and the hydroxybromobenzoquinone are essentially the same.As a general rule,the nucleophilic reactivity of the ortho-,meta-and para-positions of the C atom at the hydroxyl group gradually increases.Furthermore,the competition between OH^–and N-MeBHA^–for the degradation and transformation reaction of OH-HBQs was examined.Competition was observed between them when the C atom at the para-position of the hydroxyl position was attached to an H atom.In contrast,there was no competition between them when the C atom was attached to a halogen atom.In addition,the nucleophilic reaction activity and the stability of the generated radicals were analysed in combination with the condensed Fukui function,the lowest unoccupied molecular orbital（LUMO）,and the spin density population analysis.The above studies provide an important theoretical basis for an in-depth investigation on the transformation and degradation of OH-HBQs in the presence of N-MeBHA-like hydroxamic acids.（2）Based on the above study,we further investigated the possible stepwise transformation pathways of various types of OH-HBQs in the presence of N-PHAHA systematically using density functional theory.As a result,except for 3-hydroxyl-2-chloro-1,4-benzoquinone（3-OH-CBQ）and 3-hydroxyl-2,5-dichloro-1,4-benzoquinone（3-OH-2,5-DCBQ）,which could generate the initial complexes via Michael addition reactions,the remaining OH-HBQs could form the initial complexes mainly via nucleophilic substitution reactions.In particular,we have further investigated the experimentally unspecified process of the quinone ring reduction reaction.It was found that the reaction proceeds starting from a Claisen-type rearrangement reaction intermediate through proton dissociation→dechlorination→OH^–attack→C-C bond break→protonation→CO₂ dissociation to form the final five-membered ring product.When different atoms or substituents are attached to the neighbouring C atoms at the nucleophilic site,different products are produced.When it is H atom,the C-C bonding product is formed mainly through Claisen-type rearrangements and keto-enol isomerization reactions.When the H atom is replaced by a Cl atom,a five-membered ring product is formed mainly through a Claisen-type rearrangement and a ring reduction reaction.When changed to hydroxyl,ring reduction reactions are almost impossible and it is difficult to form five-membered ring products.The reaction process for the degradation and transformation of 2,5-DCBQ by N-PhAHA was also investigated and compared with experiments.In addition,the competition between OH^–and N-PhAHA^–for the degradation conversion reaction of OH-HBQs was also investigated.In the case of 3-OH-CBQ,there was competition between them,whereas in the case of the other OH-HBQs,there was no competition between them.