Theoretical Investigation into Rate-Determining Factors in Electrophilic Aromatic Halogenation

Revisiting the chloramination of phenolic compounds: Formation of novel high-molecular-weight nitrogenous disinfection byproducts

Disinfection is critical for ensuring water safety; however, the potential risks posed by disinfection byproducts (DBPs) have raised public concern. Previous studies have largely focused on low-molecular-weight DBPs with one or two carbon atoms, leaving the formation of high-molecular-weight DBPs (HMW DBPs, with more than two carbon atoms) less understood. This study explores the formation of HMW DBPs during the chloramination of phenolic compounds using a novel approach that combines high-resolution mass spectrometry with density functional theory (DFT) calculations. For the first time, we identified nearly 100 previously unreported HMW nitrogenous DBPs (N-DBPs), with nearly half of those being halogenated N-DBPs. These N-DBPs were tentatively identified as heterocyclic (e.g., pyrrole and pyridine analogs) and coupling heterocyclic N-DBPs. Through detailed structure analysis and DFT calculations, the key formation steps of heterocyclic N-DBPs (monochloramine-mediated ring-opening reactions of halobenzoquinones) and new bonding mechanisms (C-N, C-O, and C-C bonding) of the coupling heterocyclic N-DBPs were elucidated. The selective formation of these novel N-DBPs was significantly influenced by factors such as contact time, monochloramine dosage, pH, and bromide concentration. Our findings emphasize the occurrence of diverse HMW heterocyclic N-DBPs, which are likely toxicologically significant, underscoring the need for further research to evaluate and mitigate their potential health risks in water disinfection.

Br-to-Cl Transformation Guided the Formation of Polyhalogenated Dibenzo-p-dioxins/Dibenzofurans

Polyhalogenated dibenzo-p-dioxins/dibenzofurans (PXDD/Fs) are commonly released into the environment as byproducts of combustion processes, accompanied by flue gases. Chlorinated (Cl) and brominated (Br) precursors play crucial roles in forming PXDD/Fs. However, the specific contributions of Cl-precursors and Br-precursors to PXDD/Fs formation have not been fully elucidated. Herein, we demonstrate that the formation of Br-precursors can increase the fraction of polychlorinated dibenzo-p-dioxins/dibenzofurans (PCDD/Fs) congeners substituted at specific positions, such as 1,2,3,4,6,7,8-HpCDD, OCDD, 2,3,4,7,8-PeCDF, and 2,3,4,6,7,8-HxCDF. This is attributed to the electrophilic chlorination reaction of the Br-precursors, which includes the Br-to-Cl transformation pathway, following the principle of regioselectivity. The observed formation of polybrominated/chlorinated dibenzo-p-dioxins/benzofurans (PBCDD/Fs) from 1,2-dibromobenzene (1,2-DiBBz) as a Br precursor provides direct evidence supporting the proposed Br-to-Cl transformation. Quantum chemical calculations are employed to discuss the principle of regioselectivity in the Br-to-Cl transformation, clarifying the priority of the position for electrophilic chlorination. Additionally, the concentration of PCDD/Fs formed from 1,2-DiBBz is 1.6 μg/kg, comparable to that of polybrominated dibenzo-p-dioxins/dibenzofurans (PBDD/Fs) (2.4 μg/kg), highlighting the potential of brominated organic pollutants as precursors for PCDD/Fs formation. This study provides three potential pathways for PCDD/Fs formation from Br-precursors, establishing a theoretical foundation for elucidating the formation mechanism of PXDD/Fs in the coexistence of Cl and Br.

Halogenation of the 3-position of pyridines through Zincke imine intermediates

Pyridine halogenation reactions are crucial for obtaining the vast array of derivatives required for drug and agrochemical development. However, despite more than a century of synthetic endeavors, halogenation processes that selectively functionalize the carbon-hydrogen bond in the 3-position of a broad range of pyridine precursors remain largely elusive. We report a reaction sequence of pyridyl ring opening, halogenation, and ring closing whereby the acyclic Zincke imine intermediates undergo highly regioselective halogenation reactions under mild conditions. Experimental and computational mechanistic studies indicate that the nature of the halogen electrophile can modify the selectivity-determining step. Using this method, we produced a diverse set of 3-halopyridines and demonstrated late-stage halogenation of complex pharmaceuticals and agrochemicals.

Mini-Review on Structure-Reactivity Relationship for Aromatic Molecules: Recent Advances

Recent advances in quantifying nucleophilic reactivities in chemical reactions and intermolecular interactions of aromatic molecules are reviewed. This survey covers experimental (IR frequency shifts induced by hydrogen bonding) and theoretical (modeling of potential energy surfaces, atomic charges, molecular electrostatic potential) approaches in characterizing chemical reactivity. Recent advances in software developments assisting the evaluation of the reactive sites for electrophilic aromatic substitution are briefly discussed.

Formation Mechanism of Iodinated Aromatic Disinfection Byproducts: Acid Catalysis with H2OI. ENVIRONMENTAL SCIENCE & TECHNOLOGY 2022;56:1791-1800. [PMID: 35061374 DOI: 10.1021/acs.est.1c05484]

Iodinated aromatic disinfection byproducts (I-DBPs) are a group of nonregulated but highly toxic DBPs. The formation of I-DBPs is attributed mainly to HOI because it is the most abundant reactive iodine species in chloraminated water. In this study, we used computational modeling of thermodynamics to examine the mechanism of iodination of aromatic contaminants, e.g., dipeptides and phenols. Computational prediction of the energy barriers of the formation of iodinated tyrosylglycine (I-Tyr-Gly) (66.9 kcal mol^-1) and hydroxylated Tyr-Gly (OH-Tyr-Gly) (46.0 kcal mol^-1) via iodination with HOI favors the formation of OH-Tyr-Gly over I-Tyr-Gly. Unexpectedly, mass spectrometry experiments detected I-Tyr-Gly but not OH-Tyr-Gly, suggesting that I-Tyr-Gly formation cannot be attributed to HOI alone. To clarify this result, we examined the thermodynamic role of the most reactive iodine species H₂OI⁺ in the formation of aromatic I-DBPs under chloramination. Computational modeling of thermodynamic results shows that the formation of a loosely bonded complex of aromatic compounds with H₂OI⁺ is the key step to initiate the iodination process. When H₂OI⁺ serves as an acid catalyst and an iodinating agent, with HOI or H₂O acting as a proton acceptor, the energy barrier of I-DBP formation was significantly lower (10.8-13.1 kcal mol^-1). Therefore, even with its low concentration, H₂OI⁺ can be involved in the formation of I-DBPs. These results provide insight into the mechanisms of aromatic I-DBP formation and important information for guiding research toward controlling I-DBPs in drinking water.

Cluster halogenation of adamantane and its derivatives with bromine and iodine monochloride

Noncatalytic halogenation of adamantane with Br2 or ICl proceeds according to the cluster mechanism featuring high kinetic order in halogen and a sharp decrease in DFT energy barrier when additional halogen molecules are involved in the system.

Structural effects on the bromination rate and selectivity of alkylbenzenes and alkoxybenzenes in aqueous solution

Aqueous free bromine species (e.g., HOBr, BrCl, Br2, BrOCl, Br2O, and H2OBr+) can react with activated aromatic compounds via electrophilic aromatic substitution to generate products with industrial applications, environmental consequences, and potentially adverse biological effects. The relative contributions of these brominating agents to overall bromination rates can be calculated via nonlinear regression analyses of kinetic data collected under a variety of solution conditions, including variations in parameters (e.g., [Cl-], [Br-], and pH) known to influence free bromine speciation. Herein, kinetic experiments conducted in batch reactors were employed to evaluate the contributions of steric and electronic effects on bromination of monosubstituted alkylbenzenes (ethyl, isopropyl, tert-butyl) and alkoxybenzenes (ethoxy, isopropoxy, tert-butoxy) and to elucidate the inherent reactivities of aqueous brominating agents towards these aromatic compounds. For bromination at the para position of alkylbenzenes, overall reactivity increased from tert-butyl < ethyl ≈ isopropyl. For bromination at the para position of alkoxybenzenes, reactivity increased from tert-butoxy < ethoxy < isopropoxy. In going from ethyl to tert-butyl and ethoxy to isopropoxy, unfavorable steric effects attenuated the favorable electronic effects imparted by the substituents. When comparing unsubstituted benzene, alkyl-, and alkoxybenzenes, the structure of the substituent has a significant effect on bromination rates, nucleophile regioselectivity, and electrophile chemoselectivity. Hirshfeld charges were useful predictors of reactivity and regioselectivity. The experimental results were also modeled using Taft equations. Collectively, these findings indicate that steric effects, electronic effects, and brominating agents other than HOBr can influence aromatic compound bromination in solutions of free bromine.

An analysis of electrophilic aromatic substitution: a "complex approach"

Electrophilic aromatic substitution (EAS) is one of the most widely researched transforms in synthetic organic chemistry. Numerous studies have been carried out to provide an understanding of the nature of its reactivity pattern. There is now a need for a concise and general, but detailed and up-to-date, overview. The basic principles behind EAS are essential to our understanding of what the mechanisms underlying EAS are. To date, textbook overviews of EAS have provided little information about the mechanistic pathways and chemical species involved. In this review, the aim is to gather and present the up-to-date information relating to reactivity in EAS, with the implication that some of the key concepts will be discussed in a scientifically concise manner. In addition, the information presented herein suggests certain new possibilities to advance EAS theory, with particular emphasis on the role of modern instrumental and theoretical techniques in EAS reactivity monitoring.

Iodination of Dimethenamid in Chloraminated Water: Active Iodinating Agents and Distinctions between Chlorination, Bromination, and Iodination

Few studies have elucidated the agent(s) that generate iodinated disinfection byproducts during drinking water treatment. We present a kinetic investigation of iodination of dimethenamid (DM), a model compound lacking acid-base speciation. Water chemistry parameters (pH, [Cl^-], [Br^-], [I^-], and [pH buffer]) were systematically varied. As pH increased (4-9), DM iodination rate decreased. Conventional wisdom considers hypoiodous acid (HOI) as the predominant iodinating agent; nevertheless, HOI (pK_HOI = 10.4) could not have produced this result, as its concentration is essentially invariant from pH 4-9. In contrast, [H₂OI⁺] and [ICl] both decrease as pH increases. To distinguish their contributions to DM iodination, [Cl^-] was added at constant pH and ionic strength. Although chloride addition did increase the iodination rate, the reaction order in [Cl^-] was fractional (≤0.36). The contribution of ICl to DM iodination remained below 47% under typical drinking water conditions ([Cl^-] ≤ 250 mg/L), implicating H₂OI⁺ as the predominant iodinating agent. Distinctions between DM iodination versus chlorination or bromination include a more pronounced role for the hypohalous acidium ion (H₂OX⁺), negligible contributions by hypohalous acid and molecular halogen (X₂), and a more muted influence of XCl, leading to lesser susceptibility to catalysis by chloride.

Electrophilic Chlorination of Naphthalene in Combustion Flue Gas

Naphthalene chlorination is an important formation mechanism of polychlorinated naphthalenes (PCNs) in combustion flue gas. In this study, a total of 21 metal chlorides and oxides were screened for their activities in the electrophilic chlorination of naphthalene. Copper(II) chloride exhibited the highest activity at 200-350 °C, followed by copper(I) chloride. Copper(II) chloride primarily acted as a strong chlorinating agent to facilitate chlorine substitution on naphthalene. Iron (II and III) chlorides were only highly active at 200-250 °C. At 250 °C, the average naphthalene chlorination efficiency over CuCl₂·2H₂O was 7.5-fold, 30.2-fold and 34.7-fold higher than those over CuCl, FeCl₃·6H₂O and FeCl₂·4H₂O, respectively. The other metal chlorides were less active. Under heated conditions, copper(II) and iron(III) chlorides were transformed to copper(I) and iron(II) chlorides via dechlorination, and then transformed to oxychlorides and oxides, thereby forming dechlorination-oxychlorination cycles of copper and iron species, respectively. The results obtained suggest that electrophilic chlorination of naphthalene in combustion flue gas is primarily driven by dechlorination-oxychlorination cycles of copper and iron species, and the reaction produces a selective chlorination pattern at 1 and 4 positions of naphthalene.

Elevated reaction order of 1,3,5-tri-tert-butylbenzene bromination as evidence of a clustered polybromide transition state: a combined kinetic and computational study

The kinetics and mechanism of concurrent bromo-de-protonation and bromo-de-tert-butylation of 1,3,5-tri-tert-butylbenzene at different bromine concentrations were studied experimentally and theoretically.

Transformation of naproxen during the chlorination process: Products identification and quantum chemistry validation

The by-products produced by pharmaceutically active compounds (PhACs) during chlorination are attracting wide concern. Thus, the transformation and toxicity of naproxen (NAP) during the chlorination process were assessed in this study. The transformation of NAP was found to follow pseudo-first-order kinetics, and the first-order rate constant was improved by increasing the NaOCl dose. High-resolution mass spectrometry (HRMS) was successfully applied to identify 14 chlorination products. This study represents the first elucidation and report of the exact structure of the primary chlorine substitution product ((2S)-2-(5-chloro-6-methoxy-2-naphthyl)propionic acid) based on HRMS and ¹H NMR. Chlorine will primarily substitute the hydrogen atom on the C7 position of the naphthalene ring to form the mono-chlorine substitution product, as further validated at the theoretical level by quantum chemical calculations. A series of HOCl-induced reactions, including substitution, demethylation, and dehydrogenation, led to the transformation of NAP during the chlorination process. ECOSAR program revealed that the potential aquatic toxicity of the transformation products is significantly higher than that of the parent NAP. Their introduction into the environment may still pose potential risks.

A Predictive Tool for Electrophilic Aromatic Substitutions Using Machine Learning

At the early stages of the drug development process, thousands of compounds are synthesized in order to attain the best possible potency and pharmacokinetic properties. Once successful scaffolds are identified, large libraries of analogues are made, which is a challenging and time-consuming task. Recently, late stage functionalization (LSF) has become increasingly prominent since these reactions selectively functionalize C-H bonds, allowing to quickly produce analogues. Classical electrophilic aromatic halogenations are a powerful type of reaction in the LSF toolkit. However, the introduction of an electrophile in a regioselective manner on a drug-like molecule is a challenging task. Herein we present a machine learning model able to predict the reactive site of an electrophilic aromatic substitution with an accuracy of 93% (internal validation set). The model takes as input a SMILES of a compound and uses six quantum mechanics descriptors to identify its reactive site(s). On an external validation set, 90% of all molecules were correctly predicted.