Hydrogen‐Abstraction/Acetylene‐Addition Exposed

Polycyclic aromatic hydrocarbons (PAHs) are omnipresent in the interstellar medium (ISM) and also in carbonaceous meteorites (CM) such as Murchison. However, the basic reaction routes leading to the formation of even the simplest PAH—naphthalene (C₁₀H₈)—via the hydrogen‐abstraction/acetylene‐addition (HACA) mechanism still remain ambiguous. Here, by revealing the uncharted fundamental chemistry of the styrenyl (C₈H₇) and the ortho‐vinylphenyl radicals (C₈H₇)—key transient species of the HACA mechanism—with acetylene (C₂H₂), we provide the first solid experimental evidence on the facile formation of naphthalene in a simulated combustion environment validating the previously postulated HACA mechanism for these two radicals. This study highlights, at the molecular level spanning combustion and astrochemistry, the importance of the HACA mechanism to the formation of the prototype PAH naphthalene.     

Unexpected Chemistry from the Reaction of Naphthyl and Acetylene at Combustion‐Like Temperatures

The hydrogen abstraction/acetylene addition (HACA) mechanism has long been viewed as a key route to aromatic ring growth of polycyclic aromatic hydrocarbons (PAHs) in combustion systems. However, doubt has been drawn on the ubiquity of the mechanism by recent electronic structure calculations which predict that the HACA mechanism starting from the naphthyl radical preferentially forms acenaphthylene, thereby blocking cyclization to a third six‐membered ring. Here, by probing the products formed in the reaction of 1‐ and 2‐naphthyl radicals in excess acetylene under combustion‐like conditions with the help of photoionization mass spectrometry, we provide experimental evidence that this reaction produces 1‐ and 2‐ethynylnaphthalenes (C₁₂H₈), acenaphthylene (C₁₂H₈) and diethynylnaphthalenes (C₁₄H₈). Importantly, neither phenanthrene nor anthracene (C₁₄H₁₀) was found, which indicates that the HACA mechanism does not lead to cyclization of the third aromatic ring as expected but rather undergoes ethynyl substitution reactions instead.     

Dynamics simulation and reaction pathway analysis of characteristics of soot particles in ethylene oxidation at high temperature

Soot particles characteristics were investigated numerically for high temperature oxidation of C₂H₄/O₂/N₂ (C/O ratio of 2.2) in a closed jet-stirred/plug-flow reactor (JSR/PFR) system. Based on the growth mechanism of polycyclic aromatic hydrocarbons (PAHs), two mechanisms were used to explore the formation pathways of soot precursors and soot. Numerical results were compared with the experimental and reference data. The simulation results show that the value predicted for small molecule intermediates within A1 gives a strong regularity, consistent trend with reference data. However, with the hydrogen-abstraction-carbon-addition (HACA) growth mechanism, the predicted value for beyond-A1 PAH macromolecules and soot volume fraction are smaller than the experimental data. The results also show that the predicted soot volume fraction is in good agreement with experimental data when a combination of the HACA and PAHs condensation (HACA + PAH-PAH) growth mechanisms is used. Analyses of the A1 sensitivity and reaction pathway elucidated that A1 are mainly formed from C₂H₃, C₂H₂, C₃H₃, C₆H₅OH, A1C₂H and A1-. The reaction 2C₃H₃ → A1 is the dominant route of benzene formation. The prediction results and an analysis of the A3 reaction pathway indicate that the growth process from benzene to larger aromatic hydrocarbons (beyond two-ring polycyclic aromatic hydrocarbons [PAHs]) goes by two pathways, i.e., HACA combined with the PAH-PAH radical recombination and addition reaction growth mechanisms.     

Selective Formation of Indene through the Reaction of Benzyl Radicals with Acetylene

The combustion of fossil fuels forms polycyclic aromatic hydrocarbons (PAHs) composed of five‐ and six‐ membered aromatic rings, such as indene (C₉H₈), which are carcinogenic, mutagenic, and deleterious to the environment. Indene, the simplest PAH with single five‐ and six‐membered rings, has been predicted theoretically to be formed through the reaction of benzyl radicals with acetylene. Benzyl radicals are found in significant concentrations in combustion flames, owing to their highly stable aromatic and resonantly stabilized free‐radical character. We provide compelling experimental evidence that indene is synthesized through the reaction of the benzyl radical (C₇H₇) with acetylene (C₂H₂) under combustion‐like conditions at 600 K. The mechanism involves an initial addition step followed by cyclization and aromatization through atomic hydrogen loss. This reaction was found to form the indene isomer exclusively, which, in conjunction with the high concentrations of benzyl and acetylene in combustion environments, indicates that this pathway is the predominant route to synthesize the prototypical five‐ and six‐membered PAH.     

Three‐dimensional hydrogen‐bonded framework in bis(melamin‐1‐ium) naphthalene‐1,5‐disulfonate melamine pentahydrate

Crystals of the title compound, 2C₃H₇N₆⁺·C₁₀H₆O₆S₂²⁻·C₃H₆N₆·5H₂O, are built up of neutral 2,4,6‐triamino‐1,3,5‐triazine (melamine), singly protonated melaminium cations, naphthalene‐1,5‐disulfonate dianions and water molecules. Two independent anions lie across centres of inversion in the space group P. The melamine molecules are connected by N—H...N hydrogen bonds into two different one‐dimensional polymers almost parallel to the (010) plane, forming a stacking structure along the b axis. The centrosymmetric naphthalene‐1,5‐disulfonate anions interact with water molecules via O—H...O hydrogen bonds, forming layers parallel to the (001) plane. The cations and anions are connected by N—H...O and O—H...N hydrogen bonds to form a three‐dimensional supramolecular framework.     

Biphenyl‐ and phenylnaphthalenyl‐substituted 1H‐imidazole‐4,5‐dicarbonitrile catalysts for the coupling reaction of nucleoside methyl phosphonamidites

Crystal structures are reported for three substituted 1H‐imidazole‐4,5‐dicarbonitrile compounds used as catalysts for the coupling reaction of nucleoside methyl phosphonamidites, namely 2‐(3′,5′‐dimethylbiphenyl‐2‐yl)‐1H‐imidazole‐4,5‐dicarbonitrile, C₁₉H₁₄N₄, (I), 2‐(2′,4′,6′‐trimethylbiphenyl‐2‐yl)‐1H‐imidazole‐4,5‐dicarbonitrile, C₂₀H₁₆N₄, (II), and 2‐[8‐(3,5‐dimethylphenyl)naphthalen‐1‐yl]‐1H‐imidazole‐4,5‐dicarbonitrile, C₂₃H₁₆N₄, (III). The asymmetric unit of (I) contains two independent molecules with similar conformations. There is steric repulsion between the imidazole group and the terminal phenyl group in all three compounds, resulting in the nonplanarity of the molecules. The naphthalene group of (III) shows significant deviation from planarity. The C—N bond lengths in the imidazole rings range from 1.325 (2) to 1.377 (2) Å. The molecules are connected into zigzag chains by intermolecular N—H...N_imidazole [for (I)] or N—H...·N_cyano [for (II) and (III)] hydrogen bonds.     

Theoretical study on the HACA chemistry of naphthalenyl radicals and acetylene: The formation of C12H8, C14H8, and C14H10 species

The hydrogen-abstraction-C₂H₂-addition (HACA) chemistry of naphthalenyl radicals has been studied extensively, but there is a significant discrepancy in product distributions reported or predicted in literature regarding appearance of C₁₄H₈ and C₁₄H₁₀ species. Starting from ab initio calculations, a comprehensive theoretical model describing the HACA chemistry of both 1- and 2-naphthalenyl radicals is generated. Pressure-dependent kinetics are considered in the C₁₂H₉, C₁₄H₉, and C₁₄H₁₁ potential energy surfaces including formally direct well-skipping pathways. On the C₁₂H₉ PES, reaction pathways were found connecting two entry points: 1-naphthalenyl (1-C₁₀H₇) + acetylene (C₂H₂) and 2-C₁₀H₇ + C₂H₂. A significant amount of acenaphthylene is predicted to be formed from 2-C₁₀H₇ + C₂H₂, and the appearance of C₁₄H₈ isomers is predicted in the model simulation, consistent with high-temperature experimental results from Parker et al. At 1500 K, 1-C₁₀H₇ + C₂H₂ mostly generates acenaphthylene through a formally direct pathway, which predicted selectivity of 66% at 30 Torr and 56% at 300 Torr. The reaction of 2-C₁₀H₇ with C₂H₂ at 1500 K yields 2-ethynylnaphthalene as the most dominant product, followed by acenaphthylene mainly generated via isomerization of 2-C₁₀H₇ to 1-C₁₀H₇. Both the 1-C₁₀H₇ and 2-C₁₀H₇ reactions with C₂H₂ form some C₁₄H₈ products, but negligible phenanthrene and anthracene formation is predicted at 1500 K. A rate-of-production analysis reveals that C₁₄H₈ formation is strongly affected by the rates of H-abstraction from acenaphthylene, 1-ethynylnaphthalene, and 2-ethynylnaphthalene, so the kinetics of these reactions are accurately calculated at the high level G3(MP2,CC)//B3LYP/6-311G^** level of theory. At intermediate temperatures like 800 K, acenaphthylene + H are the leading bimolecular products of 1-C₁₀H₇ + C₂H₂, and 1-acenaphthenyl radical is the most abundant C₁₂H₉ isomer due to its stability. The predicted product distribution of 2-C₁₀H₇ + C₂H₂ at 800 K, in contrast to the results of Parker et al is predicted to consist primarily of species containing three fused benzene rings—for example, phenanthrene and anthracene—as the leading products, indicating HACA chemistry is valid from two to three ring polycyclic aromatic hydrocarbons under some conditions. Further experiments are needed for validation.     

2‐(2‐Naphthyl­oxy)­acetate derivatives. II. A new class of antiamnesic agents

The title compounds, 4‐(2‐naphthyl­oxy­methyl­carbonyl)­morpholine, C₁₆H₁₇NO₃, (I), and 4‐methyl‐1‐(2‐naphthyl­oxy­methyl­carbonyl)­piper­azine, C₁₇H₂₀N₂O₂, (II), are potential antiamnesics. The morpholine ring in (I) and the piperazine ring in (II) adopt chair conformations. In (I), the mol­ecules are linked by weak intermolecular C—H⃛O interactions into chains that have a graph‐set motif of C(10), while in (II), the mol­ecules are linked by weak intermolecular C—H⃛O interactions that generate two C(7) graph‐set motifs. The dihedral angle between the naphthalene moiety and the best plane through the morpholine ring is 20.62 (4)° in (I), while the naphthalene moiety is oriented nearly perpendicular to the mean plane of the piperazine ring in (II).     

Dibromomethyl- and bromomethyl- or bromo-substituted benzenes and naphthalenes: C—Br…Br interactions

The structures of six benzene and three naphthalene derivatives involving bromo, bromomethyl and dibromomethyl substituents, namely, 1,3-dibromo-5-(dibromomethyl)benzene, C₇H₄Br₄, 1,4-dibromo-2,5-bis(bromomethyl)benzene, C₈H₄Br₆, 1,4-dibromo-2-(dibromomethyl)benzene, C₇H₄Br₄, 1,2-bis(dibromomethyl)benzene, C₈H₆Br₄, 1-(bromomethyl)-2-(dibromomethyl)benzene, C₈H₇Br₃, 2-(bromomethyl)-3-(dibromomethyl)naphthalene, C₁₂H₉Br₃, 2,3-bis(dibromomethyl)naphthalene, C₁₂H₈Br₄, 1-(bromomethyl)-2-(dibromomethyl)naphthalene, C₁₂H₉Br₃, and 1,3-bis(dibromomethyl)benzene, C₈H₆Br₄, are presented. The packing patterns of these compounds are dominated by Br…Br contacts and C—H…Br hydrogen bonds. The Br…Br contacts, shorter than twice the van der Waals radius of bromine (3.7 Å), seem to play a crucial role in the crystal packing of all these compounds. The occurrence of Type I and Type II interactions is also discussed briefly, considering the effective atomic radius of bromine, as is their impact on the packing of molecules in the individual structures.     

Quantitative tests of mixed crystal excition theory. I. Naphthalene monomer1B2u and 3B1u spectra

A quantitative, computer processed spectroscopic study, using photon counting, on the first excited triplet and singlet states of dilute isotopic mixed crystals of naphthalene at 2 K is presented for C₁₀H₈; 1-DC₁₀H₇; 2-DC₁₀H₇; 1,4-D₂C₁₀H₆; 1,4,5-D₃C₁₀H₅; 1,4,5,8-D₄C₁₀H₄; 1,2,4,5,8-D₅C₁₀H₃; a β-D₄C₁₀H₄ and a β₂-D₆C₁₀H₂ as guests in C₁₀D₈ host crystals (and, for comparison, also for the same guests in a durene host crystal). The guest—host relative polarization Rashba formula has been verified quantitatively, and, as an added bonus, the elusive polarization ratio of the pure naphthalene crystal singlet Davydov components has been found to be 80 ± 20 (b/a), which is in poor agreement with the transition octupole—transition octupole model. The experimental guest energies and their concomitant quasiresonance shifts for bound singlet states (as well as the occurrences of unbound states) are in excellent quantitative agreement (about 1 cm^?1) with those calculated using a Green's function formalism based on the ideal mixed crystal approximation and on a restricted Frenkel type dispersion relation derived from resonance pairs. The same Green's function also accounts quantitatively (within 10%) for the guest singlet state exciton localizations (guest excitation amplitudes). The triplet exciton state reveals an orientational site splitting (about 0.7 cm^?1) for the 0—0 transition of the I-DC₁₀H₇ guest in C₁₀D₈ host. The order of the α and β substituted deuteronaphthalenes in the triplet state is reversed from that of the singlet state. The last two observations are related to the different nature of the lowest Π-Π^* singlet and triplet states of naphthalene.     

Weak C—H...O hydrogen bonds in anisaldehyde,salicylaldehyde and cinnamaldehyde

In situ cryocrystallization has been employed to grow single crystals of 4‐methoxybenzaldehyde (anisaldehyde), C₈H₈O₂, 2‐hydroxybenzaldehyde (salicylaldehyde), C₇H₆O₂, and (2E)‐3‐phenylprop‐2‐enal (cinnamaldehyde), C₉H₈O, all of which are liquids at room temperature. Several weak C—H...O interactions of the types C_aryl—H...O, C_formyl—H...O and Csp³—H...O are present in these related crystal structures.     

Binuclear Aromatic C−H Bond Activation at a Dirhenium Site

The electronically unsaturated dirhenium complex [Re₂(CO)₈(μ‐H)(μ‐Ph)] ( 1 ) has been found to exhibit aromatic C?H activation upon reaction with N,N‐diethylaniline, naphthalene, and even [D₆]benzene to yield the compounds [Re₂(CO)₈(μ‐H)(μ‐η¹‐NEt₂C₆H₄)] ( 2 ), [Re₂(CO)₈(μ‐H)(μ‐η²‐1,2‐C₁₀H₇)] ( 3 ), and [D₆]‐ 1 , respectively, in good yields. The mechanism has been elucidated by using DFT computational analyses, and involves a binuclear C?H bond‐activation process.     

Gas‐Phase Synthesis of the Benzyl Radical (C6H5CH2)

Dicarbon (C₂), the simplest bare carbon molecule, is ubiquitous in the interstellar medium and in combustion flames. A gas‐phase synthesis is presented of the benzyl radical (C₆H₅CH₂) by the crossed molecular beam reaction of dicarbon, C₂(X¹Σ_g⁺, a³Π_u), with 2‐methyl‐1,3‐butadiene (isoprene; C₅H₈; X¹A′) accessing the triplet and singlet C₇H₈ potential energy surfaces (PESs) under single collision conditions. The experimental data combined with ab initio and statistical calculations reveal the underlying reaction mechanism and chemical dynamics. On the singlet and triplet surfaces, the reactions involve indirect scattering dynamics and are initiated by the barrierless addition of dicarbon to the carbon–carbon double bond of the 2‐methyl‐1,3‐butadiene molecule. These initial addition complexes rearrange via multiple isomerization steps, leading eventually to the formation of C₇H₇ radical species through atomic hydrogen elimination. The benzyl radical (C₆H₅CH₂), the thermodynamically most stable C₇H₇ isomer, is determined as the major product.     

The 1:1 proton‐transfer compounds of 4‐(phenyldiazenyl)aniline (aniline yellow) with 3‐nitrophthalic, 4‐nitrophthalic and 5‐nitroisophthalic acids

The structures of the anhydrous 1:1 proton‐transfer compounds of the dye precursor aniline yellow [4‐(phenyldiazenyl)aniline], namely isomeric 4‐(phenyldiazenyl)anilinium 2‐carboxy‐6‐nitrobenzoate, C₁₂H₁₂N₃⁺·C₈H₄NO₆⁻, (I), and 4‐(phenyldiazenyl)anilinium 2‐carboxy‐4‐nitrobenzoate, C₁₂H₁₂N₃⁺·C₈H₄NO₆⁻, (II), and 4‐(phenyldiazenyl)anilinium 3‐carboxy‐5‐nitrobenzoate monohydrate, C₁₂H₁₂N₃⁺·C₈H₄NO₆⁻·H₂O, (III), have been determined at 130 K. In (I) the cation has longitudinal rotational disorder. All three compounds have substructures comprising backbones formed through strong head‐to‐tail carboxyl–carboxylate hydrogen‐bond interactions [graph set C(7) in (I) and (II), and C(8) in (III)]. Two‐dimensional sheet structures are formed in all three compounds by the incorporation of the 4‐(phenyldiazenyl)anilinium cations into the substructures, including, in the cases of (I) and (II), infinite H—N—H to carboxylate O—C—O group interactions [graph set C(6)], and in the case of (III), bridging through the water molecule of solvation. The peripheral alternating aromatic ring residues of both cations and anions give only weakly π‐interactive step features which lie between the sheets.     

A twofold interpenetrating three‐dimensional zinc–organic framework built from naphthalene‐1,4‐dicarboxylate and 4,4′‐bipyridine ligands

A novel three‐dimensional Zn^II complex, poly[[(μ₂‐4,4′‐bipyridine)(μ₄‐naphthalene‐1,4‐dicarboxylato)(μ₂‐naphthalene‐1,4‐dicarboxylato)dizinc(II)] dimethylformamide monosolvate monohydrate], {[Zn₂(C₁₂H₆O₄)₂(C₁₀H₈N₂)]·2C₃H₇NO·H₂O)}_n, has been prepared by the solvothermal assembly of Zn(NO₃)·6H₂O, naphthalene‐1,4‐dicarboxylic acid and 4,4′‐bipyridine. The two crystallographically independent Zn atoms adopt the same four‐coordinated tetrahedral geometry (ZnO₃N) by bonding to three O atoms from three different naphthalene‐1,4‐dicarboxylate (1,4‐ndc) ligands and one N atom from a 4,4′‐bipyridine (bpy) ligand. The supramolecular secondary building unit (SBU) is a distorted paddle‐wheel‐like {Zn₂(COO)₂N₂O₂} unit and these units are linked by 1,4‐ndc ligands within the layer to form a two‐dimensional net parallel to the ab plane, which is further connected by bpy ligands to form the three‐dimensional framework. The single net leaves voids that are filled by mutual interpenetration of an independent equivalent framework in a twofold interpenetrating architecture. The title compound is stable up to 673 K. Excitation and luminescence data observed at room temperature show that it emits bright‐blue fluorescence.     

Structural characterization and photochromic behaviour of a novel compound based on a π‐acidic naphthalene diimide derivative and a double hydroxide‐bridged dinuclear AlIII aqua ion cluster

Noncovalent interactions, such as π–π stacking interactions, C—H…π interactions and hydrogen bonding, are important driving forces for self‐assembly in the construction of functional supermolecules and materials, especially in multicomponent supramolecular systems. Herein, a novel compound based on a π‐acidic naphthalene diimide derivative and a double hydroxide‐bridged dinuclear Al³⁺ aqua ion cluster, namely bis[N,N′‐bis(2‐sulfonatoethyl)‐1,4,5,8‐naphthalene diimide] di‐μ‐hydroxido‐bis[tetraaquaaluminium(III)] tetrahydrate, (C₁₈H₁₂N₂O₁₀S₂)₂[Al₂(OH)₂(H₂O)₈]·4H₂O, was obtained using the above‐mentioned common noncovalent interactions, as well as uncommon lone‐pair–π interactions. Functional molecular modules were connected by these noncovalent interactions to generate obvious photochromic properties. The compound was prepared by the self‐assembly of N,N′‐bis(2‐sulfoethyl)‐1,4,5,8‐naphthalene diimide and Al(NO₃)₃·9H₂O under mixed solvothermal conditions, and was characterized in detail by single‐crystal X‐ray diffraction, powder X‐ray diffraction and FT–IR spectroscopy. The thermal stability and photochromic properties were also investigated; furthermore, in‐situ solid‐state UV–Vis absorption spectroscopy and electron spin resonance (ESR) were used to clarify the photochromic mechanism.     

(1‐{[2‐(6‐Methoxynaphthalen‐1‐yl)ethyl]amino}ethylidene)oxidanium bromide monohydrate

The title salt, C₁₅H₁₈NO₂⁺·Br⁻·H₂O, is an analogue of the antidepressant drug agomelatine. The cation is protonated at the carbonyl O atom of its amide group. The side chain at the 1‐position adopts an extended conformation, with all non‐H atoms lying in the same plane as the naphthalene ring. This is in contrast with the crystal structures known for three agomelatine polymorphs, and also with two known cocrystals containing agomelatine. The structure displays three types of hydrogen bond, namely C=O—H...O, N—H...Br and O—H...Br, which define a two‐dimensional network parallel to the (100) plane. The naphthalene rings interdigitate in a `zipper‐like' fashion between these hydrogen‐bonded networks, forming an offset arrangement. Direct face‐to‐face π–π contacts between naphthalene rings are not present in the structure.     

Comparative structural analysis of 2,7‐diethoxy‐1,8‐bis(4‐phenoxybenzoyl)naphthalene and its homologues: orientation of the 4‐phenoxybenzoyl groups at the 1‐ and 8‐positions of the naphthalene ring

The title molecule, C₄₀H₃₂O₆, possesses crystallographically imposed twofold symmetry, with the central two C atoms of the naphthalene unit sited on the rotation axis. The two 4‐phenoxybenzoyl groups in the molecule are twisted away from the attached naphthalene unit, with a torsion angle of 66.76 (15)° between the naphthalene unit and the carbonyl group (C—C—C=O), and are oriented in mutually opposing directions (anti orientation). There is an apparent difference in the conformations of the 4‐phenoxybenzoyl groups at the 1‐ and 8‐positions of the naphthalene ring between the title molecule and its methoxy‐bearing homologue [Hijikata et al. (2010). Acta Cryst. E 66 , o2902–o2903]. Whilst the 4‐phenoxybenzoyl groups in 2,7‐diisopropoxy‐1,8‐bis(4‐phenoxybenzoyl)naphthalene [Yoshiwaka et al. (2013). Acta Cryst. E 69 , o242] are situated in the same anti orientation as the title molecule, those of 2,7‐dimethoxy‐1,8‐bis(4‐phenoxybenzoyl)naphthalene are oriented in the same direction with respect to the naphthalene ring system, i.e. in a syn orientation.     

2‐(2‐Naphthyloxy)acetate derivatives. I. A new class of antiamnesic agents

The title compounds 1‐(2‐naphthyloxymethylcarbonyl)piperidine, C₁₇H₁₉NO₂, (I), and 3‐methyl‐1‐(2‐naphthyl­oxy­methyl­carbonyl)­piperidine, C₁₈H₂₁NO₂, (II), are potential antiamnesics. In (II), the methyl‐substituted piperidine ring is disordered over two conformations. The piperidine ring has a chair conformation in both compounds. In (I), the mol­ecules are linked by weak intermolecular C—H⃛O interactions to give networks represented by C(4), C(6) and (18) graph‐set motifs, while in (II), weak intermolecular C—H⃛O interactions generate (5), C(4) and C(7) graph‐set motifs. The dihedral angle between the naphthalene moiety and the piperidine ring is 33.83 (7)° in (I), while it is 31.78 (11) and 19.38 (19)° for the major and minor conformations, respectively, in (II).     

One‐dimensional hydrogen‐bonded structures in the 1:1 proton‐transfer compounds of 4,5‐dichlorophthalic acid with 8‐hydroxyquinoline, 8‐aminoquinoline and quinoline‐2‐carboxylic acid (quinaldic acid)

The structures of the 1:1 proton‐transfer compounds of 4,5‐dichlorophthalic acid with 8‐hydroxyquinoline, 8‐aminoquinoline and quinoline‐2‐carboxylic acid (quinaldic acid), namely anhydrous 8‐hydroxyquinolinium 2‐carboxy‐4,5‐dichlorobenzoate, C₉H₈NO⁺·C₈H₃Cl₂O₄⁻, (I), 8‐aminoquinolinium 2‐carboxy‐4,5‐dichlorobenzoate, C₉H₉N₂⁺·C₈H₃Cl₂O₄⁻, (II), and the adduct hydrate 2‐carboxyquinolinium 2‐carboxy‐4,5‐dichlorobenzoate quinolinium‐2‐carboxylate monohydrate, C₁₀H₈NO₂⁺·C₈H₃Cl₂O₄⁻·C₁₀H₇NO₂·H₂O, (III), have been determined at 130 K. Compounds (I) and (II) are isomorphous and all three compounds have one‐dimensional hydrogen‐bonded chain structures, formed in (I) through O—H...O_carboxyl extensions and in (II) through N⁺—H...O_carboxyl extensions of cation–anion pairs. In (III), a hydrogen‐bonded cyclic R₂²(10) pseudo‐dimer unit comprising a protonated quinaldic acid cation and a zwitterionic quinaldic acid adduct molecule is found and is propagated through carboxylic acid O—H...O_carboxyl and water O—H...O_carboxyl interactions. In both (I) and (II), there are also cation–anion aromatic ring π–π associations. This work further illustrates the utility of both hydrogen phthalate anions and interactive‐group‐substituted quinoline cations in the formation of low‐dimensional hydrogen‐bonded structures.