Quinoline vs Isoquinoline: How “Where the Nitrogen Sits” Changes Reactivity and Applications

I.Quinoline vs Isoquinoline: Changing the N Position Changes Three Things at Once

In drug discovery, catalysis, and materials chemistry, quinoline and isoquinoline are often treated as “pretty similar N-heteroaromatics.” But in real experiments, the differences most likely to trigger rework often come from one key factor: the position of the ring nitrogen. That single change can shift three classes of variables together:

1. Salt formation & solubility (salt form / crystallization window):

Both are benzopyridine isomers. Their nitrogen is pyridine-type, so both can be protonated to form salts. However, because the N position changes electron distribution, molecular dipole, and solvation, you can see different salt forms, crystallization behavior, mother-liquor partitioning, and solubility even under “the same acid + the same solvent system” (especially near the solubility/crystallization boundary).

2. Regioselectivity in functionalization (can you hit the target position in one go?):

Both are fused aromatic N-heterocycles, but the N position redistributes π-electron density and reaction polarity, changing which sites are more prone to nucleophilic attack or activation via coordination/directing effects. As a result, under different mechanisms—electrophilic aromatic substitution (EAS), nucleophilic substitution/addition, directed metalation, transition-metal C–H activation—their “preferred reactive positions” often diverge, directly determining whether you must change protecting groups, directing strategies, or even substrates.

3. Interactions with metals (coordination strength & system behavior):

When used as N-donor ligands or coordination fragments, differences in donor vector/orientation, adjacent sterics, and molecular dipole influence coordination stability, conformational preferences (especially with ortho substitution or potential chelation), and therefore performance in catalysis, luminescence, sensing, and related systems.

II.What Are Quinoline and Isoquinoline?

Quinoline and isoquinoline are the simplest benzopyridines—aromatic N-heterocycles formed by fusing a benzene ring with a pyridine ring.

They are constitutional isomers, differing only in which position within the fused system contains the pyridine nitrogen. Using common numbering: quinoline has N at position 1 (N-1), while isoquinoline has N at position 2 (N-2). Quinoline is often described as benzo[b]pyridine, and isoquinoline as benzo[c]pyridine.

Basic identifiers:

Item	Quinoline	Isoquinoline
Molecular formula	C₉H₇N	C₉H₇N
Molecular weight	129.1586	129.1586
CAS RN	91-22-5	119-65-3

III.Structural Comparison: How N Position Drives Acidity/Basicity, Regioselectivity, and Coordination/Interaction Direction

Dimension	Quinoline	Isoquinoline	Selection / practical notes
Core scaffold (shared)	Fused aromatic heterocycle: benzene + pyridine (“benzannulated pyridine”)	Same (same formula and fusion type; only N position differs)	Planar aromatic scaffold → strong π–π interactions; N introduces polarity / an H-bond acceptor site (not purely hydrophobic).
Type of nitrogen (shared)	Pyridine-type N: protonatable; can coordinate as a Lewis base	Same	Can form salts and coordinate metals—common in salt screening, acid–base extraction, and ligand/catalyst systems.
Nitrogen position (root of the difference)	N at position 1 of the quinoline ring system (by quinoline numbering)	N at position 2 of the isoquinoline ring system (by isoquinoline numbering)	The spatial orientation/relative geometry of N changes: if you want to attach a side chain or “handle” near N, the numbering and synthetic route often cannot be copied directly between the two.
Aqueous pKaH (pKa of the conjugate acid BH⁺; commonly cited compiled value)	pKaH ≈ 4.85	pKaH ≈ 5.14	Isoquinoline is slightly more readily protonated (difference is small, but in certain pH windows it can amplify into distribution/salt-form differences). Note: pKaH values are literature-compiled aqueous values and can shift slightly with temperature, ionic strength, and cosolvents.
Converting pKaH to “fraction protonated at a given pH” (estimate)	pH 7: ≈ 0.70%; pH 5: ≈ 41.5%; pH 2: ≈ 99.86%	pH 7: ≈ 1.36%; pH 5: ≈ 58.0%; pH 2: ≈ 99.93%	Near neutral pH, both are mainly neutral. Differences are largest when pH ≈ pKaH (about 4–6), affecting extraction partitioning and salt-form boundaries. In strong acid (e.g., pH ≤ 2), both are almost fully protonated, so the difference becomes small.
Electrophilic aromatic substitution (EAS): which ring/positions react first?	Mostly on the benzenoid ring side; commonly 5/8 positions	Similar trend: still mainly benzenoid ring; commonly 5/8	For nitration/halogenation/sulfonation EAS: by default, check the benzenoid ring first (especially 5/8). To do EAS on the “N-containing ring side,” you usually need a different strategy.
Nucleophilic reactions / dearomatization: which positions are attacked first?	Tends toward α/γ positions on the N-containing ring (often corresponding to C2/C4; many nucleophilic additions/aminations favor the α position)	Also favors α/γ positions, but the α position often corresponds to C1 in isoquinoline (many nucleophilic additions prefer C1)	If you need functionalization on the N-containing ring, the preferred attack positions often differ—so “the same site scan” typically requires separate route design and intermediate selection. Note: these are common trends under activating conditions; many systems require quaternization, N-oxide formation, strong nucleophiles, or promoters to proceed efficiently.
Common ways to enable functionalization on the “N-containing ring”	N-oxide strategy: oxidize N → N-oxide to reshape electron distribution/site preference → functionalize the harder positions → reduce/deoxygenate back to quinoline; or switch to metalation / cross-coupling routes	Same strategy applies	To substitute the N-containing ring, don’t force a benzenoid-EAS mindset; more commonly use N-oxide or metalation/cross-coupling handles to reach the target position.

IV.Practical R&D Classification: From Substitution and Charge State to N-Oxides and Hydrogenation—What to Change and How to Use It

Category (what you change)	What changes structurally	The most visible practical change	Where it’s most commonly used
A Ring substitution / functional “handles”	Introduce halogens, alkyl/aryl groups, O/N-containing functional groups, etc. on quinoline/isoquinoline (i.e., “install a handle at a specific position”)	Changes electronic effects and site reactivity; shifts lipophilicity/polarity and metabolic stability; determines which downstream coupling/interconversion routes are available	SAR scanning; coupling handles (halides, boronic acids, Sn/Zn precursors, etc.); tuning functional molecules/ligands (solubility, stability, binding strength)
B Make N charged: protonated salts vs quaternary salts	Protonated salt (BH⁺): reversible salt formation by adding acid; quaternary salt (N-alkylation): permanent positive charge	Protonated salts: solubility/crystallization/partitioning are pH-tunable; quaternary salts: solubility, ionic interactions, and interfacial behavior become “locked in,” typically more hydrophilic and more strongly ionic	Salt screening & purification (especially protonated salts); aqueous operations / extraction partitioning; quaternary salts in ionic materials, phase transfer/ion pairs, antimicrobials, dyes, electrochemical systems, etc.
C N-oxides (N → O)	Oxidize ring N to an N-oxide (ring system remains, but electron distribution is reorganized)	Strongly increased polarity; N→O alters coordination/protonation behavior and electron distribution (often reflected in O-end participation and stronger solvation), reshaping regioselectivity and reactivity	Directed functionalization intermediates (to enable reactions on the otherwise difficult “N-ring side” / specific positions); also used for property tuning and control comparisons
D Partial hydrogenation: tetrahydroquinoline / tetrahydroisoquinoline, etc.	Partially hydrogenate the aromatic system (planar aromatic → partially saturated)	sp² → sp³; increased 3D character and conformational freedom; basicity/saltability and solubility windows often change; chirality/enantiomers may become relevant	Lead optimization (increase 3D character, reduce “over-planarity”); building alkaloid/natural-product-like scaffolds; improving solubility/exposure and selectivity (must be assessed case-by-case with substitution pattern)

V.High-Frequency Application Lines: Choose the Core Scaffold First, Then Match the Use Case

1. Working on a quinoline scaffold (including “signature chelating fragments” such as 8-hydroxyquinoline) → see Table 5A

2. Working on an isoquinoline scaffold (natural products/alkaloids, and biisoquinoline ligands) → see Table 5B

5A | Common Applications of Quinoline (Quinoline)

High-frequency use line	Key structural handle	Core problem it solves	Typical representatives / keywords	Selection tips
Drug scaffold (highly reusable N-heteroaromatic)	Aromatic hydrophobic scaffold + ring pyridine-type N (protonatable / H-bond acceptor); many drugs further add a side-chain amine to strengthen salt formation and solubility tuning	Provides both a hydrophobic/π binding surface + a tunable charge/salt-form & solubility handle (common division of labor: ring N for “binding/positioning,” side-chain amine for “salt/solubility”)	4-aminoquinolines (e.g., chloroquine family); aminoquinoline	When you need to retain an aromatic binding surface yet want physicochemical properties to be tunable via salt formation, quinoline is a frequent choice. In SAR, two tracks usually run in parallel: ring-substitution site scan + side-chain amine/salt-form optimization (avoid assuming the ring N alone will solve solubility).
Chelation & analysis (signature fragment)	8-hydroxyquinoline: ring N + ortho O forms an N,O bidentate site (chelation often accompanies phenolic deprotonation)	Provides a ready-made stable bidentate chelating handle, enabling metal complexation, separations, and analytical applications (strength/selectivity often vary with pH and metal)	8-hydroxyquinoline (oxine/oxinate); metal chelation / extraction / analytical reagent	If you need not just an “N-heteroaromatic,” but a built-in N,O chelating site, prioritize 8-hydroxyquinoline and derivatives. For method/extraction work, treat the pH window as a key variable and design around it.
Coordination / catalysis / materials (general N-donor unit + site-control tool)	Quinoline N can act as a monodentate ligand, or be combined with other N/O/S donors into bi-/polydentate ligands; N-oxide serves as a synthetic “tool state” to reshape electron distribution and install substitution at target positions (then reduce back to quinoline)	As a ligand unit: provides a scalable N-donor module. As a synthetic tool: makes otherwise difficult regioselective functionalization more controllable, “locking in” substitution positions for downstream coordination/material design	quinoline-based ligands; N-oxide (oxidation → functionalization → reduction)	For coordination/materials: first choose a ligand framework based on whether you need mono- vs bi-/polydentate binding. If the bottleneck is “the position is hard to functionalize,” a common route is N-oxide (or cross-coupling) to fix the position first, then return to the target quinoline scaffold.

5B | Common Applications of Isoquinoline (Isoquinoline)

High-frequency use line	Key structural handle	Core problem it solves	Typical representatives / keywords	Selection tips
Drug / natural-product scaffold: a common parent core in alkaloid families—binds well and is tunable for properties	Aromatic planar isoquinoline scaffold + one pyridine-type N (protonatable to salts / participates in interactions); ample space for ring substitution and side-chain extension, enabling systematic derivative series	Highly prevalent in natural products and drug-like molecules: aromatic core provides hydrophobic/π interactions; N charge state and substitution offer tunable knobs for solubility, partitioning, interaction strength, etc.	benzylisoquinoline alkaloids (e.g., papaverine branch); isoquinoline alkaloids (e.g., berberine branch, often as quaternary/salt forms)	If you want “alkaloid-like” scaffold expansion or natural-product-inspired scaffold replacement, prioritize isoquinoline. First distinguish two common routes: benzylisoquinolines (expandable side chain/substitution) vs quaternary/salt-form isoquinolines (fixed positive charge; properties/interactions are more strongly ionic).
Chelation / analysis: isoquinoline is more often used as a ligand unit and needs “assembly” into polydentate sites	A single isoquinoline typically provides only one pyridine-type N donor (monodentate); unlike 8-hydroxyquinoline, it does not inherently contain an N,O bidentate chelating site	Valuable when isoquinoline serves as part of a larger ligand framework: incorporation into larger N,N or multidentate ligands yields more stable coordination/chelation. If you need an off-the-shelf strong bidentate chelator, a single isoquinoline is usually not the direct solution	N-donor ligand subunit; combined with other N donors to form N,N / multidentate ligands	If you need a ready-made strong chelating site: prioritize 8-hydroxyquinoline (quinoline side). If you need an expandable ligand framework: use isoquinoline as an N-donor building unit and assemble it into bi-/polydentate ligands.
Coordination / materials: bidentate branches enable new coordination geometry and spatial constraints	Biisoquinoline and other N,N bidentate isoquinoline ligands; often featuring clear geometric traits (e.g., axial chirality / conformational locking)	Compared with a “single quinoline/isoquinoline N donor,” bidentate ligands offer more stable and predictable bite-angle coordination geometry; geometry/configuration reshapes the steric environment around the metal center, enabling different coordination behavior and materials/catalytic performance	biisoquinoline; N,N bidentate ligand	When you need a clearly defined N,N bidentate ligand to control coordination geometry, or want spatial constraints from configuration/chirality, prioritize biisoquinoline-type bidentate branches. Do not treat isoquinoline as a simple drop-in equivalent of quinoline.

VI.Product Navigation Table | Quickly Locate Quinoline/Isoquinoline-Related Chemicals by Research Task (Tables 1–5)

Research task / experimental need	Recommended table to check first	Table-selection logic	Typical product types you may use
Antibacterial activity references, susceptibility testing (MIC/MBC), or building/validating LC-MS/HPLC quantitation methods (antibiotic-related)	Table 1	These experiments most need standardized API reference compounds (well-defined structures and fixed use scenarios), often with impurity/degradation/stability method development; Table 1 focuses on quinolone and anti-TB representatives, closest to susceptibility and analytical needs	Fluoroquinolones (norfloxacin/ofloxacin/ciprofloxacin/levofloxacin/gatifloxacin/moxifloxacin); early quinolone representatives; anti-TB quinoline (bedaquiline)
Antimalarial efficacy/resistance references, combination screening, or antimalarial drug analysis (metabolites/impurities/stability)	Table 2	Antimalarial work often selects references by drug lineage: 4-aminoquinolines, 8-aminoquinolines, quinoline methanols, and Cinchona alkaloids differ in mechanisms/usage; Table 2 assembles these key lineages for “set-based” selection	Chloroquine/hydroxychloroquine/amodiaquine; primaquine/tafenoquine; mefloquine; quinine/quinidine/cinchonine/cinchonidine
Chiral resolution, phase-transfer/asymmetric catalysis screening (Cinchona “toolbox”-centered)	Table 2	Chiral experiments first look for entries that can be used directly as chiral bases/resolving agents/catalyst parents; Table 2 concentrates Cinchona alkaloids, enabling set comparisons across quinine/quinidine/cinchonine/cinchonidine	(−)-Quinine, quinidine, cinchonine, cinchonidine (for racemate resolution/chiral induction/derivatized catalytic systems)
Natural-product/alkaloid pharmacology screening, cell experiments, or reference/QC standards (mainly isoquinoline alkaloids)	Table 3	This need prioritizes natural alkaloid / salt-form standards, often involving aqueous preparation and batch-to-batch consistency; Table 3 centralizes berberine/palmatine/sanguinarine/papaverine/noscapine, aligning with pharmacology and QC	Isoquinoline alkaloids (berberine, palmatine, sanguinarine, noscapine, papaverine) and their hydrochloride salts/hydrates; rotundine
Neuroscience/metabolic pathway work: kynurenine-pathway quantitation; THIQ/endogenous-related tool-molecule references	Table 3	Core requirement is endogenous/pathway-related small-molecule standards with reproducible references; Table 3 includes quinolinic acid and norharmane-type entries that lean toward neuro/metabolic tools	Quinolinic acid; norharmane; (and THIQ-related alkaloid tool molecules)
“Quinoline/isoquinoline scaffold derivatization”: site scans, coupling libraries (Suzuki/Buchwald/Sonogashira), rapid substituent expansion	Table 4	Library building and site scans first need couplable/substitutable handle building blocks (halides, carboxylic acids, N-oxides, etc.); Table 4 groups parent cores, halides, acids, quinolinones and key intermediates for route setup	6-Bromoquinoline, 8-bromoquinoline, 4,7-dichloroquinoline; 1-chloro/1-bromo/3-bromo/5-bromoisoquinoline; quinoline-2/4-carboxylic acids; isoquinoline-3-carboxylic acid; isoquinoline N-oxide
Salt form / solubility / crystallization behavior of aza-arenes (process development, salt screening, analytical controls)	Table 4	You typically need both free base/parent core and salt-form controls; Table 4 includes quinoline/isoquinoline parent cores and salt entries such as quinoline hydrochloride, making it convenient for paired comparison designs	Quinoline, isoquinoline, quinoline hydrochloride (and related derivable salt forms/tautomeric systems: 2-quinolinol, quinolinone, etc.)
8-Hydroxyquinoline system: metal-ion chelation, extraction/analytical reagents, coordination chemistry	Table 5	If chelation/coordination is the core task, start from 8-hydroxyquinoline and its salts/derivatives; Table 5 centralizes 8-HQ, hemisulfate, and nitro/halo derivatives, suitable for comparing electronic effects/lipophilicity	8-Hydroxyquinoline; 8-hydroxyquinoline hemisulfate; 5-nitro-8-hydroxyquinoline; 5-chloro-8-hydroxy-7-iodoquinoline; 5,7-diiodo-8-hydroxyquinoline
Organic optoelectronic / coordination-material references (e.g., 8-hydroxyquinoline metal-complex systems)	Table 5	Materials work often needs either pre-formed metal complexes or high-purity ligands; Table 5 includes representative complexes such as tris(8-hydroxyquinolinato)aluminum, enabling direct fabrication and property benchmarking	Tris(8-hydroxyquinolinato)aluminum (Alq₃, high metal-basis purity); ligands and derivatives for coordination tuning

Note: The quinolones/fluoroquinolones in Table 1 belong to the 4-quinolone core (quinolone scaffold), not the quinoline/isoquinoline (benzopyridine) parent cores discussed in this article. They are included for anti-infective API references/method development, not as representatives of the quinoline core itself.

Table 1 | Anti-infective Drug APIs (Antibacterial: Quinolones/Fluoroquinolones + Anti-TB Quinoline)

Category	CAS No.	Aladdin Cat. No.	Name	Spec / Purity	Product features & applications
Drug API | Quinolone-related antibacterial (nalidixic acid lineage)	389-08-2	N104920	Nalidixic acid	≥98%	Classic early representative of the quinolone lineage (nalidixic acid family): used for antibacterial activity reference, method development (HPLC/LC-MS), and impurity/degradation studies; suitable for lineage comparison versus fluoroquinolones.
Drug API | Fluoroquinolone antibacterial (quinolone scaffold)	70458-96-7	N114262	Norfloxacin (MK-0366)	Moligand™, ≥98%	Early fluoroquinolone representative: used for method development and quality studies, susceptibility-testing references, and impurity/degradation & stability studies; common in series comparison experiments.
Drug API | Fluoroquinolone antibacterial (quinolone scaffold)	82419-36-1	O102014	Ofloxacin	Moligand™, ≥98%	Classic fluoroquinolone: widely used as a microbial susceptibility reference, for analytical method establishment, and for stability & degradation-pathway studies.
Drug API | Fluoroquinolone antibacterial (quinolone scaffold)	85721-33-1	C129896	Ciprofloxacin	Moligand™, ≥98%	Common fluoroquinolone reference drug: used for mechanism studies, standardized susceptibility testing, HPLC/LC-MS quantitation, and impurity profiling.
Drug API | Fluoroquinolone antibacterial (quinolone scaffold)	112811-59-3	G129827	Gatifloxacin	Moligand™, ≥98%	Fluoroquinolone antibacterial: used for antibacterial references, drug analysis, and impurity/degradation studies; suitable for structure–activity/property comparisons within quinolone series.
Drug API | Fluoroquinolone antibacterial (quinolone scaffold)	100986-85-4	L473790	Levofloxacin	Moligand™, anhydrous grade, ≥98%	Fluoroquinolone antibacterial: commonly used for activity references, method development (HPLC/LC-MS), impurity/degradation studies, and formulation compatibility work; mechanistic studies often focus on inhibition of DNA gyrase / Topo IV.
Drug API | Fluoroquinolone antibacterial (quinolone scaffold)	151096-09-2	M302962	Moxifloxacin	Moligand™, ≥98%	Newer-generation fluoroquinolone: used for antibacterial reference, drug analysis, and impurity/degradation studies; useful for comparing PK/property differences driven by structural variation.
Drug API | Anti-tuberculosis (quinoline class: diarylquinoline)	843663-66-1	B177578	Bedaquiline	Moligand™, ≥97%	Representative anti-TB quinoline drug: used for TB drug R&D, susceptibility/resistance studies, analytical reference work, and impurity/degradation studies; often used as a new-mechanism reference.

Table 2 | Antimalarial / Antiparasitic (4-Aminoquinolines + 8-Aminoquinolines + Quinoline Methanols + Cinchona Alkaloids)

Category	CAS No.	Aladdin Cat. No.	Name	Spec / Purity	Product features & applications
Natural alkaloid | Cinchona alkaloids (quinoline scaffold)	130-95-0	Q431754	(−)-Quinine	Moligand™, used for resolution of racemates in synthesis	Classic quinoline-type natural product: widely used as a chiral resolving agent / chiral base and as a tool for asymmetric synthesis (e.g., quinine/quinidine-derived phase-transfer catalysis, chiral induction); also a representative reference compound in antimalarial-related studies.
Natural alkaloid | Cinchona alkaloids (quinoline scaffold)	56-54-2	Q109702	Quinidine	Moligand™, ≥98%, contains 5–15% dihydroquinidine	Representative Cinchona alkaloid: commonly used for chiral resolution and building asymmetric synthesis/catalysis systems; also a classic pharmacologically active molecule for related reference and method-development studies.
Natural alkaloid | Cinchona alkaloids (quinoline scaffold)	485-71-2	C109632	Cinchonidine	Moligand™, ≥98%	Cinchona alkaloid: commonly used for chiral resolution / chiral induction and asymmetric synthesis systems; also used in comparative reference studies (often benchmarked against quinine/quinidine).
Natural alkaloid | Cinchona alkaloids (quinoline scaffold)	118-10-5	C106926	Cinchonine	≥98%	Cinchona alkaloid: commonly used as a chiral resolving agent / chiral base and as a reference in asymmetric synthesis; together with quinine/quinidine/cinchonidine forms the frequently used “Cinchona toolbox.”
Drug API | Antimalarial / antiparasitic (8-aminoquinoline)	90-34-6	P187973	Primaquine	Moligand™, ≥95%	Classic 8-aminoquinoline antimalarial: used for antimalarial research and analytical referencing; often compared with close analogs (e.g., tafenoquine) for structure/property and experimental-performance benchmarking.
Drug API | Antimalarial / antiparasitic (8-aminoquinoline)	106635-80-7	T614303	Tafenoquine	Moligand™, ≥98%	8-aminoquinoline antimalarial: commonly used in antimalarial drug research and as an analytical reference for metabolism/impurities; representative for Plasmodium-related assays (susceptibility, mechanism, combination-therapy evaluation).
Drug API | Antimalarial / antiparasitic (4-aminoquinoline)	54-05-7	C193834	Chloroquine	Moligand™, ≥97%	Classic 4-aminoquinoline antimalarial: used for mechanism and resistance studies, drug analysis/QC, and method references; a benchmark reference across quinoline antimalarial lineages.
Drug API | Antimalarial / antiparasitic (4-aminoquinoline)	118-42-3	C586514	Hydroxychloroquine	Moligand™, ≥98%	Representative 4-aminoquinoline: used as an antimalarial-related reference; also widely used as a tool compound in studies involving weakly basic amines (lysosomal enrichment / intracellular acidic compartments) and as a method reference.
Drug API | Antimalarial / antiparasitic (4-aminoquinoline)	86-42-0	A331454	Amodiaquine	Moligand™, ≥98%	4-aminoquinoline antimalarial: used for antimalarial drug research, susceptibility references, and impurity/metabolism-related analysis; suitable for series comparisons with chloroquine/hydroxychloroquine.
Drug API | Antimalarial / antiparasitic (quinoline methanol class)	53230-10-7	M611781	Mefloquine	Moligand™	Representative antimalarial compound: used in efficacy/resistance studies, analytical referencing, and method development; also commonly used for property and SAR-style comparisons across quinoline antimalarial series.

Table 3 | Isoquinoline Alkaloids and Bio/Neuro Tool Molecules (Including Salt Forms / Hydrates)

Category	CAS No.	Aladdin Cat. No.	Name	Spec / Purity	Product features & applications
Natural alkaloid | Isoquinoline (protoberberine-type)	2086-83-1	B414323	Berberine	Moligand™, ≥99%	Typical protoberberine-type isoquinoline alkaloid: used for pharmacological screening/mechanism studies, analytical referencing, and quality control; also commonly used as a tool molecule in fluorescence/binding assays (e.g., interactions with nucleic acids and proteins).
Natural alkaloid | Isoquinoline (protoberberine / palmatine-type)	10605-02-4	P274975	Palmatine hydrochloride	Moligand™, ≥98%	Isoquinoline alkaloid salt (improved water solubility; easier formulation): used for pharmacology studies, analytical referencing, and comparisons of formulation/solubility and salt-formation behavior (within the related berberine-like scaffold family).
Natural alkaloid | Isoquinoline (protoberberine) salt form	633-65-8	B139120	Berberine hydrochloride	≥95%	Berberine HCl: used for pharmacological studies, analytical referencing, and stability/impurity work; salt form facilitates aqueous experiments and method development (paired comparisons with free base/hydrate).
Natural alkaloid | Isoquinoline (protoberberine) salt (hydrate)	141433-60-5	B107342	Berberine hydrochloride hydrate	≥98%	Hydrated salt form of berberine: closer to certain aqueous/formulation conditions; used for pharmacology studies, analytical referencing, impurity/stability work, and salt-form behavior comparisons (vs anhydrous HCl salt/free base).
Natural alkaloid | Isoquinoline (benzophenanthridine / sanguinarine-type)	2447-54-3	S303178	Sanguinarine	Moligand™, ≥97%	Isoquinoline alkaloid: commonly used for bioactivity screening, mechanism studies, and analytical referencing; also useful for comparing structure–activity differences among isoquinoline alkaloids.
Natural alkaloid | Isoquinoline (benzylisoquinoline derivative)	912-60-7	N639558	Noscapine hydrochloride	≥99%, chp	Typical isoquinoline alkaloid salt: used for pharmacology/mechanism studies, analytical referencing, and quality work; hydrochloride form is convenient for aqueous preparation and method development.
Drug API / tool molecule | Isoquinoline alkaloid (papaverine) salt form	61-25-6	P275480	Papaverine hydrochloride	≥95%	Typical benzylisoquinoline alkaloid (pharmacologically active molecule) in salt form: used for drug research, analytical referencing, and quality/impurity & degradation studies; HCl salt is better suited for aqueous preparation and quantitation.
Natural alkaloid | Tetrahydroisoquinoline (analgesic / neuropharmacology tool)	483-14-7	R136626	Rotundine	≥98%	Tetrahydroisoquinoline alkaloid (pharmacologically active molecule): commonly used in analgesia/sedation-related studies, activity referencing, and analytical method development; also a structure–activity reference for THIQ scaffolds.
Natural / endogenous-related | THIQ alkaloid (salsolinol)	27740-96-1	S695071	Salsolinol	≥98%	Also known as salsolinol (6,7-dihydroxy-1-methyl-1,2,3,4-tetrahydroisoquinoline): widely used as a tool molecule and analytical reference in neurochemistry/biogenic-amine-related studies; a representative THIQ scaffold member.
Neuro / metabolic tool compound | Kynurenine pathway (quinolinic acid-related metabolite)	492-27-3	K120012	Quinolinic acid	Moligand™, ≥97%	Endogenous metabolite standard (kynurenine pathway): used for quantitation and reference in neuroscience/immunometabolism research (e.g., pathway metabolite profiling; receptor/neurotransmission-related experimental benchmarking).

Table 4 | Parent Cores & General Scaffolds / Salt Forms + Key Synthetic Building Blocks (Halides / Carboxylic Acids / Quinolinones / N-Oxides, etc.)

Category	CAS No.	Aladdin Cat. No.	Name	Spec / Purity	Product features & applications
Parent core | Quinoline (Quinoline)	91-22-5	Q110656	Quinoline	AR, ≥96%	Quinoline parent core: commonly used for heterocycle teaching/method development, as a starting point and reference for substituted quinoline series syntheses; also used to establish spectral/purity reference baselines for quinoline scaffolds.
Parent core | Isoquinoline (Isoquinoline)	119-65-3	I105573	Isoquinoline	≥97%	Isoquinoline parent core: used as the starting point and reference for isoquinoline series synthesis (spectra/purity/reactivity), and as a benchmark heteroaromatic scaffold in medicinal and coordination chemistry.
Parent core | Quinoline salt (salt form / solubility control)	530-64-3	Q134192	Quinoline hydrochloride	≥98%	Salt-form reference for quinoline: used in comparisons of salt formation/solubility and crystallization behavior, and for dosing/quantitation of the acid-salt form in process development; also serves as an analytical reference for quinoline systems (salt vs free base).
Base scaffold | Partially hydrogenated quinoline (N-containing saturated scaffold)	635-46-1	T107484	1,2,3,4-Tetrahydroquinoline	≥97%	Common N-containing saturated scaffold: used in medicinal chemistry to increase sp³ content / alter conformation and salt-form behavior; also serves as a starting material for oxidation back to quinoline or further substitution / ring fusion.
Base scaffold | Partially hydrogenated isoquinoline (THIQ)	91-21-4	T139420	1,2,3,4-Tetrahydroisoquinoline	≥95%	THIQ is a high-frequency medicinal-chemistry scaffold: often used in Pictet–Spengler / reductive amination and related ring fusion/side-chain builds; used to increase sp³, tune conformation, and adjust salt/solubility behavior.
Synthetic building block | Alkylquinoline (Quinaldine)	91-63-4	Q107985	2-Methylquinoline	≥98%	Common starting point for quinoline core modification: enables benzylic (2-methyl) functionalization, oxidation/halogenation/coupling derivatives; used to build 2-substituted series for structure–property comparisons.
Synthetic building block | Hydroxyquinoline (2-position) / tautomeric system	59-31-4	Q160822	2-Quinolinol	≥98%	Quinoline building block featuring 2-hydroxy/2-keto tautomerism: used for H-bond/tautomerism controls, O-alkylation/acylation derivatization, and construction of 2-substituted quinoline/quinolinone-related fragments.
Synthetic building block | Quinoline carboxylic acid (functional handle)	486-74-8	Q123917	Quinoline-4-carboxylic acid	≥98%	Quinoline scaffold + carboxylic-acid “connection handle”: used to introduce side chains via amidation/esterification, or as a ligand/coordination fragment (N + CO₂H) for metal binding and property tuning.
Quinoline carboxylic acid | 2-position acid (Quinaldic acid)	93-10-7	Q110294	Quinaldic acid	≥98%	Quinoline-2-carboxylic acid (quinaldic acid): commonly used as a coordination/chelation fragment and synthetic intermediate; carboxylic acid enables amide/ester derivatization and is often used in structure–property comparisons across quinoline series.
Synthetic building block | Isoquinoline carboxylic acid (functional handle)	6624-49-3	I177102	Isoquinoline-3-carboxylic acid	≥97%	Isoquinoline scaffold + carboxylic-acid handle: used for side-chain installation via amidation/esterification to build comparable 3-substituted isoquinoline series; also used in coordination-fragment design (N + CO₂H).
Quinolinone-related building block | 4-hydroxy/4-keto tautomeric system	611-36-9	Q160821	4-Hydroxyquinoline	≥98% (HPLC)	Classic 4-hydroxy/4-keto tautomeric parent: used for constructing and benchmarking quinolone-like structures (4-oxo-1,4-dihydroquinoline), and as a model for coordination/H-bonding interactions.
Quinolinone-related | Quinolin-4(1H)-one core (key for quinolone lineage)	529-37-3	Q303900	Quinolin-4(1H)-one	≥95%	Core 4-quinolone parent scaffold: closely tied to quinolone antibacterial lineages; commonly used in building quinolone-like structures, tautomer/H-bond model studies, and method-development comparisons.
Synthetic intermediate | Halogenated quinoline (downstream antimalarial precursor)	86-98-6	D155356	4,7-Dichloroquinoline	≥98% (GC)	Key halogenated quinoline intermediate: widely used as a scaffold precursor for 4-aminoquinoline antimalarials (e.g., chloroquine/hydroxychloroquine); suitable as a reference for process-route development and impurity-source tracing.
Synthetic building block | Halogenated quinoline (8-position handle)	16567-18-3	B107744	8-Bromoquinoline	≥97%	8-halogenated quinoline: used in coupling to build 8-substituted quinolines (ligands, medicinal fragments, materials units); also a key building block for position-scan series.
Synthetic building block | Halogenated quinoline (coupling handle)	5332-25-2	B101593	6-Bromoquinoline	≥96%	Heteroaryl bromide: used in Suzuki / Buchwald–Hartwig / Sonogashira couplings to rapidly build 6-substituted quinoline series; often used for site scans and property optimization in medicinal/ligand development.
Synthetic building block | Halogenated isoquinoline (activated C1 position)	19493-44-8	C103592	1-Chloroisoquinoline	≥98%	Activated haloisoquinoline: commonly used for C1 nucleophilic substitution (amination/alkoxylation/thio-substitution) and Pd-coupling to build 1-substituted isoquinolines; a frequently used “handle” for rapid scaffold modification in medicinal chemistry.
Synthetic building block | Halogenated isoquinoline (activated C1 position)	1532-71-4	B120013	1-Bromoisoquinoline	≥95%	Activated C1 haloisoquinoline: used for nucleophilic substitution and Pd-coupling to build 1-substituted isoquinolines; key intermediate for rapidly introducing amine/alcohol/thiol substituents.
Synthetic building block | Halogenated isoquinoline (coupling handle)	34784-02-6	B588817	3-Bromoisoquinoline	≥98%	Typical heteroaryl bromide: used in Suzuki / Buchwald–Hartwig / Sonogashira couplings to rapidly access 3-substituted isoquinoline series; common in site scans during lead optimization.
Synthetic building block | Halogenated isoquinoline (coupling handle)	34784-04-8	B102307	5-Bromoisoquinoline	≥98%	Frontline building block for coupling to construct 5-substituted isoquinolines; suitable for series expansion (aryl/amine/alkynyl variants) to compare activity and physicochemical properties.
Synthetic building block | Isoquinoline N-oxide (directing/activation intermediate)	1532-72-5	I167508	Isoquinoline N-oxide	≥98%	N-oxides are often used in directed C–H functionalization (enabling regioselective modification of the isoquinoline ring) and as a transient activated form that can be reduced back to the parent core; suitable for method development in site-selective functionalization.
General synthetic reagent | o-Nitrobenzylating reagent (not a quinoline compound)	612-23-7	N139079	2-Nitrobenzyl chloride	≥98% (GC)	Common benzylation/protecting-group installation reagent: used to introduce o-nitrobenzyl protecting groups onto substrates (N/O/S, etc.); frequently used as a general alkylation reagent in quinoline/isoquinoline derivatization (e.g., N-oxides; protection and downstream transformations of alcohols/phenols/amines).

Table 5 | 8-Hydroxyquinoline System and Metal Complexes (Including Derivatives / Salt Forms)

Category	CAS No.	Aladdin Cat. No.	Name	Spec / Purity	Product features & applications
Coordination / analysis / materials building block | 8-Hydroxyquinoline (chelating agent)	148-24-3	H104638	8-Hydroxyquinoline	AR, ≥99%	Classic metal chelator/analytical reagent: used for metal-ion extraction/titration, coordination chemistry, and materials-precursor studies; the core ligand for constructing 8-hydroxyquinoline metal complexes (luminescent/transport materials, etc.).
8-Hydroxyquinoline system | Salt form (improved water solubility / handling)	134-31-6	Q106374	8-Hydroxyquinoline hemisulfate	≥98%	Salt form of 8-hydroxyquinoline: convenient for aqueous preparation and quantitative handling; used for metal-ion chelation/extraction and analysis, coordination-chemistry studies, and preparation of precursors to 8-hydroxyquinoline metal complexes.
8-Hydroxyquinoline derivative | Nitro substitution (chelation / antimicrobial tool)	4008-48-4	H107536	5-Nitro-8-hydroxyquinoline	≥97%	8-hydroxyquinoline derivative: used for comparative studies of metal-chelation properties and antimicrobial/antibacterial screening; nitro substitution can markedly change electronic effects and chelation/solubility behavior, making it suitable for structure–property comparisons.
8-Hydroxyquinoline derivative | Halogenated / chelation and tool compound	130-26-7	C123531	5-Chloro-8-hydroxy-7-iodoquinoline	≥98% (T)	Halogenated 8-hydroxyquinoline derivative: commonly used in studies of metal chelation/complexation behavior and as a parent for further functionalization; also frequently compared as a tool compound in directions such as antimicrobial preservation and metal-homeostasis topics in neurodegeneration.
8-Hydroxyquinoline derivative | Heavy halogen substitution (chelation / antimicrobial tool)	83-73-8	D592266	5,7-Diiodo-8-hydroxyquinoline	≥94%	Heavy-halogen 8-hydroxyquinoline: used for comparing metal chelation and the effects of increased hydrophobicity; also used as an antimicrobial/tool compound in structure–activity comparisons; suitable for series controls alongside 5-nitro and 5-chloro-7-iodo analogs.
Materials / coordination chemistry | 8-Hydroxyquinoline metal complex (OLED-related)	2085-33-8	T114511	Aluminum 8-hydroxyquinolinate	≥99.995% metals basis	Representative 8-hydroxyquinoline metal-complex system: widely used in organic optoelectronic materials (emission/electron transport) research and benchmarking (e.g., the Alq₃ material concept); high metals-basis purity helps reduce device-to-device variation caused by trace metal impurities.

Note: The above are representative Aladdin products. For additional specifications, please refer to the product list at the end of the article or search the Aladdin website using “product name / CAS / catalog number.”

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