Quinoline: The Benzopyridine Heterocycle at the Heart of Antimalarials, Antibiotics, and OLED Materials — Complete Guide

# Quinoline: The Benzopyridine Heterocycle at the Heart of Antimalarials, Antibiotics, and OLED Materials — Complete Guide

CAS: 91-22-5 | MF: C9H7N | MW: 129.16 g/mol | PubChem CID: 7047

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What is quinoline and why does this benzopyridine heterocycle appear in more FDA-approved drugs than almost any other fused-ring system?

Quinoline (CAS 91-22-5) is a benzopyridine heterocycle — formally, a benzene ring fused to a pyridine ring at the [b] face — that has earned its status as one of medicinal chemistry's truly privileged scaffolds. With molecular formula C9H7N and molecular weight 129.16 g/mol ^[001], quinoline is a colorless to pale yellow, high-boiling liquid (bp 237 °C) with a distinctive, persistent odor. Its privileged status derives from a convergence of favorable properties: the nitrogen atom provides a site for hydrogen bonding, salt formation, and metal coordination; the aromatic system enables π-stacking interactions with biological targets; and the well-developed synthetic chemistry of quinoline — ranging from classical Skraup and Doebner-von Miller syntheses to modern cross-coupling and C–H functionalization methods — allows systematic exploration of structure-activity relationships around every position of the ring system. The global quinoline market was valued at USD 3.14 billion in 2025 and is projected to reach USD 4.91 billion by 2035 at a CAGR of 4.55% ^[002], driven by pharmaceutical demand (antimalarials, fluoroquinolone antibiotics, kinase inhibitors), agrochemicals, and specialty applications including OLED materials. Quinoline and its derivatives appear in over 25 FDA-approved drugs, including chloroquine, ciprofloxacin, levofloxacin, bosutinib, and elvitegravir.

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Key Statistics

- The global quinoline market was valued at USD 3.14 billion in 2025 and is projected to reach USD 4.91 billion by 2035, growing at a CAGR of 4.55% ^[002].

- Quinoline and its derivatives appear in over 25 FDA-approved small-molecule drugs, spanning antimalarials, antibiotics, kinase inhibitors, and antivirals ^[001].

- Quinoline's boiling point of 237.1 °C and pKa of 4.90 (conjugate acid) ^[001] place it as a moderately basic heterocycle, between pyridine (pKa 5.23) and isoquinoline (pKa 5.40), enabling selective extraction strategies during synthesis workup.

- Quinoline has a molecular complexity score of 111 ^[001] — moderate relative to drug-like molecules but high enough to support diverse substitution patterns for SAR exploration in medicinal chemistry.

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What Are the Verified Physical and Chemical Properties of Quinoline?

Quinoline (CAS 91-22-5) is characterized by the following verified physicochemical parameters, all confirmed through PubChem PUG REST API query on 2026-06-19 ^[001]:

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Frequently Asked Questions

What are the classical methods for synthesizing quinoline?

The three foundational quinoline syntheses are: (1) Skraup synthesis — aniline + glycerol + H2SO4 + oxidizing agent, producing quinoline directly through a dehydration-cyclization-oxidation sequence; (2) Doebner-von Miller synthesis — aniline + α,β-unsaturated carbonyl compound under acid catalysis; and (3) Friedlander synthesis — o-aminobenzaldehyde + enolizable ketone/aldehyde under base or acid catalysis — the most versatile method due to the wide availability of substituted partners. Modern variants employ microwave heating, solid-supported reagents, and transition metal-catalyzed cyclizations. ^[003]

How does quinoline compare to isoquinoline in terms of reactivity?

Quinoline (benzo[b]pyridine) and isoquinoline (benzo[c]pyridine) are constitutional isomers with significantly different reactivity profiles. Quinoline undergoes electrophilic aromatic substitution preferentially at the 5- and 8-positions (benzene ring), while nucleophilic substitution occurs at the 2- and 4-positions (pyridine ring). Isoquinoline undergoes electrophilic substitution at the 5- and 8-positions but nucleophilic substitution at the 1-position. These differences are strategically exploited in drug design.

Why are fluoroquinolones so effective as antibiotics?

Fluoroquinolones (ciprofloxacin, levofloxacin, moxifloxacin) target bacterial DNA gyrase and topoisomerase IV — enzymes essential for DNA replication that are absent in human cells. The quinoline-3-carboxylic acid core chelates Mg2+ ions in the enzyme active site, while the C6-fluorine and C7-piperazine substituents determine potency, spectrum, and pharmacokinetics. The fluorine at C6 enhances both DNA gyrase binding (10–100× improved potency over non-fluorinated nalidixic acid) and cell penetration.

What role does quinoline play in OLED and materials chemistry?

Quinoline derivatives — particularly 8-hydroxyquinoline metal complexes (e.g., Alq3) — were the foundational electron-transport and emitting materials in early OLED devices. Alq3, with its green emission (λmax ~530 nm), adequate electron mobility (~10⁻⁵ cm²/V·s), and excellent thin-film forming properties, was the workhorse emitter in the first generation of commercial OLED displays. The quinoline ligand framework provides tunable HOMO/LUMO levels through metal selection and substitution.

What are the key physical and spectroscopic properties of quinoline?

Quinoline (C9H7N, MW 129.16 g/mol ^[001]) is a colorless to pale yellow liquid (mp -15 °C, bp 237.1 °C, density 1.093 g/mL). Its UV spectrum shows characteristic absorption at λmax 313 nm (log ε 3.56) in ethanol. In 1H NMR (CDCl3), quinoline shows H2 as a doublet (~8.9 ppm, J = 4.2 Hz), H3 as a doublet of doublets (~7.4 ppm), and H4 as a doublet (~8.1 ppm). The parent ion in EI-MS appears at m/z 129 with characteristic fragment ions at m/z 102 (loss of HCN).

How is quinoline typically purified for demanding applications?

Commercial quinoline (typically 96–98% purity) contains isoquinoline and alkylquinolines from coal tar as primary contaminants. For demanding applications (e.g., electronic materials), quinoline is purified by: (1) formation of the phosphate salt, recrystallization, and regeneration with NaOH; (2) fractional distillation at reduced pressure through a 30+ theoretical plate column; or (3) zone refining for ultra-high purity (99.999%) material for OLED applications. Water content must be rigorously excluded for metal-catalyzed reactions.

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PRODUCT AVAILABILITY

Quinoline (CAS 91-22-5) is available in the CoreyChem product catalog.

[View Product →](https://careerchemical.com/product/91-22-5)

CoreyChem offers this compound with Certificate of Analysis (CoA), SDS/MSDS, and on request: ICH Q3D elemental impurity profiling, residual solvent analysis per USP, and TSE/BSE compliance statements.

References & Sources