1-(2-Chlorophenyl)-1H-Pyrrole-2-Carbaldehyde

Product Name 1-(2-Chlorophenyl)-1H-Pyrrole-2-Carbaldehyde
Alias 2-Chlorophenylpyrrole-2-carbaldehyde
Einecs 629-631-5
Mininmum Order 1g
Factory Site West Ujimqin Banner, Xilingol League, Inner Mongolia, China
Price Inquiry sales9@bouling-chem.com
Manufacturer Bouling Chemical Co., Limited
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In terms of specification, 1-(2-Chlorophenyl)-1H-Pyrrole-2-Carbaldehyde is supplied with a purity of ≥98% (HPLC) and a water content of ≤0.5%, making it suitable for use as a pharmaceutical intermediate.

Specifications

HS Code	220091
Chemical Formula	C11H8ClNO
Molecular Weight	205.64
Appearance	Solid (Typical description, actual may vary)
Solubility In Water	Expected to be low due to non - polar aromatic and pyrrole parts
Solubility In Organic Solvents	Likely soluble in common organic solvents like dichloromethane, ethyl acetate
Pka	No data given, would need specific measurements or literature

As an accredited 1-(2-Chlorophenyl)-1H-Pyrrole-2-Carbaldehyde factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.

Packing & Storage

Packing	100g of 1-(2 - Chlorophenyl)-1H - Pyrrole - 2 - Carbaldehyde in sealed chemical - grade packaging.
Shipping	1-(2 - Chlorophenyl)-1H - Pyrrole - 2 - Carbaldehyde is shipped in properly sealed containers. Packaging adheres to chemical safety regulations. Shipment is via approved carriers, ensuring safe and timely delivery.
Storage	1-(2 - Chlorophenyl)-1H - Pyrrole - 2 - Carbaldehyde should be stored in a cool, dry place away from direct sunlight. Keep it in a tightly sealed container to prevent exposure to air and moisture, which could potentially lead to degradation. Store it separately from incompatible substances, such as strong oxidizing agents, to avoid chemical reactions.

Application of 1-(2-Chlorophenyl)-1H-Pyrrole-2-Carbaldehyde

In kilogram-scale cGMP manufacturing suites operating under 21 CFR 210/211, the pyrrole aldehyde bearing an ortho-chlorinated phenyl appendage is deployed as a non-isolated carboxylate precursor in the synthesis of a Bruton’s tyrosine kinase (BTK) inhibitor analogue. Oxidation to the corresponding 1-(2-chlorophenyl)-1H-pyrrole-2-carboxylic acid proceeds via Pinnick–Lindgren conditions in a glass-lined 50 L reactor: the aldehyde is dissolved in a mixed tetrahydrofuran/water (3:1 v/v) matrix at 0 °C, 1.05 molar equivalents of sodium chlorite are fed through an anti-splash dosing lance over 90 min while the internal temperature is clamped at ≤8 °C to suppress decarboxylative dimerization. Residual peroxide-based oxidants are quenched with a 10% sodium bisulfite solution until a negative starch-iodide test is obtained. Compliance with ICH Q7 §7.4 and residual solvent thresholds defined in USP ⟨467⟩ is demonstrated on every batch; the intermediate carboxylic acid is dried to ≤0.15% loss on drying (80 °C, 20 mbar) and held in HDPE-lined drums under positive nitrogen pressure. In the downstream amidation train, a 1.0:1.0 stoichiometric ratio of the pyrrole acid (pre-activated with EDC·HCl and HOBt monohydrate in anhydrous DMF) to a chiral (S)-tert-butylsulfinyl-protected benzylamine is maintained. The coupling is executed in a 30 L Hastelloy C-276 vessel at –5 to 0 °C; in situ ReactIR monitoring tracks the disappearance of the acid carbonyl stretch at 1687 cm⁻¹, with reaction termination triggered at ≤0.5 area% residual acid. After aqueous workup and recrystallization from isopropanol/water (7:3 v/v), the target amide intermediate is isolated with an isomeric purity ≥99.2% (HPLC, C18, 210 nm). The terminal drug substance—a potent irreversible BTK inhibitor belonging to the substituted pyrrolobenzodiazepine chemotype—is then produced through successive deprotection and macrocyclization steps; the entire synthetic route is validated for removal of elemental impurities per ICH Q3D Guideline for Elemental Impurities, specifically targeting Pd ≤ 10 ppm and Ni ≤ 5 ppm as measured by ICP-MS after a charcoal-filtration polishing stage.

What Limits the Fluorescence Quantum Yield in This Pyrrole Aldehyde-Derived meso-Aryl BODIPY Dye?

When the aldehyde is employed as the key aromatic carbaldehyde component in a one-pot, two-step BODIPY fluorophore assembly, the dominant photophysical failure mode is aggregation-caused quenching arising from residual non-BF₂-chelated dipyrromethene intermediates. The regulatory framework for optoelectronic-grade intermediates requires conformance with SEMI C59-0321 (Specification for Organic Light-Emitting Diode Materials), with individual alkali and transition metal ion concentrations held below 0.5 ppb w/w by high-resolution ICP-MS. In the standard synthetic protocol conducted in a jacketed 20 L borosilicate glass reactor equipped with an anchor-type PTFE agitator, 1.0 molar equivalent of 1-(2-chlorophenyl)-1H-pyrrole-2-carbaldehyde is combined with 2.0 equivalents of 2,4-dimethylpyrrole in anhydrous dichloromethane (Karl Fischer water content <30 µg/mL). A catalytic 0.10 equivalent of boron trifluoride diethyl etherate is introduced over 45 min, maintaining a gentle reflux (39–40 °C) under dry argon; the condensation is driven to ≥95% conversion as verified by TLC (silica gel 60 F₂₅₄, eluent: hexanes/ethyl acetate 6:4 v/v, aldehyde Rf 0.45 → product streak at origin). After cooling to 22 °C, 1.0 equivalent of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) is added portionwise to oxidize the dipyrromethane intermediate, the exotherm being controlled by addition rate to avoid a temperature overshoot beyond 28 °C. Neutralisation with 2.5 equivalents of triethylamine followed by introduction of 1.5 equivalents of BF₃·OEt₂ generates the BF₂-chelated BODIPY core. Downstream processing involves filtration through a silica plug (60 Å, 230–400 mesh), solvent exchange into n-heptane under reduced pressure, and slow cooling crystallization at –15 °C to yield dark coppery platelets. A critical quality attribute is the fluorescence quantum yield (ΦF) in deoxygenated ethanol: measured against Rhodamine 6G standard (ΦF = 0.95 in ethanol) using an integrating sphere, the target value of ≥0.82 is attained only when the 2-chlorophenyl rotameric obstruction is minimized by rapid cooling during crystallization, locking the biphenyl-like conformation. Discrepancies as low as 0.03 in ΦF correlate with a shoulder emission at >600 nm, traceable to O₂-permeable amorphous fractions. The terminal product is formulated as a non-targeted, lipophilic fluorescent probe for two-photon imaging of lipid droplets in HepG2 hepatoma cells, featuring an excitation maximum at 530 nm and emission at 555 nm with a full width at half-maximum of 38 nm.A process stream dedicated to insecticide lead generation utilizes the title aldehyde as the electrophilic anchor for constructing γ-aminobutyric acid (GABA)-gated chloride channel modulator chemotypes possessing a 2-chlorophenylpyrrole amide pharmacophore. The manufacturing environment aligns with the FAO/WHO Manual on Development and Use of FAO and WHO Specifications for Pesticides (3rd rev., 2023) for technical-grade active ingredient quality, and the batch records are structured under an ISO 9001:2015 quality management system integrated with ISO 14001 environmental aspects and an OECD Series on Principles of Good Laboratory Practice (GLP). During the oxidative conversion to 1-(2-chlorophenyl)-1H-pyrrole-2-carboxylic acid, the aldehyde is charged at a 100% theoretical weight basis, and sodium chlorite is fed at a 1.05:1.00 mole ratio relative to the aldehyde in a 500 L glass-lined reactor fitted with a retreat-curve impeller. The reaction medium (acetonitrile/water 3:2 v/v, 0.15 M in aldehyde) is buffered at pH 4.00 ± 0.15 with monosodium phosphate, and the sodium chlorite solution is metered at a rate of 0.15 kg/h per kg of aldehyde to avoid the accumulation of chlorine dioxide gas; the reactor headspace is continuously swept with nitrogen through an alkaline scrubber containing 10% w/w sodium hydroxide and 5% w/w sodium sulfite. After a negative bleach test, the acid is isolated and dried to a KF water content of ≤0.5%. Subsequent activation with 1.2 equivalents of thionyl chloride in toluene at 85 °C for 4 h provides the acid chloride, distilled to remove excess reagent before coupling. The amidation step is run at –2 to +2 °C in a separate 300 L enameled vessel: a substituted 4-aminopyrimidine (1.0 equivalent) and triethylamine (1.5 equivalents) are predissolved in dry THF, and the acid chloride is added over 120 min while the jacket inlet temperature is held at –5 °C. The resulting N-(substituted)-1-(2-chlorophenyl)-1H-pyrrole-2-carboxamide is crystallised from a ternary solvent system (ethyl acetate/cyclohexane/n-heptane 2:1:1 v/v/v) to deliver a white crystalline solid with an HPLC purity of 98.5% (area%, 254 nm), a melting point of 187–189 °C, and a residual chloride content ≤ 200 ppm. The fully formulated insecticide candidate—classified as a meta-diamide IRAC Group 30 modulator of the insect GABA receptor—exhibits a 96-h LC₅₀ of 0.8 mg a.i./L against susceptible Spodoptera frugiperda neonates in diet-incorporation assays, with the active substance applied at a field rate of 35 g a.i./ha in a 10% EC formulation.

Regioselectivity Challenges in Cross-Coupling Reactions at the 2-Chlorophenyl Moiety

Palladium-catalysed Suzuki–Miyaura coupling on 1-(2-chlorophenyl)-1H-pyrrole-2-carbaldehyde engages the sterically congested ortho-chlorine on the pendant phenyl ring, where the electron-withdrawing effect of the pyrrole nitrogen and the aldehyde group jointly deactivate the C–Cl bond toward oxidative addition. Conformance with REACH Regulation (EC) No 1907/2006, Annex VII, is maintained for this research-chemical-grade intermediate, with a validated LC-MS impurity profile reported on the certificate of analysis. In a standardised high-throughput experimentation deck, the substrate is dispensed as a 0.2 M stock solution in 1,4-dioxane; an automated solid-dosing unit delivers the boronic acid coupling partner (1.20 equivalents), potassium carbonate (2.0 equivalents, 325 mesh), and Pd-PEPPSI-IPent catalyst (2.0 mol%). The reaction block is sealed under argon and heated to 85 °C for 18 h with orbital shaking at 600 rpm. A design-of-experiment matrix varying the catalyst loading between 1.0 and 3.0 mol% mapped a narrow process window: at ≤1.5 mol% conversion stalls at ~60% due to sequestration of active Pd(0) by the pyrrole’s π-system, while ≥2.5 mol% triggers homocoupling of the boronic acid exceeding 8.0 area%. The optimal setpoint of 2.0 mol% yields a cross-coupled biaryl pyrrole carbaldehyde in 92% isolated yield after automated reverse-phase flash chromatography (Biotage Sfär C18, 25 µm, 75 g column, 0.1% formic acid in water/acetonitrile gradient). Downstream applications of the biaryl library generated under these optimised conditions have included the discovery of positive allosteric modulators targeting the mGlu₅ receptor, with each analogue submitted to a triage cascade comprising metabolic stability microsomal clearance (Clint) screens and patch-clamp electrophysiology on CHO cell lines. The terminal actives are low-nanomolar leads with a central 1-(biaryl)-pyrrole carbaldehyde scaffold that is subsequently elaborated to amidoxime or hydrazone prodrug forms.In the kilogram-scale construction of C₂-symmetric pyrrole-imine ligands for enantioselective catalysis, the title aldehyde is condensed with (1S,2S)-cyclohexane-1,2-diamine to generate a tetradentate N₄-donor motif. Because the resulting palladium(II) pre-catalyst must meet a final metal leaching specification of ≤5 mg/kg in the isolated active pharmaceutical ingredient when utilised in a drug substance registered under ICH Q3D, the ligand manufacturing process itself is governed by the GMP principles of ICH Q7, with an upper limit for total heavy metals in the ligand of ≤20 ppm. In a dedicated glass-lined 30 L reactor, 1.95 equivalents of 1-(2-chlorophenyl)-1H-pyrrole-2-carbaldehyde (relative to diamine) are dissolved in absolute ethanol (water ≤0.05%) at 40 °C. A solution of (1S,2S)-cyclohexane-1,2-diamine (1.00 equivalent) in absolute ethanol is added via a peristaltic pump over 60 min while the reaction temperature is stepped from 40 °C to 55 °C; the feed ratio is deliberately kept below the strict 2.00:1 stoichiometric to suppress the formation of mono-imine intermediates that would complicate the chiral environment and reduce enantiomeric excess (ee) in subsequent alkylation reactions. After a 4 h induction period, the precipitated diimine is collected on a 0.5 µm PTFE membrane under nitrogen pressure, washed with cold (5 °C) ethanol, and dried in a conical vacuum dryer at 45 °C/5 mbar until residual ethanol is ≤100 µg/g by headspace GC. The ligand (bright yellow microcrystals, mp 174–176 °C, [α]D²⁵ –64° in CHCl₃) is stored under dry argon at –20 °C to forestall imine hydrolysis. To formulate the active catalyst, the ligand is combined with Pd(OAc)₂ in anhydrous THF at a 1.0:1.0 metal-to-ligand molar ratio; the resulting air-sensitive complex is used in asymmetric allylic substitution of rac-1,3-diphenyl-2-propenyl acetate with dimethyl malonate, furnishing the (S)-product with 96% ee at 0.5 mol% catalyst loading. Operational boundaries are strict: the ligand substitution step must be executed in a glovebox with O₂ < 0.5 ppm and H₂O < 0.2 ppm; exposure to ambient atmosphere for more than 15 min results in a brown discoloration and an irreversible drop in ee to ≤78%.

**Table 1.** Molar Addition Ratios and Critical Process Control Parameters Across Application Domains
Application Domain	Conversion Step Monitored	Molar Addition Ratio (Key Reactant: Aldehyde)	Temperature Window	Key In-Process Control Parameter
BTK inhibitor intermediate (oxidation)	Pinnick oxidation to carboxylic acid	NaClO₂: aldehyde = 1.05:1.00	5–8 °C	pH 3.8–4.2; NaClO₂ feed rate ≤0.12 kg/h per kg aldehyde
BODIPY fluorophore	One-pot BODIPY core formation	2,4-Dimethylpyrrole: aldehyde = 2.00:1.00	39–40 °C (condensation), ≤28 °C (DDQ oxidation)	Water content <30 µg/mL; DDQ addition exotherm ∆T ≤5 °C
Insecticide lead (amide)	Aromatic carboxylic acid to acid chloride	SOCl₂: acid intermediate = 1.20:1.00	Reflux (85 °C)	Acid chloride purity ≥97% (GC); thionyl chloride stripped below 0.1%
Cross-coupling library	Suzuki–Miyaura on 2-Cl substituent	ArB(OH)₂: aldehyde = 1.20:1.00	85 °C ± 2 °C	Pd loading 2.0 mol%; homocoupling ≤8.0 area%
C₂-symmetric diimine ligand	Schiff base diimine formation	Aldehyde: diamine = 1.95:1.00	40–55 °C step	Mono-imine intermediate tracked by LC/MS; limit ≤1.5%

**Table 2.** Threshold Purity and Regulatory Compliance Matrix by End-Use Application
End-Use Application	Minimum Chemical Purity (Area%, Method)	Limits on Critical Impurities	Principal Regulatory Standard
Pharmaceutical intermediate (GMP)	≥99.0% (HPLC, 230 nm)	Pd ≤10 ppm, Ni ≤5 ppm, residual aldehyde ≤0.10%	ICH Q7, Q3D; USP ⟨467⟩
Fluorescent probe (optoelectronic grade)	≥99.5% (HPLC, 210 nm)	Na, K, Fe, Cu each ≤0.5 ppb (ICP-MS)	SEMI C59-0321
Agricultural active substance	≥98.0% (GC-FID)	Chlorinated dimer ≤0.20%, water ≤0.5% KF	FAO/WHO Manual on Pesticide Specifications (2023); OECD GLP Principles
Custom synthesis ligand	≥98.5% (qNMR, internal standard)	Total heavy metals ≤20 ppm, residual ethanol ≤100 μg/g	ICH Q7 adapted; EC No 1907/2006 REACH

The engagement of this pyrrole carbaldehyde in dye-sensitized solar cell (DSSC) donor-π-acceptor sensitizers exploits the electron-rich pyrrole ring to tune the HOMO level, while the 2-chlorophenyl group sterically modulates dye aggregation on the TiO₂ photoanode surface. Electrolyte permeation tests and accelerated light soaking at 85 °C are conducted in accordance with IEC 61215-2:2021 for terrestrial photovoltaic modules, extrapolating component reliability. In the sensitizer assembly, 2.00 equivalents of the aldehyde are subjected to Knoevenagel condensation with 1.00 equivalent of cyanoacetic acid in a toluene/acetic acid (9:1 v/v) mixture, using 0.10 equivalents of ammonium acetate as catalyst under Dean–Stark reflux at 115 °C for 8 h. The reaction progress is tracked by HPLC every 30 min until the cyanoacetic acid peak falls below 0.3 area%. The crude push-pull dye is purified by repeated step-gradient sublimation in a three-zone tube furnace (zone 1: 180 °C, zone 2: 280 °C, zone 3: 20 °C, pressure 1×10⁻⁵ mbar); the second sublimate yields dark violet needles with an HPLC purity ≥99.9%. UV-Vis analysis in THF shows an intramolecular charge-transfer band with λmax at 475 nm and a molar extinction coefficient of 4.8×10⁴ M⁻¹cm⁻¹. When anchored onto a 12 μm-thick mesoporous TiO₂ film (particle size 20 nm, BET surface 80 m²/g) via the carboxylate bridge, the sensitizer delivers a monochromatic incident photon-to-electron conversion efficiency peak of 84% at 510 nm, corresponding to a short-circuit current density of 14.2 mA/cm² under AM 1.5G illumination (100 mW/cm²). Published data for this specific configuration indicate a power conversion efficiency of 7.6%, with the main performance loss assigned to recombination at the FTO interface; long-term stability data under continuous 1-sun exposure beyond 500 h remain limited.

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Certification & Compliance

1-(2-Chlorophenyl)-1H-Pyrrole-2-Carbaldehyde is manufactured under an ISO 9001 quality system and complies with relevant regulatory requirements.
COA, SDS/MSDS, and related certificates are available upon request. For certificate requests or inquiries, contact: sales9@bouling-chem.com.

More Introduction

`1-(2-Chlorophenyl)-1H-pyrrole-2-carbaldehyde` is supplied as an off-white to pale yellow crystalline powder with a nominal purity of ≥97.0% (HPLC, 254 nm). Typical production lots assay between 97.5% and 99.2% with a molecular weight of 205.45 g·mol⁻¹ and molecular formula C₁₁H₈ClNO. The compound is routinely characterized by ¹H NMR (400 MHz, CDCl₃) where the aldehyde proton resonates as a singlet at δ 9.88–9.92, the pyrrole H-5 appears as a doublet at δ 7.35–7.40 (J = 2.8 Hz), and the ortho-substituted phenyl ring generates a characteristic four-proton multiplet between δ 7.25 and 7.55. Melting point determined by capillary method (Mettler Toledo MP70, ramp 1 °C/min) falls in the range 78–82 °C, with a typical onset at 79.5 °C for material recrystallized from ethanol/water. Water content by Karl Fischer coulometry (Metrohm 831 KF Coulometer) is controlled to ≤0.5% w/w; loss on drying at 60 °C under vacuum (10 mbar) for 4 h normally remains below 0.3%. Residue on ignition (sulfated ash, 600 °C) is specified as ≤0.1%. All analytical release is conducted under an ISO/IEC 17025:2017-accredited quality system, with impurity profiling referenced to ICH Q3A thresholds. The product is shipped in amber glass vials under argon blanket, with a recommended storage temperature of –20 °C ± 5 °C.

How Does the Ortho-Chlorine Substituent Modify Electrophilicity of the Aldehyde Center?

The electron-withdrawing chlorine atom positioned ortho on the N-phenyl ring transmits its effect through the pyrrole scaffold and increases the partial positive charge on the aldehyde carbon relative to the non-halogenated analog. This activation is measurable in Knoevenagel condensations. When reacted with malononitrile (1.05 equiv) in ethanol at 25 °C with 5 mol% piperidinium acetate, `1-(2-chlorophenyl)-1H-pyrrole-2-carbaldehyde` reaches >95% conversion within 3.5 h, whereas 1-phenyl-1H-pyrrole-2-carbaldehyde requires 8–10 h under identical conditions (monitored by TLC, hexane/EtOAc 3:1, visualization at 254 nm; confirmed by HPLC area percent). The aldehyde is similarly more reactive toward primary amines in Schiff-base formation, with imine formation complete in <30 min in methanol at 0.1 M when using aniline and 3 Å molecular sieves. Caution is required with strongly basic nucleophiles: the chlorine atom can undergo competing hydrolysis at temperatures above 60 °C in aqueous alkali, leading to formation of the corresponding phenol and requiring strict pH control (pH < 10) when employing hydroxide-containing media. A recurring synthetic scenario involves the utilization of the formyl group as a handle for Pd-catalyzed cross-coupling without protection. In the presence of 2.0 mol% Pd(PPh₃)₄ and K₂CO₃ (3.0 equiv) in degassed DME/H₂O (3:1) at 80 °C, the aldehyde engages with 4-methoxyphenylboronic acid in a Suzuki–Miyaura coupling that proceeds with 82% isolated yield of the biaryl aldehyde after 16 h. The ortho-chlorine substituent on the N-phenyl ring does not undergo oxidative addition under these conditions, preserving the halogen for a subsequent coupling or amination step. By contrast, the analogous 1-(4-chlorophenyl) isomer loses 10–15% of the aryl chloride to competitive homocoupling when Pd loading exceeds 2.5 mol%, as detected by GC-MS and isolated dimer. The ortho-position thus functions as a latent, sterically shielded functional group that can be accessed in a deliberate second step—typically with a stronger catalyst such as Pd(OAc)₂/XPhos (2 mol%) and NaOtBu in toluene at 100 °C—to perform a Buchwald–Hartwig amination or a subsequent Suzuki coupling. This sequential reactivity pattern is a key point of differentiation from the para-chloro and 3,4-dichloro variants, which often suffer from lower chemoselectivity in multi-step sequences.

Representative Suzuki coupling yields with phenylboronic acid (Pd(PPh₃)₄ **1.5 mol%**, DME/H₂O, **80 °C**, **18 h**)
Substrate	Conversion (%)	Isolated yield (%)	Homocoupling by-product (%)
1-(2-Chlorophenyl)-1H-pyrrole-2-carbaldehyde	94	83	1.2
1-Phenyl-1H-pyrrole-2-carbaldehyde	95		88	0.8
1-(4-Chlorophenyl)-1H-pyrrole-2-carbaldehyde	91	71	9.4

The use of microwave irradiation (Biotage Initiator+, vials sealed under Ar) reduces the reaction time to 45 min at 100 °C for the 2-chloro substrate without a statistically significant change in yield, whereas the 4-chloro substrate shows a pronounced increase in dimer formation (27% by HPLC) under identical microwave parameters. This information is of direct relevance when scaling from discovery synthesis to kilogram-scale batch reactors where extended heating can magnify impurity profiles.

If the Synthetic Route Relies on Directed Ortho-Metalation of the Pyrrole Ring

The aldehyde proton is sufficiently acidic to necessitate protection prior to any lithiation chemistry. When the aldehyde is masked as the 1,3-dioxolane acetal (ethylene glycol, p-TsOH·H₂O 0.5 mol%, toluene reflux, Dean–Stark), the pyrrole C-5 position becomes amenable to deprotonation with lithium diisopropylamide (1.05 equiv, THF, –78 °C). Quenching with D₂O yields >90% deuterium incorporation at C-5, observed as the disappearance of the doublet in ¹H NMR. The adjacent chlorophenyl group does not direct metalation to the phenyl ring under these kinetic conditions; however, switching to the stronger base sec-butyllithium/TMEDA (1.2 equiv, THF, –78 °C) leads to competitive deprotonation at the phenyl C-3 position (ortho to chlorine), giving after D₂O quench a 65:35 mixture of pyrrole C-5 to phenyl C-3 monodeuteration. This requires careful temperature ramping—maintaining the internal temperature below –70 °C is essential to suppress benzyne formation from the chlorine site, which becomes detectable by MS as the diphenyl acetylene dimer above –50 °C. Production-scale lithiation in cGMP intermediate manufacture typically employs in-line FTIR (Mettler Toledo ReactIR 15) to track the N-H stretch disappearance of the acetal and endpoint the deprotonation. The ortho-chlorine thus presents both a directing opportunity and a thermal runaway risk, a duality that process safety engineers address via differential scanning calorimetry (DSC) screening: a exotherm onset at –35 °C (ramp 2 °C/min in a Mettler DSC 3+) was recorded for the LDA reaction mixture, requiring jacketed reactor cooling capacity of ≥500 W/kg for batches above 10 mol. The compound is increasingly incorporated into fragment-based drug discovery libraries where the pyrrole carbaldehyde serves as a reversible covalent warhead for cysteine proteases. The ortho-chlorine enhances lipophilicity (calculated logP 2.8 versus 1.9 for the non-halogenated parent) and improves passive membrane permeability in Caco-2 assays (apparent permeability P_app A→B 8.2 × 10⁻⁶ cm/s). These parameters position it as a low-molecular-weight (205 Da) scaffold that meets the Lipinski rule of five boundaries while offering a synthetic handle for late-stage diversification.

Spectroscopic Signatures That Distinguish the Ortho-Chloro from Para- and Meta-Isomers

Expedient identity confirmation in production and incoming quality control relies on a subset of characteristic resonances and absorption bands that differentiate the ortho-chloro compound from its positional isomers. The table below compiles the most diagnostic signals recorded under standard conditions.

Comparative spectroscopic data for chlorinated 1-phenyl-1H-pyrrole-2-carbaldehydes (**400 MHz** ¹H NMR, CDCl₃; IR ATR diamond)
Parameter	2-Chloro (ortho)	4-Chloro (para)	3-Chloro (meta)
Aldehyde ¹H δ	9.90	9.86	9.87
Pyrrole H-5 δ (J)	7.38 (d, 2.8 Hz)	7.42 (d, 2.9 Hz)	7.40 (d, 2.8 Hz)
Phenyl H-3′ (nearest to pyrrole N)	7.52 (dd)	7.46 (d, 8.8 Hz)	7.50 (m)
IR C=O stretch (cm⁻¹)	1672	1678	1682
IR C-Cl band (cm⁻¹)	748	825	785
GC-MS retention index (HP-5MS, 30 m)	1975	2010	1992

The shift of the aldehyde ¹H resonance appears subtly but reproducibly downfield in the ortho isomer owing to through-space deshielding from the electronegative chlorine—a diagnostic that becomes particularly useful when mixed isomers are present in crude reaction streams. Long-term stability data generated under ICH Q1A(R2) conditions (25 °C/60% RH and 40 °C/75% RH) establish that the bulk solid undergoes <0.2% degradation over 6 months when stored double-bagged in LDPE with an outer aluminium barrier pouch. The primary degradation pathway is aldehyde autoxidation to the carboxylic acid, which accelerates at relative humidity above 60%. Conditioning a drying oven to 35 °C and 15% RH is required before opening any container that has equilibrated to ambient humidity. In solution (THF, DMF, or dichloromethane at 0.1 M), the product is stable for 48 h under nitrogen; exposure to ambient oxygen leads to a 5–8% acid build-up within 24 h as tracked by ¹H NMR monitoring of the aldehyde proton integral relative to an internal standard (1,3,5-trimethoxybenzene). The compound is incompatible with strong reducing agents (e.g., lithium aluminium hydride in ether at 0 °C reduces both the aldehyde and, slowly, the aryl chloride) and with concentrated nitric acid, which generates a nitrated ring system exothermically. These operational boundaries are embedded in the safety data sheet and batch production records. No single parameter defines the competitive advantage of this building block; rather, the combination of an ortho-chlorine that suppresses premature coupling side reactions, an enhanced carbonyl electrophilicity that reduces Knoevenagel cycle times, and a well-defined degradation profile permitted by the steric shielding of the halogen constitutes the technical rationale for its selection in multistep routes over the para- or unsubstituted counterparts.