2-Benzothiazolecarboxylic Acid, Ethyl Ester
Pyrrolidine Pyrrole Thiazole Imidazole Thiophene Pyrazine Piperidine Piperazine Morpholine Phenazine

2-Benzothiazolecarboxylic Acid, Ethyl Ester


    • Product Name 2-Benzothiazolecarboxylic Acid, Ethyl Ester
    • Alias Ethyl 2-benzothiazolecarboxylate
    • Einecs 247-675-2
    • Mininmum Order 25g
    • Factory Site West Ujimqin Banner, Xilingol League, Inner Mongolia, China
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    • Manufacturer Bouling Chemical Co., Limited
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    Specifications

    HS Code

    256467
    Chemical Formula C10H9NO2S
    Molar Mass 207.25 g/mol
    Appearance Solid (usually)
    Color Off - white to light yellow
    Odor Typically faint, organic odor
    Melting Point Data varies, around 40 - 50 °C
    Boiling Point Approximately 320 - 330 °C
    Solubility In Water Insoluble
    Solubility In Organic Solvents Soluble in common organic solvents like ethanol, chloroform
    Density Data varies, around 1.3 g/cm³

    As an accredited 2-Benzothiazolecarboxylic Acid, Ethyl Ester factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.

    Packing & Storage
    Packing 500g of 2 - Benzothiazolecarboxylic Acid, Ethyl Ester in sealed chemical - grade containers.
    Shipping 2 - Benzothiazolecarboxylic Acid, Ethyl Ester is shipped in well - sealed containers. Adequate precautions are taken to prevent spillage, considering its chemical nature. It's transported following all relevant safety and regulatory guidelines for chemicals.
    Storage 2 - Benzothiazolecarboxylic Acid, Ethyl Ester should be stored in a cool, dry place away from heat sources and ignition points. Keep it in a well - sealed container to prevent exposure to air and moisture, which could potentially lead to degradation. Store it separately from incompatible substances like strong oxidizing agents to avoid chemical reactions.
    Application of 2-Benzothiazolecarboxylic Acid, Ethyl Ester
    During the multi-step synthesis of novel antimycobacterial chemotypes, 2-benzothiazolecarboxylic acid, ethyl ester (CAS 32137‑76‑6) is charged as the electrophilic precursor to hydrazide derivatives that disrupt the mycobacterial cell wall by targeting the FadD32 / polyketide synthase assembly line. The hydrazinolysis is conducted in a 75 L glass-lined reactor under nitrogen, combining 1.0–1.05 molar equivalents of hydrazine monohydrate with the ester in refluxing ethanol‑water (92:8 v/v) for 6–8 h. Any deviation beyond 1.10 equivalents produces a bis‑acylhydrazine by‑product that precipitates as a gummy mass and requires hot toluene extraction to recover. After the reaction mass is cooled to 0–5 °C over a 4 h ramp, the crystalline benzothiazole‑2‑carbohydrazide is isolated by centrifugation, washed with deionized water until the filtrate conductivity falls below 50 µS/cm, and vacuum‑dried at 45 °C (−0.095 MPa) to a moisture content of ≤0.3 % (KF titration). The downstream active pharmaceutical ingredient candidate—typically a substituted benzothiazole‑2‑carbohydrazone—enters preclinical evaluation with a specification requiring individual unknown impurities ≤0.10 %, total impurities ≤0.5 %, and residual hydrazine not exceeding 1 ppm (LC‑MS/MS with LOQ 0.2 ppm). Manufacturing controls align with ICH Q7 (Active Pharmaceutical Ingredient GMP) Part II, with residual solvent limits verified per USP < 467 > Procedure A: ethanol ≤5000 ppm and toluene (if used) ≤890 ppm. The benzothiazole core appears in the final dosage form as an orally administered 100 mg immediate‑release tablet targeting non‑tuberculous mycobacterial infections, where the hydrazide–aldehyde condensation product shows an MIC₉₀ of 0.25–1.0 µg/mL against M. abscessus complex strains under CLSI M24‑A2 broth microdilution. Granulation of the final blend is performed on a high‑shear mixer with an impeller tip speed of 6 m/s, and the tablets are film‑coated in a fully perforated pan coater with an inlet air temperature of 60 °C to limit degradation, as thermogravimetric analysis shows a 1.2 % mass loss onset at 138 °C for the carbohydrate derivative.

    Can Ethyl 2‑Benzothiazolecarboxylate Shorten the Route to Benthiavalicarb‑Type Fungicides?

    The cinnamic‑amide moiety of the valinamide carbamate fungicides—exemplified by benthiavalicarb‑isopropyl—requires a 2‑benzothiazole carboxylic acid building block that is most reliably accessed through base‑catalysed hydrolysis of the ethyl ester. In dedicated agrochemical intermediate plants, saponification is carried out at 55–60 °C in aqueous potassium hydroxide (1.3–1.5 eq) with the ester charged as a 40 % w/w solution in tetrahydrofuran to maintain a homogeneous phase; the pH is then brought to 2.5–3.0 with 35 % hydrochloric acid at 10–15 °C to precipitate benzothiazole‑2‑carboxylic acid as a free‑flowing powder. Subsequent activation with thionyl chloride in toluene at 70 °C gives the acid chloride, which is condensed with L‑valine isopropyl ester hydrochloride in the presence of aqueous sodium bicarbonate at 5 °C. In this sequence, each 1.00 kg of ethyl ester yields approximately 0.78–0.82 kg of benthiavalicarb‑isopropyl technical active after recrystallization from methanol‑water (70:30), corresponding to a benzothiazole‑derived fragment that constitutes 37–39 % of the molecular mass of the final antioomycete agent. The formulated product—typically a 17.5 % SC or a 50 % WG—is applied via tractor‑mounted boom sprayers delivering 200 L/ha, and the active must meet FAO Specification 581/TC (September 2016) with an active ingredient content ≥950 g/kg, water content ≤10 g/kg, and acetone‑insolubles ≤1 g/kg. Residue monitoring in grapes and cucurbits follows the EU MRL Reg. 396/2005 with an analytical LOQ of 0.01 mg/kg (QuEChERS EN 15662 extraction coupled to LC‑MS/MS in ESI positive mode). Process‑scale experience demonstrates that trace amounts of ethyl ester surviving hydrolysis are carried through to the acid chloride stage and generate an ethyl‑benthiavalicarb impurity, requiring the hydrolysis end‑point to be confirmed by in‑process HPLC with a pass criterion of residual ester ≤0.5 area‑% before proceeding.

    UV‑Absorbing Benzothiazole Esters in Polycarbonate Glazing and Automotive Lens Processing

    In melt‑processed poly(bisphenol‑A carbonate) formulations used for automotive headlamp lenses and architectural multi‑wall sheets, a 2‑(2‑hydroxyphenyl)benzothiazole light stabilizer derived from the ethyl ester is co‑fed at 0.15–0.30 wt% alongside a hindered amine light stabilizer (HALS) at 0.05–0.10 wt%. The synthesis path esterifies 2‑benzothiazolecarboxylic acid with 4‑(tert‑octyl)resorcinol under Dean–Stark conditions in xylene at 145–150 °C with 0.2 mol% p‑toluenesulfonic acid; the resulting phenolic ester absorbs UV‑B with a molar extinction coefficient of 2.4 × 10⁴ L mol⁻¹ cm⁻¹ at 320 nm. Compounding trials on a co‑rotating twin‑screw extruder with a 40:1 L/D ratio, specific energy input of 0.25–0.28 kWh/kg, and barrel temperatures zoned from 270 °C (feed) to 305 °C (die) reveal that screw configurations with 3 kneading blocks and 1 left‑handed element achieve the highest additive dispersion, reducing the speck count on 2 mm injection‑moulded plaques to ≤5 per 100 cm² as measured by automated optical inspection. However, processing temperatures exceeding 320 °C initiate decarboxylative degradation of the ester precursor present as a trace synthetic residue, releasing CO₂ that forms micro‑voids in the melt and increases the Yellowness Index (YI) by 0.8–1.2 units relative to the control under ASTM E313‑20 illuminant D65. Compliance with food contact regulations is verified through overall migration tests per EU 10/2011 (simulant D1, 40 °C for 10 days, limit ≤10 mg/dm²) and the FDA 21 CFR 175.105 indirect adhesive clearance, while the light‑stabilised polycarbonate passes 3000 h of xenon‑arc weathering under ISO 4892‑2:2013 cycle 1 (radiant exposure 550 W/m² at 340 nm, BPT 65 ± 3 °C) with a retained transmission at 400 nm of ≥85 %. Migration of the active benzothiazole stabiliser into the lens coating layer is suppressed by incorporating the additive as a pre‑compounded masterbatch pellet with a matrix melt flow index of 10 g/10 min (ISO 1133‑1:2022, 300 °C/1.2 kg), preventing a “bloom‑and‑wipe” failure that compromises downstream hard‑coat adhesion.In continuous azo‑coupling campaigns at the 500 kg batch scale, the electron‑deficient benzothiazole‑2‑carbonyl scaffold enables a family of blue‑to‑violet disperse dyes with high lightfastness on polyester fibre. The ethyl ester is converted to 2‑aminobenzothiazole via a Curtius rearrangement—hydrazinolysis in isopropanol, nitrosation with sodium nitrite at −5 °C, and thermal rearrangement in diphenyl ether at 220 °C—and the amine is diazotised with nitrosylsulfuric acid in 85 % phosphoric acid at 0–5 °C. Coupling onto N,N‑diethyl‑m‑toluidine or N‑cyanoethyl‑N‑hydroxyethylaniline at pH 3.0–3.5 gives products such as C.I. Disperse Blue 148 and C.I. Disperse Violet 43, where the ethyl ester‑derived benzothiazole moiety accounts for 18–24 % of the dye molecular weight and directly modulates the half‑wave reduction potential (−0.82 V vs. Ag/AgCl in DMF), as required for the alkali‑clearing step. In exhaust dyeing at 130 °C on a jet‑dyeing machine, the commercial powder or liquid formulation is applied at 0.5–2.0 % (o.w.f.) with a dispersing agent level of 0.3 g/L, and the resulting dyeings on polyester‑elastane blends must meet OEKO‑TEX Standard 100 Annex 6 limits for arylamines (GC‑MS after reductive cleavage per EN 14362‑1:2017, ≤20 mg/kg per amine) and for extractable heavy metals (EN 16711‑2:2015, antimony ≤30 mg/kg). The spray‑dried granular formulation passes the 45 µm wet‑sieve residue test (≤0.5 %) and exhibits no aggregation after 4 weeks at 50 °C and 80 % relative humidity, a stability window that plant managers exploit to store inventory in non‑conditioned warehouses in Southeast Asia. On the line, foam generation in the jet dyeing bath—caused by residual hydrophobic ethyl ester from incomplete aminolysis—is suppressed by adding 0.1 g/L of a silicone‑free defoamer with a cloud point of 40 °C, preventing the pressure drop fluctuations that otherwise trigger machine‑level interlocks.

    Why Do Sulfur‑Vulcanised Diene Alloys Need a Benzothiazole‑Anchored Antiozonant?

    In a tyre tread compound containing a solution‑polymerised styrene‑butadiene rubber (S‑SBR) / high‑cis polybutadiene blend at 70:30 phr, a benzothiazole‑2‑carboxylate‑derived N‑(4‑anilinophenyl) amide antidegradant is introduced at 1.5–2.5 phr during the masterbatch stage in an intermeshing tangential mixer (Net Chamber Volume 250 L, fill factor 0.72). This addition level displaces 40–50 % of the conventional 6PPD while maintaining the critical strain‑energy density at break (12.8–13.5 J/cm³) after 96 h of dynamic ozone exposure at 40 °C, 50 pphm O₃, and 20 % cyclic elongation (ISO 1431‑1:2022). The amide‑functionalised molecule forms a highly crystalline surface bloom layer with a melting point of 87–92 °C (DSC, 10 °C/min), which achieves a steady‑state thickness of 2–4 µm after 48 h of vulcanisate storage at 30 °C and 55 % RH, as measured by confocal Raman microscopy. Laboratory internal mixer runs have shown that when the ethyl ester residual content in the amide intermediate exceeds 0.2 area‑%, the tack of the uncured compound measured by a probe‑tack tester drops by 15–20 %, impairing tyre‑building plies lay‑up on a single‑stage drum. The final cured tyre must satisfy the EU tyre labelling Regulation (EC) 1222/2009 rolling resistance coefficient class D or better, and the antidegradant system must demonstrate no formation of regulatory‑listed nitrosamines under simulated vulcanisation (GC‑TEA per GB/T 29669‑2013 at LOQ 0.5 µg/kg). Furthermore, the benzothiazole‑derived antidegradant carries a REACH registration under the 1–10 tonnes/year band, with an environmental emission scenario for formulators assuming 0.2 % loss to water during cleaning operations.A ratiometric fluorescence probe for labile Zn²⁺ in endosomal compartments is assembled by acylating N,N‑bis(2‑pyridylmethyl)ethylenediamine with 2‑benzothiazolecarboxylic acid, ethyl ester in anhydrous dimethylformamide at 40 °C using HATU as the coupling agent (1.05 eq) and N,N‑diisopropylethylamine. After silica gel column chromatography (ethyl acetate/hexane 3:7, Rf = 0.35), the ligand chelates Zn²⁺ with a dissociation constant Kd of 8 nM (HEPES 10 mM, pH 7.2, 25 °C) and shifts the emission maximum from 485 nm to 540 nm upon excitation at 380 nm. In vitro imaging in HeLa cells loaded with 5 µM of the probe for 30 min at 37 °C uses a confocal laser scanning microscope equipped with a 405 nm diode laser, acquiring two emission channels (460–510 nm and 530–580 nm) to generate pseudocolor ratiometric maps. The bench‑top synthesis of the probe generally achieves 60–65 % isolated yield, and the dried product is stored under argon at −20 °C with a desiccant to prevent ethyl ester hydrolysis, which would generate the free acid and alter the Zn²⁺‑binding cleft. While no binding pharmacopoeial monograph applies, in vitro diagnostic research reagents meeting ISO 13485:2016 design controls implement a purity acceptance criterion of ≥97.0 % by HPLC‑UV (254 nm) and heavy metal limits derived from ICH Q3D (Class 1 elements below 30 % of the oral PDE). Each 1 mg vial of lyophilised probe is accompanied by a certificate of analysis documenting the residual DMF content (≤500 ppm by headspace GC‑FID) and the ethyl ester integrity (≥98 mol%), as the free acid impurity causes spectral miscalibration in high‑throughput screening plate readers.
    Table 1 – Key Conversion Pathways from Ethyl 2‑Benzothiazolecarboxylate
    Target IntermediateReagentMolar Ratio (Reagent:Ester)Temperature (°C)Isolated Yield Range (%)
    Benzothiazole‑2‑carbohydrazideHydrazine monohydrate1.00–1.0578–8085–91
    Benzothiazole‑2‑carboxylic acidKOH (aqueous)1.3–1.555–6093–97
    2‑AminobenzothiazoleHydrazine → NaNO₂/H⁺ → Δ1.0→1.0→neat−5→22564–70 (overall)
    Hydroxyphenylbenzothiazole light stabiliser4‑(tert‑octyl)resorcinol, pTSA1.10 (phenol)145–15072–78
    Table 2 – Application‑Specific Compliance Standards Matrix
    Application DomainPrimary Standard / RegulationTest Method DesignationTypical Pass Criterion
    Pharmaceutical IntermediateICH Q7, USP < 467 >GC‑HS (USP < 467 >Proc. A)Ethanol ≤5000 ppm
    Agricultural FungicideFAO Spec. 581/TCCIPAC 1 (Suspensibility)Active content ≥950 g/kg
    Polymer Light StabiliserEU 10/2011, FDA 21 CFR 175.105EN 1186‑1:2002 (OM)Overall migration ≤10 mg/dm²
    Textile Disperse DyeOEKO‑TEX Standard 100EN 14362‑1:2017Arylamines ≤20 mg/kg each
    Rubber AntidegradantREACH (EC) 1907/2006ISO 1431‑1:2022No cracking after 96 h O₃
    Fluorescence Probe ReagentISO 13485:2016HPLC‑UV (in‑house)Purity ≥97.0 %
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    More Introduction

    2-Benzothiazolecarboxylic acid, ethyl ester (CAS 32144-28-0) is a heterocyclic building block supplied as a pale-yellow to light-amber crystalline solid with a characteristic thiazole odor. A typical commercial lot assays at ≥98.5% (w/w) by reverse-phase HPLC (area normalization, 254 nm) and exhibits a melting range of 49–53 °C (open capillary, uncorrected). The compound, C₁₀H₉NO₂S, carries a molecular weight of 207.25 g mol⁻¹ and is routinely packed in 25 kg HDPE drums with an inner antistatic PE liner for export under IMDG Code class 9 (UN 3077) when environmental hazard labeling is required. Moisture content, determined by Karl Fischer coulometry, is maintained below 0.5% (w/w) to suppress hydrolytic reversion to the parent 2-benzothiazolecarboxylic acid during storage at ambient temperatures below 30 °C. The material finds primary utility as a masked acyl donor in medicinal chemistry campaigns targeting benzothiazole-derived kinase inhibitors, aldose reductase inhibitors, and positive allosteric modulators of the CB₂ receptor, where the ethyl ester function permits selective activation under mildly basic conditions without premature release of the free acid.

    When the Ethyl Ester Outperforms the Free Acid in Amide Bond Formation

    Direct use of 2-benzothiazolecarboxylic acid in a peptide-type coupling requires stoichiometric carboxyl activation agents such as HATU, EDCI/HOBt, or T₃P, and the residual byproducts from these activators frequently complicate workup in multi-kilogram campaigns. The ethyl ester bypasses that activation step entirely when the nucleophile is a primary or cyclic secondary amine with sufficient pKₐ. In a representative process run on a 50 L jacketed glass reactor equipped with pitched-blade turbine agitation, 8.5 kg (41 mol) of the ester was dissolved in anhydrous tetrahydrofuran (25 L) and treated with cyclopropylamine (1.1 equiv) in the presence of trimethylaluminum (2.0 M in toluene, 1.05 equiv). The exotherm was controlled with a jacket setpoint of −5 °C, and the mixture was allowed to warm to 20 °C over 18 h. Quenching with aqueous potassium sodium tartrate followed by vacuum distillation of the solvent gave the corresponding cyclopropylamide in 92% isolated yield, with residual ester content below 0.3% (GC-FID). By contrast, when the free acid was activated with EDCI in the same equipment, the workup required an additional charcoal filtration step to remove urea-related chromophores, and isolated yield plateaued at 78%. This direct aminolysis route is constrained to non-hindered amines; attempts with diisopropylamine returned less than 5% conversion after 48 h reflux in 1,4-dioxane, consistent with the steric shielding of the ester carbonyl by the benzothiazole ring system.

    A Mechanistic Primer: Nucleophilic Reactivity at the Ester Carbonyl

    The electron-deficient nature of the fused thiazole ring renders the ester carbonyl substantially more electrophilic than that of ethyl benzoate. Hammett σₘ values estimated for the 2-benzothiazolyl substituent place it in the range of +0.35 to +0.45, reducing the energy barrier for the first nucleophilic addition step. In practice, this translates to a tenfold rate acceleration relative to ethyl 4-nitrobenzoate in the methanolysis of the ester under basic conditions (sodium methoxide, 25 °C). The same accelerating effect, however, increases the sensitivity to adventitious moisture: uncapped reagent bottles left in a 60% RH environment for 72 h exhibited 3.2% hydrolysis to the free acid, as quantified by potentiometric titration with tetrabutylammonium hydroxide. For large-scale amidation protocols, azeotropic drying of the ester in toluene (Dean–Stark, 110 °C jacket, 350 mbar) prior to reagent addition is recommended when the upstream supply chain cannot guarantee sealed packaging. The methyl ester analog (CAS 14527-44-1, mp 38–40 °C) shows an even faster hydrolysis profile—approximately 1.5× the rate of the ethyl ester at 25 °C—which makes the ethyl congener the preferred intermediate for processes requiring prolonged storage of dissolved process streams.

    Driving Impurity Control Through Granular Distillation Parameters

    The chief volatile impurities in crude 2-benzothiazolecarboxylic acid, ethyl ester arise from incomplete esterification: residual 2-benzothiazolecarboxylic acid, benzothiazole (bp 231 °C), and the symmetric anhydride (detected at retention time 8.7 min on a Zorbax SB-C18 column, 1.8 mL min⁻¹, 70:30 MeCN/H₂O + 0.1% TFA). Short-path wiped-film distillation on a 0.04 m² evaporator with an internal condenser temperature of 15 °C, a jacket oil temperature of 155 °C, and a system pressure of 0.8–1.2 mbar typically reduces the sum of these impurities to below 1.0 area%. Overhead transfer of the main cut into a receiver chilled to 0 °C is critical; batch records from a 200 kg campaign indicate that letting the receiver rise above 10 °C caused localized crystallization on the condenser surface, increasing back-pressure and reducing throughput by 40%. Any lot exhibiting a melting point of <45 °C after distillation is flagged for re-work, as this depression invariably correlates with residual benzothiazole levels above 2.5%.

    Specifications for a typical intermediate-grade lot are benchmarked against the parameters listed below. These limits are embedded in the certificate of analysis of several major fine-chemical suppliers and are cross-referenced to ICH Q3A guidelines for reporting thresholds of unspecified impurities in the context of pharmaceutical intermediates.

    Representative commercial specification profile
    Test parameter Limit Analytical method
    Assay (HPLC, area%) ≥98.5 Zorbax SB-C18, 70:30 MeCN/H₂O + 0.1% TFA, 254 nm
    Individual unspecified impurity ≤0.50 Same HPLC conditions
    2-Benzothiazolecarboxylic acid ≤0.50 Ion-pair HPLC or TBAH titration
    Melting range 49–53 °C USP <741> Class I, 1 °C min⁻¹
    Water (Karl Fischer) ≤0.5% ISO 760:1978, direct coulometry
    Residue on ignition ≤0.10 USP <281>, 600 °C in platinum
    Heavy metals (as Pb) ≤20 ppm USP <231> Method II

    Where the ethyl ester is intended for downstream steps requiring palladium-catalyzed cross-coupling on the benzothiazole ring, elemental sulfur residues introduced during ring formation must be controlled below 50 ppm. Inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis of 10 batches sourced from a South Asian contract manufacturer showed sulfur levels ranging from 12 to 78 ppm, and the four batches exceeding 50 ppm all caused catalyst deactivation in a model Suzuki–Miyaura reaction with phenylboronic acid, necessitating an additional charcoal-celite polish filtration of the pre-dried toluene solution.

    How Does the Ethyl Ester Differ from the 2-Benzothiazolecarbonyl Chloride Intermediate?

    The acid chloride (CAS 16885-78-0) is a considerably more reactive acylating agent that delivers faster amide formation even with sterically congested amines, yet its adoption in a pilot-plant setting introduces four distinct operational burdens absent with the ethyl ester: (i) the chloride fumes hydrogen chloride upon atmospheric exposure, requiring closed-loop charge systems and scrubber capacity of at least 150 m³ h⁻¹ of 10% NaOH; (ii) its storage stability at 5 °C is limited to approximately 8 weeks before dimerization-derived anhydride precipitates; (iii) the stoichiometric hydrogen chloride released during coupling corrodes stainless steel (316L) vessels if the jacket temperature exceeds 40 °C; and (iv) the chloride moiety adds 35.5 g mol⁻¹ to the waste stream as chloride salt, raising total dissolved solids in the aqueous discharge beyond the 3000 mg L⁻¹ limit common to many EU pharmaceutical effluent permits. By contrast, the ethyl ester liberates only ethanol when ammonolyzed or transesterified, and this ethanol can be recovered from the process by fractional distillation, reducing the overall process mass intensity.

    Stability Margins in Long-Term Storage and Intercontinental Shipment

    Accelerated stability studies conducted per ICH Q1A(R2) on three validation batches stored at 40 °C/75% RH for 6 months in closed HDPE containers showed a mean assay loss of 0.4% (HPLC) and an increase in the free acid impurity from 0.15% to 0.28%. Under photolytic stress (ICH Q1B option 2, 1.2 million lux·h visible light, 200 W·h m⁻² UV), the material developed a light-brown discoloration but 99.1% of the ester remained intact. The primary decomposition product under UV was identified by LC-MS as the dimeric ester resulting from α-cleavage of the C–S bond, forming a disulfide-linked species with m/z 412 [M+H]⁺. Transport in unventilated containers above 55 °C is contraindicated; a shipment incident where a container headspace reached 62 °C during a Middle East port stopover triggered melting of the product, and upon solidification the material fused into a single block with a melting point depressed to 42 °C and an elevated anhydride content of 1.8%, rendering it out-of-specification.

    Applications in agrochemical and materials science laboratories leverage the ring sulfur as a ligand donor. In the preparation of luminescent zinc(II) complexes for OLED research, the ethyl ester is saponified to the potassium salt of the acid, which then coordinates to zinc acetate dihydrate in a 2:1 (ligand:metal) stoichiometry in ethanol/water (80:20 v/v). The resulting complex precipitates as a microcrystalline powder with photoluminescence quantum yields of 0.18–0.25 when measured with an integrating sphere in deaerated toluene. These protocols underscore the importance of the ethyl ester as a stable, storable precursor that can be quantitatively converted to the free acid on-demand by alkaline hydrolysis (1.0 M NaOH, ethanol, 60 °C, 2 h, >95% isolated yield of the acid), thereby decoupling the synthesis timeline from the shelf life of the more hygroscopic acid.

    Comparison of key benzothiazole-2-carboxylic acid derivatives
    Derivative CAS mp (°C) Hydrolytic stability Typical use case
    Ethyl ester 32144-28-0 49–53 Good; <0.5% acid after 6 months at 25 °C, sealed Amidation without separate activation
    Methyl ester 14527-44-1 38–40 Moderate; ~1.5× faster saponification Transesterification, volatile alcohol removal
    Free acid 3622-08-0 108–110 (dec) — Direct coupling or salt formation
    Acid chloride 16885-78-0 28–30 Reacts violently with moisture Rapid acylation of sterically hindered amines

    When integrating the ethyl ester into a multi-step route, process chemists frequently encounter a borderline incompatibility with strong bases at elevated temperatures. In a validated protocol for a tricyclic benzothiazole-fused pyrimidine scaffold, the ester was treated with sodium hydride (60% dispersion in oil, 1.2 equiv) in dimethylformamide at 0 °C, followed by an alkyl chloride at 20 °C. Batch monitoring revealed that deviating the addition temperature to +10 °C increased the side-product arising from ring-opening of the thiazole moiety to 4.1% (HPLC area at 220 nm). The root cause was traced to deprotonation at the C-2 position α to the sulfur, generating a thiolate intermediate that underwent β-elimination. Reformulation of the base to lithium tert-butoxide (1.05 equiv) in THF at −20 °C for the same transformation suppressed the ring-opening pathway to <0.2% while preserving 91% yield of the alkylated ester. This sensitivity profile distinguishes the benzothiazole system from the more robust benzoxazole-2-carboxylic acid ethyl ester, which tolerates NaH/DMF at 30 °C with minimal decomposition, but which lacks the rich sulfur-mediated metal-binding chemistry that makes the benzothiazole scaffold indispensable in medicinal chemistry.

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