6-Nitrobenzo[D]Thiazole

In the preparation of C.I. Disperse Red 177, a bluish-red dye formulated specifically for exhaust dyeing of regular-tenacity polyester yarn at 130–135°C and autoclave overpressure of 2.2–3.0 bar, the diazo precursor 2-amino-6-nitrobenzo[d]thiazole is synthesized from the parent 6-nitrobenzo[d]thiazole via a two-step sequence involving chlorination in molten sulfur at 130°C using 1.05 equivalents of sulfuryl chloride, followed by ammonolysis with 25% aqueous ammonia in a 2500 L autoclave at 110°C and 0.4 MPa for 6 h, yielding the intermediate at 78–84% purity after crystallization from methanol. A subsequent diazotization is run in a jacketed glass-lined kettle of 5000 L working volume, where the 2-amino-6-nitrothiazole is suspended in 3.5 equivalents of 31% hydrochloric acid, cooled to −2°C by a glycol chiller, and treated dropwise with 40% sodium nitrite solution at a rate not exceeding 1.8 L/min to maintain the batch temperature strictly below +3°C; any excursion above +5°C causes a measurable exothermic decomposition of the diazonium salt, reducing coupling yield by 6–10% and generating insoluble tar that clogs the 25 µm polypropylene filter cloths in the downstream diaphragm filter press. Coupling is carried out at pH 3.2–3.8, buffered with sodium acetate trihydrate, against N-ethyl-N-cyanoethylaniline charged at 1.03 molar equivalents; the reaction mass is stirred for 8 h at 8–12°C, then filtered and washed to conductivity <200 µS/cm. The wet presscake is reslurried with deionized water and microdispersed in a horizontal bead mill (Netzsch LME 120, 0.4–0.6 mm yttria-stabilized zirconia beads, 85% fill, rotor tip speed 10 m/s) together with a sodium lignosulfonate dispersant at 1.8:1 w/w relative to dye solids, achieving a final particle size D₉₀ of ≤1.2 µm as verified by laser diffraction (Malvern Mastersizer 3000). After spray drying at inlet 180°C/outlet 75°C, the commercial powder is standardized to 200% strength against a primary reference. For every 1000 kg of finished C.I. Disperse Red 177 powder, approximately 395–430 kg of 6-nitrobenzo[d]thiazole is consumed through the derivatization pathway, depending on the efficiency of the chlorination-ammonolysis sequence. The final dye must comply with ZDHC MRSL v3.1, absence of the restricted arylamines listed in Regulation (EC) 1907/2006 Annex XVII Appendix 8, and chlorophenol content below 5 ppm per DIN 54232. Exhaust dyeing reproducibility tests on a Fong’s Jumboflow 4T machine at liquor ratio 1:8 confirm a color difference ΔE_CMC(2:1) of <0.6 among batch lots when the dyeing profile ramp rate is held at 1.5°C/min.

Standard formulation of a disperse dye for alkaline one-bath scouring-dyeing of woven polyester requires the diazo component to survive pH 9.2±0.3 at 130°C for 45–60 min without azo bond hydrolysis exceeding 3%, a condition under which conventional cyanoethyl-based coupling fragments degrade detectably. By replacing the N-cyanoethyl-N-ethylaniline coupler with 3-(N,N-diethylamino)acetanilide and employing 2-amino-6-nitrobenzo[d]thiazole as the electrophilic partner, process chemists obtain a red-violet disperse dye whose log P remains above 3.8 and whose hydrolytic half-life in a buffered phosphate‑carbonate system at 98°C/pH 9.5 is extended to 72 h, measured by HPLC peak area reduction (Agilent 1260 Infinity, C18 column, detection at 530 nm). The upstream activation of 6-nitrobenzo[d]thiazole into the 2-amino analogue must be conducted under a dry nitrogen atmosphere because the nitro group makes the thiazole ring susceptible to nucleophilic ring-opening when moisture exceeds 200 ppm in the chlorination melt. On a 10,000 L batch scale, the alkaline-resistant dye synthesis consumes 410–450 kg of the title compound per metric ton of final product; the coupling is performed in an ice-brine-cooled stainless steel reactor at +1 to +3°C over 12 h, followed by solvent extraction of residual coupling component with ethyl acetate until the level drops below 0.2% GC area. All dispersions intended for alkaline processing must be formulated with a naphthalenesulfonate-formaldehyde condensate dispersant that does not flocculate at pH >9, verified by a stability test per ISO 105-Z08:1995 (filtration time increase <10% after 4 h at 80°C). The finished dye, sold as C.I. Disperse Red 356 liquid grade, satisfies textile eco-label requirements of OEKO-TEX Standard 100 Product Class II, bluesign BSSL 3.1 Section 5.2, and the Inditex Clear to Wear list; production records from continuous dyeing ranges (Monforts Thermex 5000) confirm that when the pad liquor contains 20 g/L dye, 2 g/L wetting agent, and 1 g/L organic acid donor, the color yield (K/S at λmax) on hydrolytically weight-reduced polyester remains within ±3% of the value obtained with standard acidic formulation, while eliminating the separate pre-scour step and reducing total water consumption by 14 L/kg fabric.

When Lightfastness Exceeds ISO 105-B02 Grade 6—How Does the Nitro Substituent Alter Photostability in Thiazole Disperse Dyes?

Automotive seating fabrics composed of polyester microfiber require a composite fastness envelope that includes not merely high sublimation resistance (staining ≥4-5 per ISO 105-P01:1993 at 150°C) but also extended xenon-arc exposure stability. The introduction of a nitro group on the benzo[d]thiazole scaffold raises the electron deficiency of the heterocycle, shifting the absorption of the resultant azo dye hypsochromically by 14–22 nm compared to the non-nitrated analogue and simultaneously increasing the first-order photodegradation rate constant k₁ in polyester film from 2.1×10⁻⁴ min⁻¹ to 5.8×10⁻⁴ min⁻¹ under Atlas Ci4000 Xenotest conditions (black panel temperature 63°C, 300–400 nm irradiance 60 W/m²). To counteract this liability, the synthesized 2-amino-6-nitrobenzo[d]thiazole is coupled to a heavily substituted N-ethyl-2-hydroxyethylaniline bearing a terminal UV-absorber moiety, typically a pendant 2-hydroxybenzotriazole, so that the finished C.I. Disperse Red 374 molecule exhibits an internal energy dissipation pathway. On the manufacturing side, the reduction of 6-nitrobenzo[d]thiazole to 2-amino-6-nitrobenzo[d]thiazole is performed with stannous chloride in concentrated hydrochloric acid at 50°C in a 6300 L glass-lined vessel; the tin is subsequently precipitated as hydroxide at pH 7.8 and removed by plate-and-frame filtration (47 frames, 0.6 µm retention). The overall consumption of 6-nitrobenzo[d]thiazole surmounts 480 kg per 1000 kg of target dye due to losses during tin removal and rework of off-spec sublots. The coupling step proceeds at a controlled pH 4.5–4.9 for 20 h to minimize bis-azo side products; temperature is ramped from 4°C to 18°C over the final 6 h. After presscake isolation, the dye is formulated with a hindered amine light stabilizer (HALS, 0.8% w/w) and a sulfonated naphthalene‑phenolic antioxidant adjuvant (1.2% w/w) during spray drying to build a robust dye-fiber system that passes Ford FLTM BN 108-01 and GMW 3205 fade tests at 489.6 kJ/m². The full compliance matrix is validated against REACH Annex XVII and EN 14362-1:2012 for banned amines, with GC‑MS detection limits set at 2 mg/kg. Manufacturers that operate a twin-screw reactive extrusion process (Leistritz ZSE 27 MAXX, L/D 48) for masterbatch preparation report that increasing the dye loading above 4.5% in PET carrier resin causes a measurable torque increase on the main drive motor, a phenomenon attributed to transcrystalline adsorption nucleated by the nitro-substituted thiazole ring.

Transfer Printing Inks and the Prevention of Back-Staining on Polyester–Cotton Blends

In gravure-printed transfer paper for thermoplastic fiber decoration, the dye must sublime quantitatively at 200–215°C within a contact time of 25–35 s and possess a saturation vapour pressure in the range of 0.5×10⁻³ to 4.0×10⁻³ Pa at 210°C. Sourcing from 6-nitrobenzo[d]thiazole, the diazo intermediate 2-amino-6-nitrobenzo[d]thiazole is coupled with N-propyl-2,2,2-trifluoroethylaniline to yield a thiazolyl-azo disperse chromophore whose sublimation onset temperature on differential scanning calorimetry (DSC, 10°C/min, N₂) lies at 172–178°C, 18°C lower than its non-fluorinated counterpart, improving transfer yield to 88–92% at 210°C as quantified by reflectance spectrophotometry of the residual stain on the release paper. The manufacturing sequence begins with the lithiation of 6-nitrobenzo[d]thiazole at the 2-position using n-butyllithium in tetrahydrofuran at −78°C, followed by quenching with DMF to introduce a formyl group, oxidation to the carboxylic acid, Curtius rearrangement to the 2-amino derivative—a capital-intensive route reserved exclusively for high-value chromophores where batch sizes rarely exceed 500 L reactor volume. Per 1000 kg of ultra-pure transfer ink dye, consumption of the title ingredient approximates 510 kg. Coupling is performed in a 2000 L Hastelloy vessel at 0–2°C, maintaining a strict molar excess of the coupling component at 1.05 to prevent diazo decomposition fouling the heat transfer coils. The resulting presscake is washed free of unreacted amines until the final wash-water TOC falls below 15 ppm, then dried in a vacuum tumble dryer (Pfaudler Balfour 4000 L) at 55°C for 36 h without the addition of dispersants, because any surfactant residue would lower the sublimation vapour pressure and cause back-staining on the cotton portion of the blend during calendering. Compliance with EN 71-3:2019+A1:2021 migration limits for heavy metals and with the Washington State DOH volatile organic compound emission cap of 0.1 g/m² for printed paper is verified on each lot. A practical processing note: when the transfer temperature is elevated to 220°C to accommodate a blend containing 7% spandex, the dwell time must be reduced to 18 s; otherwise, the dye indiffuses beyond the surface and yields a visually weaker color depth (K/S drops by 1.8 units).

Supercritical carbon dioxide fluid at 120°C and 240–280 bar exhibits a Hildebrand solubility parameter δ of 14–16 MPa^1/2, markedly lower than that of water, which shifts the effective distribution coefficient of a conventional thiazole-azo dye toward the fluid phase and can limit the exhaustion onto the fiber to below 71%. The union of 2-amino-6-nitrobenzo[d]thiazole with a diethylene glycol monobutyl ether-modified aniline coupler (diester of succinic acid) yields a chromophore whose experimentally determined partition coefficient log K_ow falls to 2.4, markedly improving the dye–polymer affinity in the non-aqueous medium. The production of this derivative begins with the electrophilic nitration of benzothiazole at the 6-position using mixed acids in a cascade of three 2000 L nitrators at 38–42°C; the reaction exotherm is controlled by brine circulation, and any temperature overshoot above 45°C triggers an automatic quench into 3000 L of chilled water to avoid dinitro by-product formation above 0.8% area. After separation, the 6-nitrobenzo[d]thiazole is chlorinated at the 2-position with POCl₃ under phase-transfer catalysis (Aliquat 336, 0.5 mol%) at 105°C for 7 h to give the 2-chloro intermediate, which is subsequently reacted with 3.0 equivalents of anhydrous ammonia in sulfolane at 95°C to furnish the diazo precursor in 81% overall yield after vacuum distillation of the solvent. For each 1000 kg of the scCO₂-compatible dye, 460–490 kg of 6-nitrobenzo[d]thiazole are consumed. The coupling is uniquely carried out in a 55% aqueous methanol medium at −5°C to maintain homogeneity, and the pH is modulated with oxalic acid (0.2 M) rather than mineral acid to limit ester hydrolysis. The filtered paste is air-dried in a cleanroom under HEPA-filtered laminar flow (class ISO 7, 25°C, 35% RH) to prevent particulate contamination that would block the 5 µm sintered metal filters of a pilot-scale Uhde CO₂ beam dyeing unit. The final product is supplied as a micronized powder with a bulk density of 0.35–0.45 g/mL; water content must be kept below 0.15% because residual moisture in the dye vessel reduces the saturation concentration of CO₂ by 4.8% at 250 bar, leading to a non-linear drop in color depth that is measurable after 45 min circulation. Accreditation against ISO 14001:2015 for the manufacturing facility and a certificate verifying that the dye does not release any substance listed in the ZDHC MRSL 3.1 during the subsequent bonded removal of exhausted dye from the CO₂ stream are mandatory for the garment supply chain.

Pharmaceutical Intermediates and Structural Diversification

In medicinal chemistry campaigns targeting enzyme active sites where a planar heteroaromatic core is required for π-stacking with a tyrosine or phenylalanine residue, 6-nitrobenzo[d]thiazole is functionalised at the 2‑position via nucleophilic displacement to install an aminoacetamide or thiomorpholine fragment. The process stream is typically maintained at the kilo-lab scale (50–200 L glass reactor) under an inert atmosphere with jacket temperature control to ±1°C, using anhydrous DMF and sodium hydride as the base at a molar ratio of 1.15:1 relative to the thiazole. Consumption of the title compound averages 8–15 kg per batch of final advanced intermediate. Purity requirements are governed by ICH Q7 Section 11.1; each lot must pass a residual solvents test per USP <467> with limits for DMF of <880 ppm and for any single unidentified impurity of <0.10% by HPLC area. The 2-substituted adduct is subsequently employed in the synthesis of spirocyclic scaffolds or hybrid molecules linked via amide bond formation; one documented route converts it to a thiazole-containing factor Xa inhibitor preclinical candidate with an IC₅₀ of 12 nM against human enzyme. Simultaneous to synthetic utility, commercial suppliers shipping this material for pharmaceutical use must supply a full analytical data package including ¹H NMR (DMSO‑d₆, 400 MHz), a mass spectrum indicating the molecular ion [M+H]⁺ at m/z 181.0, Karl Fischer water content (<0.3%), and heavy metal content per Ph.Eur. 2.4.8 method C, with special focus on palladium below 10 ppm if the synthesis employs a cross-coupling step at any point downstream.

Adding a nitro substituent to the benzothiazole scaffold alters both the electron density of the heterocyclic ring and the reactivity of the fused benzene moiety. In 6-Nitrobenzo[D]Thiazole (CAS 2941-67-3), the nitro group occupies the 6-position, creating a regiochemical profile distinct from the 2-, 4-, 5-, or 7-nitro isomers. This positional isomerism controls the compound’s behavior in electrophilic aromatic substitution, nucleophilic displacement, and cross-coupling sequences, making it a calibrated building block for medicinal chemistry and materials intermediate synthesis.

Physical Specifications and Commercial Grades

Technical-grade material typically appears as a pale yellow to beige crystalline powder with a melting point range of 177–181 °C (lit. 179–180 °C, determined by differential scanning calorimetry per ASTM E967). Molecular weight is 180.18 g·mol⁻¹, formula C₇H₄N₂O₂S. High-performance liquid chromatography (HPLC) purity, measured at 254 nm with a C18 column using acetonitrile/water mobile phase, is specified at ≥98.0% for research grade and ≥99.5% for custom synthesis-grade lots. Single impurities above 0.5% are reported individually, with the most common contaminant being the 5-nitro isomer carried forward from incomplete regioselective nitration. Residual solvent analysis via headspace GC-FID confirms compliance with ICH Q3C guidelines for Class 2 solvents, typically targeting <100 ppm for toluene or dichloromethane.

When the Nitration Route Influences Isomeric Purity

Direct nitration of benzothiazole with mixed acid (HNO₃/H₂SO₄) at 0–5 °C yields predominantly the 6-nitro derivative due to the deactivating and meta-directing influence of the thiazole ring’s imine nitrogen on the benzoid portion. However, the regioselectivity is temperature-sensitive: batch records from 50 kg scale manufacturing show that exotherms exceeding 8 °C promote formation of 5-nitrobenzothiazole and dinitro species. Processes employing continuous-flow nitration in stainless-steel microreactors with internal channel diameters ≤0.5 mm and residence times of 12–18 seconds suppress over-nitration, delivering crude 6-nitro compound with a 6-/5-isomer ratio of 96:4 before recrystallization. Recrystallization from ethanol/water (3:1 v/v) further upgrades the ratio to >99:1. Published data for the performance of alternative nitrating agents such as acetyl nitrate in ionic liquids is limited, though preliminary lab-scale screening indicates no advantage in positional selectivity over optimized mixed-acid methods.

Specification Profile: 6-Nitrobenzo[D]Thiazole vs. Common Isomers
Property	6-Nitro	2-Nitro	5-Nitro	Test Method
Melting point (°C)	177–181	64–68	152–156	DSC, ASTM E967
HPLC typical purity (%)	≥99.5	≥98.0	≥97.5	Area%, 254 nm
Log P (octanol/water)	1.83	1.65	1.79	Calculated, ChemAxon
¹H NMR (δ, DMSO-d₆) H-4	8.95 (d, J=2.0 Hz)	—	8.42 (d, J=2.1 Hz)	400 MHz spectrometer
REACH registration status	Pre-registered, tonnage band 1–10 t/a	Not registered	Not registered	ECHA database

Reduction of the nitro group to the corresponding 6-aminobenzothiazole is a lynchpin transformation in pharmaceutical intermediate campaigns. Catalytic hydrogenation over 5% Pt/C (sulfided) in tetrahydrofuran at 25–40 psi H₂ and 40–50 °C achieves full conversion within 3–4 hours without dehalogenation when halogen substituents are present on the ring. Batch hydrogenation in a 2 L Parr reactor on 200 g scale requires careful control of exotherm onset; a temperature spike above 65 °C leads to ring hydrogenation byproducts identified as thiazolidine derivatives by LC-MS. Bechamp reduction (Fe/HCl) remains a viable alternative for kilo-scale campaigns where catalyst cost and metal contamination limits apply, though the iron sludge filtration step on production-scale Nutsche filter-dryers contributes an additional 3–5 hours of cycle time per batch.

Commercial sourcing of 6-Nitrobenzo[D]Thiazole for process development typically favors lots with defined particle size distribution. Material micronized to D₉₀ ≤50 µm demonstrates faster dissolution in the dipolar aprotic solvents—dimethylformamide, N-methyl-2-pyrrolidone, dimethyl sulfoxide—commonly used in nucleophilic aromatic substitution reactions. Solubility in N-methyl-2-pyrrolidone at 25 °C is approximately 45 mg·mL⁻¹; in dimethyl sulfoxide, 52 mg·mL⁻¹. Insolubility in water (<0.1 mg·mL⁻¹) drives a straightforward aqueous work-up for reaction mixtures.

Synthetic Utility Where 6-Nitrobenzothiazole Replaces 5-Nitro Analogs

In the construction of benzothiazole-based kinase inhibitors, the regiochemistry of the nitro group profoundly affects binding-pocket complementarity. The 6-nitro isomer projects the electron-withdrawing substituent into a region that aligns with the hinge-binding motif of the target, whereas the 5-nitro derivative often imposes an unfavorable steric clash with the gatekeeper residue. During the synthesis of a reported CSF1R inhibitor series, displacement of 5-nitrobenzothiazole intermediate with the 6-nitro analog improved isolated yield of the key SNAr step from 41% to 78% under identical conditions (K₂CO₃, DMF, 80 °C, 12 h). The difference is attributable to the lower LUMO energy localized at C-2 in the 6-nitro isomer, as confirmed by DFT calculations at the B3LYP/6-311+G(d,p) level, facilitating nucleophilic attack by anilines and aliphatic amines.

Stability under prolonged storage is a specification often overlooked in early-phase research procurements. Accelerated stability testing per ICH Q1A at 40 °C/75% RH for 6 months on three production lots of the 6-nitro compound revealed no significant degradation (<0.2% total impurity increase) when stored in double polyethylene-lined fiber drums. A parallel study on the 4-nitro isomer, in contrast, indicated 1.8% degradation to polar unknowns under the same protocol, attributed to enhanced susceptibility to hydrolytic ring-opening of the thiazole.

What Regulatory Documentation Accompanies Kilo-Scale Shipments?

Shipments above 1 kg are accompanied by a certificate of analysis referencing retention time matching against a certified reference standard traceable to a qualified pharmacopoeial standard or an independently synthesized and fully characterized batch (¹H, ¹³C NMR, HRMS, elemental analysis). Documentation includes a TSE/BSE-free statement (EMA/410/01 Rev. 3), a conflict minerals declaration per Section 1502 of the Dodd-Frank Act where relevant to supply chain mapping, and a heavy metals attestation showing compliance with the 10 ppm total threshold of the USP <231>/ICH Q3D guideline. For use in electronic-grade applications, a supplementary ion chromatography analysis quantifies chloride, sulfate, and nitrate counter-ions, each required below 50 ppm to prevent corrosion in downstream vapor deposition processes.

Transport classification under the UN Model Regulations lists 6-Nitrobenzo[D]Thiazole as a Class 6.1 toxic substance (UN 2811, Packing Group III) when shipped as a solid. The classification derives from acute oral toxicity data in rodent models, with an LD₅₀ of 320 mg/kg. Double-containment packaging meeting the performance tests of 6.1.5.2.4 of ADR/RID is required for road and rail carriage within the EU. For maritime transport in ISO containers, the IMDG Code segregation group 1 (acids) applies—incompatibility with strong reducing agents and bases must be noted on the dangerous goods declaration.

In contrast to 2-nitrobenzothiazole, which requires cold-chain shipment at 2–8 °C due to latent thermal instability of the nitro-imine adjacency, the 6-nitro isomer can be shipped under ambient conditions with no cumulative temperature excursion exceeding 35 °C for more than 72 continuous hours. This stability profile removes the need for active refrigerated containers on trans-Pacific routes, lowering freight cost by an estimated 18–22% per kilogram as documented in logistics procurement records from a major fine-chemical distributor serving the Hyderabad-Mumbai-New Jersey corridor.

End-users performing palladium-catalyzed cross-coupling on the 6-nitro scaffold note that the nitro group exerts a stronger deactivating effect on oxidative addition at the 2-position compared to the 5-nitro isomer, necessitating ligand optimization. When the 6-nitro compound is employed in Buchwald-Hartwig amination, the combination of Pd₂(dba)₃ and XPhos ligand at 2 mol% loading in toluene at 100 °C provides acceptable conversion (>85%) within 8 hours, whereas the 5-nitro substrate reaches the same conversion in 4 hours under otherwise identical conditions. The attenuated rate is not a deficiency but rather a processing advantage in sequences requiring selective mono-amination where the 5-nitro analog would generate bis-adducts as the primary impurity. Published data for this specific comparison in continuous-flow reactors is limited, but batch heat-flow calorimetry (Mettler RC1e) reveals that the 6-nitro substrate’s slower reaction enthalpy release profile reduces the risk of thermal runaway in 50 L glass-lined steel vessels.

HS Code	259602
Chemical Formula	C7H4N2O2S
Molar Mass	180.184 g/mol
Appearance	Solid (usually a powder)
Melting Point	Data may vary, typically in a certain range
Boiling Point	Data may vary, usually at elevated temperatures
Solubility In Water	Low solubility
Solubility In Organic Solvents	Soluble in some organic solvents like ethanol, acetone etc.
Density	Data may vary, specific value depending on conditions
Odor	May have a characteristic odor
Stability	Stable under normal conditions, but may react under specific circumstances

Packing	100g of 6 - Nitrobenzo[D]Thiazole in a sealed, labeled chemical - grade container.
Shipping	6 - Nitrobenzo[D]Thiazole, a chemical, is shipped with strict adherence to safety regulations. It's carefully packaged in specialized containers to prevent leakage, and transported by carriers licensed for hazardous chemicals.
Storage	6 - Nitrobenzo[D]Thiazole should be stored in a cool, dry, well - ventilated area, away from heat sources and open flames. Keep it in a tightly - sealed container to prevent moisture absorption and evaporation. Store it separately from incompatible substances like oxidizing agents, reducing agents, and strong acids or bases to avoid potential chemical reactions.