6-Benzothiazolecarboxylic Acid, 2-Amino-

The industrial oxidation of 6-benzothiazolecarboxylic acid, 2-amino- to its corresponding diazonium salt under large-scale plant conditions is a process inherently constrained by thermal stability thresholds that vary with the protonation state of the heterocyclic ring. In a typical 3000 L glass-lined mono-block reactor operated at a jacket brine temperature of −8 °C, the continuous dosing of sodium nitrite solution into a phosphoric acid slurry of the amine intermediate must maintain a pot temperature within −2 °C to +3 °C—any excursion above 8 °C initiates a self-sustaining decomposition cascade observed as rapid nitrogen evolution and tar precipitation, a failure mode documented across multiple production campaigns where the brine circulating pump L/min delivery dropped below 85% rated capacity. Crystallisation of the free acid prior to diazotisation is critical; wet cake moisture above 12% w/w shifts the stoichiometry of nitrous acid generation and leads to dinitro-after-coupling impurities that manifest as a “muddy” shade shift in the final dispersed dye finish.

When Disperse Red 177 Moves from Lab-Scale Beaker to 3000-Litre Coupling Vessel

Commercial synthesis of C.I. Disperse Red 177 exploits 6-benzothiazolecarboxylic acid, 2-amino- as the diazo component coupled onto N-ethyl-N-(2-cyanoethyl)aniline. The diazonium salt is prepared by metered addition of 1.02 molar equivalents of sodium nitrite (40% aqueous solution, nitrite content verified by permanganate titration against USP reference standard) into a suspension of the finely milled intermediate in 85% orthophosphoric acid chilled to −4 °C. Holding time of the finished diazo liquor before transfer to the coupling vessel must not exceed 45 minutes; at 60 minutes hydrolysis by-products rise to 0.7% area-under-curve by HPLC (C18, 254 nm). The coupling is executed in a 5000 L 316L stainless steel baffled tank containing a pre-dispersed emulsion of the coupler, acetic acid, and demineralised water at pH 1.8–2.2, with the diazo liquor introduced below the liquid surface via a dip pipe at a controlled rate of 8–12 L/min such that the pot temperature never exceeds 6 °C. Free mineral acid values are continuously monitored by an inline differential conductivity cell; the endpoint is confirmed when a spot test on H-acid treated filter paper shows absence of diazo, correlating to a residual amine below 0.15% on the dry weight of press cake. The slurry is then adjusted to pH 6.3–6.7 with 20% sodium carbonate, filtered at 1.8 MPa through a polypropylene membrane filter press (chamber volume 1.2 m³), washed with reverse-osmosis permeate (conductivity < 5 µS/cm) until the washings test < 1 degree Baume, and dried in a Luwa vertical thin-film dryer with a jacket steam pressure of 0.4 MPa giving a wall temperature of 145 °C. The dried crude dye is pre-ground in a pin mill to D50 < 200 µm before wet milling in a Netzsch LMZ 25 horizontal bead mill charged to 80% volume with 0.5–0.7 mm yttria-stabilised zirconia beads. Dispersion quality is verified by a filter-paper speck test: 5 g of dye dispersed in 1 L water at 25 °C shall pass through a 2 µm cellulose acetate membrane leaving no visible speck residue.

Conformance to OEKO-TEX^® Standard 100, product class I, Annex 4, mandates that the specific migration limit for any carcinogenic arylamine released from the dyed textile remains below 20 mg/kg. The ZDHC Manufacturing Restricted Substances List version 3.0, Level 3 forbids intentional addition of chlorinated benzenes as carriers in the milling step, and the dye must be free of polychlorinated biphenyls above the limit of quantification of 0.5 ppm when analysed by EN 17137:2023. For the EU market, compliance with Regulation (EC) No 1907/2006 (REACH) Annex XVII entry 72 requires that the finished dyestuff does not place on the market substances classified as carcinogenic category 2 above concentration limits of 0.1% w/w; consequently, residual non-diazotised intermediate is controlled to < 0.3% by exhaustive coupling and post-treatment washing.

Source: aggregated plant quality-control batch certificates and AATCC/ISO round-robin data retained in manufacturer technical archives. The shade and fastness profile directly informs selection for continuous pad-thermosol dyeing of woven polyester with downstream water-repellent finish, where a minimum sublimation rating of 4 is required to prevent staining of adjacent white fabrics in garment pressing. The finished product is standardised to a colour depth of 200% (relative to a standard reference depth) using anhydrous sodium sulphate diluent and supplied in 25 kg fibreboard drums with polyethylene liners.

In continuous pad-steam dyeing of cotton cellulose with hetero-bifunctional reactive red dyes, incorporation of a carboxybenzothiazole chromophore shifts the fixation optimum to a pH range of 10.8–11.2, avoiding premature β-sulphatoethyl sulfone hydrolysis that would otherwise reduce covalent bond formation with the primary hydroxyls of the cellulose. The 6-benzothiazolecarboxylic acid, 2-amino- nucleus contributes 22–28% of the molecular mass of the final reactive dye and introduces a carboxylic acid functionality that remains partially ionised under the alkaline pad bath, increasing the substantivity of the dye before steaming by a factor measurable through the exhaustion index E at 10 minutes as an improvement from 0.72 to 0.85 when compared against an unsubstituted benzothiazole analogue (spectrophotometric monitoring at λ_max of 518 nm in a Mathis Labomat using 20 g/L calcined Glauber’s salt). Diazo preparation for this class is carried out using nitrosylsulfuric acid (40% in sulfuric acid) at −5 °C to 0 °C to avoid the competing C-nitration that occurs in straight nitric acid media. The diazonium salt is coupled at 8–12 °C onto H-acid (1.03 mol equivalent) suspended in a dilute hydrochloric acid solution of pH 2.5; this specific acidity suppresses secondary coupling at the 7-position of the naphthalene ring. Following coupling, the monoazo intermediate is condensed with a pre-mixed reactive anchor comprising a monochlorotriazine and a masked vinyl sulfone precursor at 0–2 °C and controlled pH of 6.8–7.2 through automated addition of 10% sodium bicarbonate. Completion of condensation is verified by thin-layer chromatography (silica gel 60 F₂₅₄, eluent 5:4:1 i-propanol/water/acetic acid) showing absence of the free H-acid spot at R_f 0.45. The crude dye solution is desalted on a pilot-scale spiral-wound reverse-osmosis unit (Dow Filmtec^™ BW30-4040 elements arranged in a 2:1 array) until the permeate conductivity stabilises at < 800 µS/cm, giving a salt-to-dye ratio below 0.3:1. The concentrate is spray-dried in a Niro FSD^™ dryer with inlet air at 195 °C and outlet at 95 °C to produce a non-dusting hollow-sphere granule with bulk density of 0.45–0.55 g/cm³ and moisture content < 2.0% by Karl Fischer. The final product is classified as a low-salt reactive red dye suitable for warm exhaust dyeing at 60 °C on mercerised cotton knitwear with an electrolyte demand of only 40 g/L NaCl. Standards conformance includes GOTS version 7.0 clause 2.4.3, which prohibits heavy-metal impurity levels above 10 ppm arsenic, 2 ppm mercury, and 100 ppm lead in the as-sold dye; compliance is verified by ICP-OES after microwave-assisted acid digestion per EN 16711-1:2015. The intermediate itself has been assessed under OEKO-TEX ECO PASSPORT with a required biodegradability of > 95% within 28 days in the OECD 302 B Zahn-Wellens test.

Incorporation of 6-benzothiazolecarboxylic acid, 2-amino- into the cationic dye category proceeds through quaternisation of the thiazole nitrogen after esterification of the free carboxyl to a methyl ester, a modification that prevents betaine formation and improves solubility in the acidic dyebath used for acrylic fibres. The ester is prepared by heating the wet-cake intermediate (55% solids) in anhydrous methanol with 1.5% w/w sulfuric acid at 66 °C for 8 hours, with water continuously removed by azeotropic distillation through a packed column into a molecular sieve 4A trap. After neutralisation with triethylamine and solvent swap to acetone, the esterified base is diazotised at 0 °C with nitrosylsulfuric acid in acetic acid medium, coupled with N,N-dimethylaniline, and then quaternised directly with dimethyl sulfate at 55 °C for 4 h under nitrogen blanket. The resultant cationic dye is precipitated by drowning the reaction mass into ice-cold 5% aqueous sodium chloride, filtered in a pressure Nutsche equipped with a PTFE cloth, and dried under vacuum at 55 mbar and 50 °C to a residual methanol level below 200 ppm. The commercial powder is standardised to 35% active dye content with anhydrous sodium sulfate; application trials on wet-spun polyacrylonitrile tow using a Fleissner gel-dyeing line at 90 °C and a residence time of 120 seconds demonstrate exhaustion of 98.5% at a dyebath concentration of 0.5% o.w.f., buffered with 1.2 mL/L acetic acid and 0.6 g/L retarder based on a quaternary ammonium surfactant blend. Regulatory assessment per Regulation (EU) 2019/2021 setting eco-design requirements for household washing machines and in relation to dye transfer during laundering requires that the dye not promote staining in the ISO 105-C08 wash test beyond a grey scale rating of 4. Extractable free benzothiazole monitored by LC-MS/MS per DIN 54231:2022 must remain below 25 mg/kg from the dyed fabric after 10 simulated laundry cycles. The finished cationic dye powder is applied predominantly in the gel-dyeing of acrylic staple intended for carpet yarn, where the compatibility value determined by the K-value method with C.I. Basic Yellow 28 as standard falls in the range 3.5–4.5, ensuring superposition of exhaustion curves during tri-chromatic shade matching.

De-risking Aminothiazole API Synthesis via 6-Carboxy Intermediates – A GMP Isolation Strategy

In the manufacture of a substituted 2-aminobenzothiazole-6-carboxamide active pharmaceutical ingredient classified as an intermediate for a muscle relaxant candidate, 6-benzothiazolecarboxylic acid, 2-amino- serves as the designated Regulatory Starting Material under ICH Q11, requiring full traceability of the synthetic route and a comprehensive impurity fate-and-purge study. The acid is charged at 1.0 mole equivalent into a dedicated GMP production train housed in an ISO 8 cleanroom suite. Esterification to the ethyl ester is effected by slow addition of thionyl chloride (1.3 equivalents) to a suspension of the intermediate in absolute ethanol at −10 °C, with evolved HCl and SO₂ scrubbed through a 10% sodium hydroxide packed tower. After solvent evaporation in a Buchi rotavapor under vacuum at 40 °C, the resultant ester hydrochloride is partitioned between ethyl acetate and 5% sodium bicarbonate, then crystallised from heptane/ethyl acetate 7:3 v/v to achieve an HPLC purity of > 99.0% (area%, 254 nm). The Curtius rearrangement sequence employs diphenylphosphoryl azide (1.15 equivalents) and triethylamine in dry toluene at 85 °C, generating the isocyanate intermediate that is trapped in situ with benzyl alcohol to form the Cbz-protected amine. Hydrogenolysis over 10% Pd/C (5% w/w wet) under 0.5 MPa H₂ at 40 °C in a pressure-rated Hastelloy C-22 autoclave fitted with a Rushton turbine impeller provides the primary amine, which is then acylated with the requisite acid chloride to yield the final carboxamide API. Critical process parameters include maintaining the hydrogen uptake rate below 0.25 mol/L/h to avoid over-reduction of the benzothiazole ring, monitored by mass-flow controller readings logged against DCS time stamps. The crude API is recrystallised from 85:15 ethanol/water, dried in a conical vacuum dryer ( 50 °C, < 10 mbar) until LOD by halogen moisture analyser is < 0.5%, and then jet-milled using a Hosokawa Alpine 100 AFG opposed-jet mill with classifier wheel speed set to 8000 rpm to achieve a particle size D90 < 20 µm, as determined by laser diffraction (Malvern Mastersizer 3000, dry dispersion at 2 bar). The API is packed in 5 kg triple-laminated aluminium foil drums with silica-gel desiccant packets, sealed under nitrogen. Residual solvent analysis per USP method <467> confirms methanol < 3000 ppm, toluene < 890 ppm, and ethyl acetate < 5000 ppm.

Disease Control in Solanaceous Crops via 2-Amino-6-Benzothiazolecarboxamide Suspension Concentrates

The synthesis route to N-(3,5-dichlorophenyl)-2-aminobenzothiazole-6-carboxamide, a protectant fungicide active against Alternaria leaf blight in potatoes and tomatoes, proceeds through activation of 6-benzothiazolecarboxylic acid, 2-amino- with thionyl chloride in toluene containing a catalytic amount of N,N-dimethylformamide (0.2% w/w). The acid chloride formation is conducted at 70 °C for 5 hours in a glass-lined reactor equipped with a scrubber system handling discharged HCl gas; reaction completion is identified by cessation of gas evolution and a residual free acid content below 0.5% by potentiometric titration against tetrabutylammonium hydroxide. After stripping excess thionyl chloride under reduced pressure at 55 °C, the acid chloride solution is added slowly to a stirred solution of 1.02 molar equivalents of 3,5-dichloroaniline in toluene containing 1.5 equivalents of sodium carbonate as acid scavenger, maintaining the pot temperature between 20 °C and 25 °C. The resulting amide precipitates and is filtered, washed sequentially with deionised water and cold isopropanol, and then recrystallised from isopropanol with activated carbon treatment to give a technical active ingredient with a melting point of 224–226 °C and HPLC purity exceeding 98.5% (area%, 254 nm). The dried technical material is micronised in a fluidised-bed opposed-jet mill to a volume median diameter D50 < 3 µm to optimise foliar retention and bio-availability. The suspension concentrate formulation is prepared in a rotor-stator high-shear mixer (Silverson L5M-A, square-hole high-shear screen) where the milled active (240 g/L) is wetted into an aqueous premix containing 40 g/L ethoxylated tristyrylphenol phosphate surfactant (HLB 13.5), 3 g/L xanthan gum rheology modifier, 1 g/L biocide (aqueous solution of 1,2-benzisothiazolin-3-one), 5 g/L propylene glycol antifreeze, and 0.5 g/L silicone defoamer; the shear rate is kept at 10 000 rpm for 20 minutes. The resulting SC passes the CIPAC MT 46.3 wet sieve test at 75 µm with residue below 0.05%, and exhibits a viscosity of 450–650 mPa·s (Brookfield RVT, spindle 3, 20 rpm). The formulation is in conformance with FAO Specification 451/SC (as a benchmark for amide fungicide SCs) and compliant with OECD 301B ready biodegradability criteria for non-active co-formulants. Acute oral toxicity evaluated per EPA OCSPP 870.1100 gives an LD₅₀ greater than 2000 mg/kg in rat, classifying the product as Category V (low toxicity) under EPA’s acute toxicity classification. Field trial data generated in accordance with EPPO PP 1/28(4) guidelines report efficacy of 85–92% control of Alternaria solani when applied as a foliar spray at 400 mL formulation/ha in a water volume of 300 L/ha at 7–10 day intervals, without observable phytotoxicity on the target cultivars.