5-Chloro-2-Methylbenzothiazole

Polyester Exhaustion Dyeing with Heterocyclic Azo Chromophores Requires Controlled Carrier-Free Migration

When 5‑Chloro‑2‑Methylbenzothiazole is nitrated, reduced, and diazotised to yield 2‑amino‑5‑chlorobenzothiazole, the resulting diazonium salt couples with N‑alkyl‑β‑cyanoethyl‑aniline derivatives to form red‑to‑blue monoazo disperse dyes with exceptional thermal stability on polyester. Industrial application of such a dye is confined to high‑temperature exhaust processes on textured filament yarns and microdenier fabrics, where the auxochrome chlorine atom raises the dipole moment of the excited state and shifts absorption into the 560–610 nm window, providing depth without after‑clearing. The standard addition rate spans 0.8–2.2% o.w.f. (on weight of fibre), depending on the target shade depth; amounts below 0.5% fail to saturate the fibre surface above 90% exhaustion under standard liquor ratios of 1:8 to 1:12. The requisite dyebath is prepared with de‑mineralised water, 1.0 g/L anionic dispersant (naphthalenesulfonate condensate, ISO 21331‑compliant), and a weak acetic acid/sodium acetate buffer to hold pH at 4.5–5.0. If the pH drifts above 5.5 during the heating ramp, hydrolysis of the cyanoethyl side chain accelerates, leading to a loss of tinctorial yield of 15–25% and a dulling of brightness measurable as a decrease in CIE L* by 3–5 points. Production‑scale execution uses a Thies‑Luf‑ro‑jet overflow machine programmed to run a 2°C/min ramp from ambient to 130°C, hold for 45‑min, and then forced‑cool to 80°C before overflow rinsing. Under these conditions, a dye‑uptake of ≥92% is routinely obtained on poly(ethylene terephthalate) fibres with a fineness of 0.8 dtex, and the subsequent reduction clearing step using 2.0 g/L sodium hydrosulfite and 2.0 g/L caustic soda at 70°C for 20 min removes all surface‑deposited dye, yielding crockfastness ratings of 4–5 under AATCC 8. The coloured goods are destined for performance‑wear end‑uses: seamless compression sportswear, automotive seat upholstery, and swimwear linings that must pass the extended lightfastness protocols of ISO 105‑B02 (xenon arc, AATCC 16.3 option) at a rating of ≥6, the multiple‑wash fastness of AATCC 61‑2A at 4 without shade change, and the sublimation fastness at 180°C for 30 s per ISO 105‑P01. Regulatory compliance for the dyestuff and the finished article is verified via OEKO‑TEX® Standard 100 (Annex 4, requirement class I for infants), REACH Regulation (EC) 1907/2006 (Annex XVII, entries 3, 22, 72), and the ZDHC Manufacturing Restricted Substances List (MRSL) Version 3.1, which caps residual arylamine content below 20 mg/kg via EN 14362‑1:2012. Process deviations observed on a Mathis AGCH pilot‑dyeing unit suggest that pump frequency modulation to maintain a fabric turn time of 60–90 s is critical; when turn time exceeds 120 s, the boundary layer at the filament surface becomes hypoxic, locally reducing dye transfer by up to 12%, detected as barrel markings on the final jigger roll.

How Does Halogen Substitution Influence Cure Kinetics in Sulfenamide‑Accelerated NR/SBR Blends?

Conversion of 5‑Chloro‑2‑Methylbenzothiazole to its 2‑mercapto analogue (5‑chloro‑2‑mercaptobenzothiazole, CMBT) via oxidation of the methyl group and subsequent amination‑thiolation steps yields a fast‑primary‑accelerator candidate whose electron‑withdrawing chlorine atom re‑balances the lability of the zinc‑thiolate complex formed during vulcanisation. In a typical natural‑rubber/styrene‑butadiene‑rubber (NR/SBR 60/40) blend, CMBT is dosed at 0.5–1.5 phr alongside 1.0–2.0 phr N‑tert‑butyl‑2‑benzothiazolesulfenamide (TBBS) and 2.0 phr sulfur, the exact ratio being determined by the MDR 2000 moving‑die rheometer at 160°C per ASTM D5289‑19. A dosage of 0.5 phr yields a Mooney scorch time t₅ (ASTM D1646) of 25.3 min at 121°C, extending processing safety for thick‑section compression moulds, while 1.5 phr shortens the scorch window to 6.8 min and drives the t₉₀ cure time to 2.9 min, acceptable only for injection‑moulded small‑cross‑section parts where demoulding is performed every 45–60 s. The mixing sequence is executed on a Farrel Banbury 1.6 L internal mixer with tangential rotors, ram pressure 0.6 MPa, dump temperature control at 140°C, and mill sheeting on a two‑roll open mill with a nip gap of 2 mm and front‑roll temperature maintained at 60°C via thermostatted water circulation. Ingredient addition follows a three‑stage protocol: mastication of NR/SBR for 60 s, incorporation of zinc oxide (5.0 phr) and stearic acid (2.0 phr) at 90 s, and delayed addition of CMBT together with sulfur only after the batch temperature drops below 100°C to prevent premature scorch that is detectable as a Mooney viscosity rise above 5 MU/min. Vulcanisation proceeds in a hydraulic compression press at 160°C and 15 MPa platen pressure, with the cure time set to t₉₀ + 2 min; physical properties of the cured pad are then tested using ASTM D412 (die C) tensile specimens, where 1.0 phr CMBT returns tensile strength of 22.4 MPa and elongation at break 480%. The table below summarises the cure‑curve trajectory across the loading range:

Finished rubber articles—such as abrasion‑resistant conveyor belt covers ( ISO 14890:2013 class F), NBR‑bonded hydraulic accumulator seals, and engine‑mount elastomers requiring fatigue resistance over 500‑000 cycles at ±15% shear strain—derive from this accelerator system. Compliance with food‑contact regulations is essential for sealing rings used in food‑processing equipment; here the formulation must satisfy FDA 21 CFR 177.2600 with a maximum CMBT content of 1.0 phr and a post‑cure aqueous extraction test confirming total extractives below 15 mg/dm² per FDA guidance for Gen-X. Additionally, REACH (EC) 1907/2006 imposes a restriction on the release of free 2‑mercaptobenzothiazole analogues under the SVHC candidate‑list screening, with a detection limit of 0.1% w/w in the article. A critical incompatibility exists with the widely‑used amine antidegradant N‑phenyl‑α‑naphthylamine (PAN): during the hot‑mixing window above 130°C, transamination generates secondary amines that scavenge the benzothiazolesulfenamide intermediates, reducing the crosslink density by 20‑30% and shifting the t_s2 scorch time unpredictably beyond 3 min, which has led to batch rejection on continuous‑vulcanisation salt‑bath lines where residence time is rigidly fixed.

Liquid Crystal Phenylbenzothiazole Dopants and Dielectric Anisotropy Tuning

Quaternisation of 5‑Chloro‑2‑Methylbenzothiazole with ethyl iodide, followed by condensation with 4‑(alkoxy)benzaldehyde and complexation with a palladium catalyst, furnishes a phenylbenzothiazole heterocyclic mesogen that functions as a positive‑Δε dopant in multi‑component nematic formulations. When dissolved at 4–15 wt% in a base matrix of bicyclohexyl‑fluorobiphenyl eutectic (clearing point 85°C, Δε +5.2 at 1 kHz), the dopant lifts the macroscopic dielectric anisotropy by +1.8 per 5‑wt% increment, measured in a 10 µm planar aligned cell using a Keithley 595 capacitance meter per IEC 61747‑1:2015 section 5.2. The compounding procedure is carried out in a nitrogen‑filled glovebox (H₂O < 1 ppm, O₂ < 1 ppm) at 80°C under magnetic agitation for 4 h; the homogeneous solution is then filtered through a 0.2 µm PTFE syringe filter directly into indium‑tin‑oxide‑coated glass cells assembled with a 5 µm bead spacer and sealed with a UV‑curable acrylic adhesive. The filled cells are annealed at 90°C for 30 min to erase flow‑induced alignment defects before electro‑optic evaluation. End‑product applications include super‑twisted nematic (STN) passenger‑information displays and industrial‑data LCD modules, where the enhanced Δε permits operation at lower multiplex drive voltages (2.8 V RMS at 1/64 duty) and frame‑rate responsiveness of 120 Hz. Each display module is assembled in a Class‑100 cleanroom under ISO 14644‑1 protocols to prevent pixel damage from sub‑micron particulate. The only regulatory prerequisite for the liquid crystal mixture itself is adherence to RoHS (2011/65/EU), verified by X‑ray fluorescence screening for restricted phthalates and heavy metals; however, the fully assembled module is additionally certified according to IEC 61747‑5:2018 for display reliability and IPC‑J‑STD‑020D for lead‑free soldering thermal history. A compositional limit emerges at dopant fractions exceeding 12 wt%: the clearing point T_NI depresses by 3 K per additional 1 wt% beyond the 10% mark, and the rotational viscosity at 25°C climbs above 140 mPa·s, which in turn widens the optical response time from 8 ms to 16 ms, rendering the mixture unsuitable for video‑rate displays. Published phase diagrams for similar benzothiazole liquid‑crystalline structures advise that the nematic range collapses entirely when the dopant mol fraction exceeds 0.18, consistent with the observed onset of smectic‑A phase induction at 72°C.

Condensation of 5‑Chloro‑2‑Methylbenzothiazole with cyanuric chloride under anhydrous dioxane, followed by a Krohnke condensation with 4,4′‑diaminostilbene‑2,2′‑disulfonic acid, produces a bis‑benzothiazolyl‑stilbene fluorescent whitening agent (FWA) whose absorption maximum at 375 nm and emission at 435 nm makes it suitable for blending with polyester‑cotton broadcloth. The FWA is applied at 0.008–0.04% on weight of the fabric (o.w.f.) in a pad‑thermosol sequence: a vertical‑bowl padding mangle with Shore‑A 70 rubber rolls set to a nip pressure of 2 bar delivers a 85‑% wet pickup using a bath containing 0.5 g/L ethoxylated nonionic penetrant and 0.2 mL/L acetic acid to maintain pH at 5.5. The padded fabric passes through an infrared pre‑dryer to reduce moisture content to 18% over a 20 s dwell, then transitions to a Benz‑type forced‑convection tenter frame where thermosol fixation occurs at 190°C for exactly 60 s; dwell‑time deviation of ±5 s can shift the whiteness index by ±3 CIE WI points, as the cis‑trans isomerisation equilibrium of the stilbene core is sensitive to thermal gradient. The whitened fabric destined for high‑white business shirting, sanitary‑napkin cover‑stock, and interlining for polyester‑filled bedding must display a CIE whiteness index ( ISO 105‑J02:2018, D65 illuminant, 10° observer) of at least 140 and a Yellowness Index ( ASTM E313‑20) below 2.0. Over‑drying above 200°C induces partial cleavage of the C‑S bond in the benzothiazole ring, yielding a yellow chromophore that reduces whiteness by 15‑20 points and cannot be corrected by re‑washing. Regulations governing food‑contact paper wrap and tea‑bag string paper made from the same FWA‑treated fibres require compliance with EU 10/2011 (overall migration limit 10 mg/dm²) and FDA 21 CFR 176.170 (components of paper in contact with aqueous and fatty foods), while the Nordic Swan Ecolabel for textiles (version 5.0) adds a restriction banning more than 0.1‑wt% of any organic brightener in the ready garment unless biodegradability within 28 days (OECD 301B) is demonstrated.

When 5‑Chloro‑2‑Methylbenzothiazole Serves as a Merocyanine Precursor in Silver Halide Emulsions

Quaternisation of 5‑Chloro‑2‑Methylbenzothiazole with methyl iodide in nitromethane at 60°C under reflux generates the 2,3‑dimethyl‑5‑chlorobenzothiazolium iodide salt that is the essential heterocyclic nucleus for synthesising red‑sensitising merocyanine dyes. The quaternary salt is condensed with a 3‑sulfopropyl rhodanine ester under a nitrogen atmosphere at 80°C in absolute ethanol containing triethylamine, yielding a dye whose J‑aggregate absorption band peaks at 650 nm when adsorbed onto 0.4±0.05 µm cubic silver‑bromide grains. Dye addition to the photographic emulsion occurs during the post‑ripening “finishing” stage in a jacketed vessel maintained at 40°C and pH 5.6, with the sensitising amount controlled to 0.02–0.08 mg of dye per gram of silver halide, calibrated spectrophotometrically via the ISO 14546:2020 method for imaging materials. Agitation at 150 rpm with a pitched‑blade impeller is sustained for 30 min to allow full equilibration of J‑aggregate formation; an abrupt increase in optical density at 650 nm measured via a Cary 5000 spectrophotometer confirms the J‑band formation efficiency. The spectrally‑sensitised emulsion is then coated onto 7‑mil ( 0.178 mm) blue‑tinted polyethylene terephthalate base using a Toshin‑style curtain coater running at 120 m/min, with coating weight held at 2.3 g Ag/m². The finished film products include orthochromatic medical X‑ray film for curved‑cassette tomography systems that must conform to the sensitometric curve demands of ISO 9236‑1:2004 for speed and ISO 8374:2001 for average gradient, and graphic‑arts contact‑duplicating film employed in phototooling for printed circuit boards where dimensional stability under ISO 18905:2009 and archival permanence (ISO 18916:2007) are mandatory. A particular processing boundary is the iodide‑ion level in the emulsion: free iodide above 0.5 mol% relative to total halide competes with the merocyanine for adsorption sites on the AgBr {100} faces, displacing the dye and causing a drop in spectral sensitivity at 650 nm by 30–40%, necessitating thorough washing of the coagulated emulsion before initiation of the dye‑sensitising step.

In the synthesis of 2‑aminobenzothiazole scaffolds deployed as hinge‑binding motifs in tyrosine‑kinase inhibitor programmes, 5‑Chloro‑2‑Methylbenzothiazole is subjected to a one‑pot oxidative amination: the methyl group is first converted to a nitrile using ammonia, oxygen, and a copper‑chromite catalyst at 250°C and 8 bar in a continuous‑flow tubular reactor (residence time 45 s), then hydrolysed to the primary amide and subsequently Hofmann‑rearranged to the 2‑amino derivative with sodium hypochlorite at −5°C. This intermediate, 2‑amino‑5‑chlorobenzothiazole, carries the halogen‑substitution pattern required for terminal‑selective Sonogashira coupling with ethynyl‑aryl fragments and is utilised in multi‑step pharmacology route scouting under current‑good‑manufacturing‑practice (cGMP) conditions compliant with ICH Q7 for API intermediates. The final API candidate molecules—often intended for the treatment of myeloproliferative neoplasms or neutrophilic dermatoses—require the benzothiazole‑amine fragment to be supplied at ≥99.5% purity (USP Monograph <621>, HPLC method) with a single‑largest unknown impurity below 0.10%, residual palladium below 10 ppm by ICH Q3D, and water content below 0.2% (Karl Fischer, USP <921> Method Ⅰa). The production process for the key intermediate is executed in a GLP‑certified facility under a Quality‑by‑Design framework; the diazotisation‑cyanuration‑reduction cascade requires strict anhydrous conditions because the intermediate diazonium salt undergoes rapid decomposition at relative humidity above 30%, generating tarry by‑products that reduce the yield by 12‑18% and necessitate wasteful column chromatography on silica gel 60 (ISO 33401:2024 reference material) with a gradient elution of heptane/ethyl acetate. A stoichiometric application rate of 0.95‑1.05 molar equivalents is targeted relative to the electrophilic coupling partner, and in‑process control by on‑line Raman spectroscopy probes the carbonyl stretch at 1685 cm⁻¹ to signal depletion of the starting material to below 0.5 area% before quench. The final small‑molecule drugs incorporating this benzothiazole moiety are currently supplied as 10‑mg and 25‑mg film‑coated tablets, and all manufacturers of the upstream intermediate must maintain a Drug Master File with the competent authority and adhere to 21 CFR Part 211 as well as the European Pharmacopoeia general monograph 2034 for active substances used in clinical trials.