2-Chloro-1,3-Benzothiazole

In a standard sulfenamide accelerator production campaign, 2-chloro-1,3-benzothiazole (assay ≥99.0% by GC, moisture ≤0.1% w/w) is suspended in demineralised water at a w/v ratio of 1:4 within a 3,000 L glass-lined reactor equipped with a retreat-curve impeller rotating at 90–110 rpm. Cyclohexylamine (1.05 molar equivalents relative to 2-Cl-BT) is injected over 45–60 min while the jacket temperature is ramped from 25°C to 40°C. The pH is brought to 10.5–11.0 by simultaneous addition of 30% sodium hydroxide solution through a dosage ring, and the exotherm is moderated to keep the internal temperature below 50°C; excursion above 52°C promotes hydrolysis to 2-hydroxybenzothiazole, a contaminant that shortens Mooney scorch time in rubber stocks by 15–25% at 0.2% inclusion. After 4 h stirring, GC analysis (DB-5, 30 m × 0.25 mm, FID) confirms residual 2-chloro starting material <0.15% area. The crude N-cyclohexyl-2-benzothiazolesulfenamide (CBS) is isolated via pressure nutsche filtration at 0.35 MPa N₂, washed with warm water (50°C) until chloride ion in the filtrate drops below 50 ppm (AgNO₃ turbidimetric test), and vacuum-dried at 60°C and ≤−0.09 MPa to a final moisture content ≤0.15%. The dried product exhibits a melting point of 96–98°C, purity >99.2% via HPLC (C₁₈, acetonitrile/water 70:30, UV 254 nm), and bulk density 0.48–0.52 g/cm³. CBS acts as a delayed-action primary accelerator in natural rubber, SBR, and BR compounds; in a typical silica-filled passenger tyre tread formulation, it is dosed at 1.2–1.8 phr alongside 2.5 phr sulfur, 0.3 phr diphenylguanidine, and 50 phr highly dispersible silica. The Mooney scorch time at 121°C per ASTM D1646 extends beyond 35 min, while the moving-die rheometer t₉₀ at 160°C (ASTM D5289, 0.5° arc) falls in the 5.0–6.5 min range, balancing long flow safety with rapid final crosslink development. For EU food-contact rubber articles, compliance with Regulation (EU) No 10/2011 and BfR Recommendation XXI demands specific migration of cyclohexylamine below 0.01 mg/kg food simulant and total primary aromatic amines below the 0.01 mg/kg detection limit, verified by LC-MS/MS after 40°C/10 days migration testing. Under REACH (EC) 1907/2006, CBS is registered as a skin sensitiser Category 1 (H317); finished articles containing CBS must not release the substance at a rate that yields a dermal exposure exceeding 0.1% w/w classification cut-off.

What Limits the Disulfide Formation Rate When Aqueous Na₂S₂ Replaces Polysulfide Reagents?

2-Chloro-1,3-benzothiazole undergoes oxidative dimerization to 2,2′-dibenzothiazyl disulfide (MBTS) when treated with sodium disulfide generated in situ from sodium sulfide nonahydrate and sulfur (molar ratio Na₂S·9H₂O : S = 1 : 1.05) in water at 70–80°C. The stoichiometric demand is precisely 2 mol of the chlorinated heterocycle per 1 mol of Na₂S₂. In a 5,000 L stainless-steel reactor, 250 kg (1,479 mol) of 2-Cl-BT is dispersed in 1,200 L water containing 0.2% w/w sodium lauryl sulfate as a wetting agent; the Na₂S₂ solution (740 mol active S₂²⁻) is added dropwise over 2 h while maintaining the temperature at 78±2°C. Air sparging at 1.5 Nm³/h through a bottom sparger ring enhances mass transfer and drives the oxidation of intermediate mercaptide to the disulfide; the off-gas is scrubbed with 10% NaOH to remove any H₂S. End-point determination relies on HPLC monitoring of residual 2-Cl-BT (<0.3% area). The slurry is cooled to 20°C, acidified with 30% H₂SO₄ to pH 3.5–4.0 to precipitate MBTS, filtered through a polypropylene plate-and-frame filter press, washed with demineralised water until conductivity falls below 100 μS/cm, and dried in a fluidised-bed dryer at inlet air temperature 80°C to a moisture content ≤0.3%. Commercial MBTS appears as a pale-yellow powder, melting range 178–186°C, purity >96.5% (HPLC), and ash <0.3%. As a medium-fast primary accelerator, MBTS finds use in thick-walled mechanical rubber goods such as engine mounts and bridge bearings, where its scorch time (Mooney t₅ at 121°C typically 15–22 min at 1.5 phr loading) provides a narrower processing safety margin than sulfenamides but permits lower cure temperatures (140°C). Regulatory alignment for MBTS parallels that of CBS under REACH and the same food-contact migration limits, with the additional requirement that batch certificates report free 2-mercaptobenzothiazole (2-MBT) content <0.5% because 2-MBT is classified as a respiratory sensitiser under CLP Regulation (EC) No 1272/2008 (H334). The MDR cure curve of an NR/SBR compound containing 1.5 phr MBTS and 2.0 phr sulfur reveals a minimum torque ML of 1.8 dNm and maximum MH 12.3 dNm at 150°C. Pre-drying of the accelerator is mandated whenever relative humidity in the storage area exceeds 60%; agglomerates formed during ocean shipment are breakable with a 500 μm sieve prior to internal mixing.

Surrogate Parameters for Alkylation Efficiency: Potassium Carbonate Particle Size and Water Content in Mefenacet Synthesis

Production of the pre-emergence herbicide mefenacet [2-(1,3-benzothiazol-2-yloxy)-N-methyl-N-phenylacetamide] proceeds via O-alkylation of N-methylhydroxyacetanilide (NMHA) with 2-chloro-1,3-benzothiazole in anhydrous N,N-dimethylformamide (DMF, water <0.05%) at 80–85°C for 6–8 h using 1.20−1.25 molar equivalents of ground potassium carbonate (325 mesh, D₅₀ 15 μm) as acid scavenger. A typical batch charges 1.0 kmol of 2-Cl-BT, 1.08 kmol of NMHA, and 1.22 kmol of K₂CO₃ into 2,500 L of DMF; agitation is set to 150 rpm via a pitched-blade turbine. The reaction progress is tracked by TLC (silica, ethyl acetate/hexane 1:1) with a target of <1% residual NMHA. Upon completion, the mixture is cooled to 10°C, quenched with 3,000 L ice-water, and the crude mefenacet is collected by centrifugation (1,200 G), washed with cold water, and recrystallised from isopropanol (1:3 w/v) to afford white crystals with m.p. 132–134°C and technical purity >98.5% (HPLC, UV 230 nm). The finished technical grade is formulated as a 500 g/L suspension concentrate (SC) or 10% wettable powder (WP) for application in paddy rice at rates of 300–600 g a.i./ha. Under Regulation (EC) No 1107/2009, the active substance specification mandates limits for relevant impurities: 2-chlorobenzothiazole <1 g/kg, NMHA <0.5 g/kg, and DMF <0.1% w/w as residual solvent (ICH Q3C Class 2). The maximum residue limit (MRL) for mefenacet in rice has been set at 0.01 mg/kg in the EU per Regulation (EC) No 396/2005 (Annex III), and the compound is not approved for use on crops intended for infant food (Directive 2006/125/EC). Production equipment requires passivation of stainless-steel surfaces with 3% citric acid prior to first batch after maintenance to prevent iron-catalysed hydrolysis of 2-Cl-BT to 2-hydroxybenzothiazole, which would otherwise form the corresponding ether impurity. The SC formulation must pass CIPAC MT 184 (suspensibility) and MT 148 (persistent foam) tests for product registration.

For the construction of 2-hydrazinyl-1,3-benzothiazole — a versatile intermediate en route to triazolobenzothiazoles with reported anticonvulsant and anti-inflammatory activity — 2-chloro-1,3-benzothiazole is condensed with hydrazine hydrate (80% aqueous, 3.5 molar equivalents) in refluxing ethanol (95%) for 5–7 h under nitrogen. The chlorinated starting material is initially dissolved at 50°C at a concentration of 1.0 mol/L; after hydrazine addition, the mixture is heated to gentle reflux (78−80°C). Conversion is monitored by HPLC until the 2-Cl-BT peak area falls below 0.1%. The resulting solution is concentrated to half volume under vacuum at 45°C, cooled to 0–5°C, and the crystalline 2-hydrazinylbenzothiazole is isolated by filtration, washed with cold ethanol, and vacuum-dried at 40°C. Typical yield is 82–88%; purity by HPLC area % exceeds 99.5%, with residual hydrazine quantified by derivatisation-GC (<10 ppm) and chloride content by ion chromatography <200 ppm. As a key starting material for active pharmaceutical ingredient (API) synthesis under ICH Q7 Good Manufacturing Practice, the lot must be accompanied by a certificate of analysis detailing residual solvents (ethanol <5,000 ppm, isopropanol <500 ppm) per ICH Q3C and heavy metals (<10 ppm Pb, <2 ppm As, <1 ppm Cd) by ICP-MS. The compound is further reacted with α-bromoketones in the presence of triethylamine to furnish triazolo[3,4-b]benzothiazoles, which have been evaluated as CRF₁ receptor antagonists. For supply into the European market, the manufacturing site must hold a valid EU GMP Part II certification; the shipment is controlled under a Technical Active Substance Master File (ASMF) with an open part complying with EMA/CHMP/QWP/227/02. Because 2-chlorobenzothiazole is sensitive to moisture and releases HCl upon hydrolysis, the warehousing of the raw material requires relative humidity <30% and storage in original 25 kg HDPE drums with desiccant bags. The hydrazine condensation is safely conducted in a 500 L glass-lined reactor with a rupture disc rated at 0.6 MPa and a dedicated scrubber for ammonia vapours.

If Ammonia Amination Precedes Diazotization, a Cationic Red Azo Dye of High Tinctorial Strength Is Obtained

When 2-chloro-1,3-benzothiazole is first converted to 2-aminobenzothiazole by treatment with 25% aqueous ammonia (molar ratio NH₃ : 2-Cl-BT ≈ 5:1) in the presence of cupric chloride dihydrate (0.05 mol %) at 120°C and 1.2 MPa in a 1,000 L Hastelloy autoclave for 8 h, the isolated 2-aminobenzothiazole (m.p. 126–128°C, purity >99.0%) serves as the diazo component. Diazotization is carried out in 25% sulfuric acid at 0–5°C with sodium nitrite (1.02 equivalents), and the resulting diazonium salt is coupled with N,N-diethylaniline (1.0 equivalent) dissolved in 10% acetic acid at pH 3.5–4.0. The coupling mass is stirred for 3 h at 10°C, neutralised with sodium acetate, and filtered; the wet cake is washed chloride-free and dried at 60°C to yield a dark red powder characterised by λ_max 522 nm in methanol and an extinction coefficient > 3.5×10⁴ L mol⁻¹ cm⁻¹. This cationic benzothiazole azo dye is applied for acrylic fibre dyeing in a bath set at pH 4.5–5.5 using 1.0–2.0% o.w.f., giving bright bluish-red shades with light fastness 5–6 (ISO 105-B02) and washing fastness 4–5 (ISO 105-C06). Dyes of this structural family must comply with Regulation (EC) No 1907/2006 Annex XVII Entry 43 on azocolourants, meaning that no reductive cleavage of the azo bond must liberate any of the 22 listed carcinogenic aromatic amines upon testing per EN 14362-1:2017; the absence of 4-aminobiphenyl, benzidine, and 2-naphthylamine is confirmed by GC-MS at reporting limits <30 mg/kg. For textile articles placed on the EU market, compliance with OEKO-TEX Standard 100 (product class I) imposes further restrictions on quinoline (<50 mg/kg) and extractable heavy metals. The intermediate 2-aminobenzothiazole is considered a potential skin sensitiser; thus, the dye synthesis operation mandates closed transfer systems and area monitoring for airborne concentrations <0.1 mg/m³ (8h TWA).