5-Methylbenzothiazole

The synthesis of 2-mercapto-5-methylbenzothiazole proceeds via thiolation of 5-methylbenzothiazole with sodium hydrosulfide under pressure in an aqueous medium at 140–160°C. The reaction is carried out in a Hastelloy C-276 autoclave with anchor-type agitation at 60–90 rpm to ensure sufficient gas-liquid contact, as the rate-limiting step is dissolution of H₂S into the molten organic phase. The molar ratio of NaHS to 5-methylbenzothiazole is maintained at 1.08:1; excess hydrosulfide shifts the equilibrium toward the thiolate but levels above 1.15:1 generate polysulfide byproducts that reduce the melting point of the crude product and cause filtration difficulties in the downstream isolation step. After a hold time of 5–6 hours, the batch is cooled to 80°C, acidified to pH 4.5–5.0 with dilute sulfuric acid, and the precipitated 2-mercapto-5-methylbenzothiazole is collected by vacuum belt filtration. The wet cake is washed with deionized water until the conductivity of the filtrate drops below 50 μS/cm and dried in a conical vacuum dryer at 45°C and 5 kPa to a moisture content of ≤0.15 wt%, which is critical because residual moisture promotes dimerization during subsequent pelletizing and storage. The final accelerator is blended with ethylene-propylene-diene monomer (EPDM) binder and paraffinic process oil in a co-rotating twin-screw extruder (L/D ratio 48:1) at a screw speed range of 180–220 rpm and barrel temperature profile 50°C→65°C→70°C→65°C→50°C to produce dust-free cylindrical pellets of 2.0–2.5 mm diameter. In a natural rubber compound formulation based on RSS-1 (ribbed smoked sheet) filled with N330 carbon black at 50 phr, the dichloromethane-extractable 2-mercapto-5-methylbenzothiazole content measured by HPLC must fall within 0.35–0.50 wt% of total compound mass to balance scorch safety and crosslink density. At loading of 0.8 phr in a sulfur cure system with 2.0 phr sulfur and 4.0 phr zinc oxide, the compound exhibits a Mooney scorch time (MS-t5 at 121°C, per ISO 289-1:2022) of 18–22 minutes and a cure rate index (per ISO 3417:2008, MDR at 150°C) of 8.5–9.5 dN·m/min. Methyl substitution on the benzothiazole ring reduces the nucleophilicity of the mercapto group relative to unsubstituted MBT, decelerating sulfidic crosslink insertion and thereby widening the safe-processing plateau by 3–5 minutes compared to formulations employing an equimolar sulfur mass fraction of MBT. This delay is exploited in the manufacture of large rubber-to-metal bonded anti-vibration mounts, where full mold filling before the onset of scorch requires a compound flow distance exceeding 600 mm in transfer molding presses with clamp force above 800 tonnes. Compliance with EU Directive 2005/69/EC (PAH restriction) is maintained via post-vulcanization acetone extraction that confirms benzo[a]pyrene content below the 1.0 mg/kg threshold. Accelerator migration into polyamide overmolding layers is suppressed when the cure temperature is limited to 145°C to avoid the exudation threshold of 150–155°C observed in TGA isothermal mass loss experiments.

Textile-grade 4,4′-bis[(4-anilino-6-morpholino-1,3,5-triazin-2-yl)amino]stilbene-2,2′-disulfonic acid derivatives are synthesized by cyanuric chloride mediated coupling, where 5-methylbenzothiazole-2-sulfonic acid serves as the anilino-group building block that extends the conjugation length and shifts the fluorescence emission maximum from 435 nm to 448 nm. Sulfonation of 5-methylbenzothiazole is performed with 20% oleum at 25–30°C, yielding the 2-sulfonic acid isomer in excess of 92% selectivity as verified by ion chromatography with conductivity detection. The condensation with cyanuric chloride is carried out in a jacketed glass-lined reactor at 0–2°C and pH 7.0–7.5, controlled by simultaneous metering of 10% sodium carbonate solution, using a mole ratio of 5-methylbenzothiazole-2-sulfonic acid to cyanuric chloride of 1.00:1.02. After the first condensation step, the second chlorine on the triazine ring is substituted with aniline-2,5-disulfonic acid at 35–40°C and pH 6.5–7.0, followed by the third chlorine displacement with morpholine at 80–85°C. The resulting triazine-stilbene fluorescent whitening agent (FWA) is isolated as a disodium salt by spray drying at an inlet temperature of 190°C and outlet of 90°C, yielding free-flowing microspheroids with a bulk density of 0.45–0.55 g/cm³. In laundry powder application at 0.08–0.15 wt% of detergent mass, the CIE whiteness index of cotton poplin fabric after 25 wash cycles at 60°C in water of 250 ppm CaCO₃ hardness is increased by 18–22 points relative to unwhitened control, as determined by a spectrophotometer with d/8° geometry per ISO 105-J02:1997. The critical aggregation concentration of the FWA in the wash liquor, measured by fluorescence polarization, is 8×10⁻⁶ mol/L; above this value the quantum yield plateaus because of excimer self-quenching. Therefore, the dosing pump stroke length on the detergent post-dosing station must be calibrated to maintain an anionic surfactant to FWA ratio of 120:1 to 150:1 to prevent precipitation as a surfactant-FWA complex. Registration for detergent use under EU Ecolabel Decision (EU) 2017/1218 requires ready biodegradability demonstrated by >70% DOC removal in a 28-day OECD 301B test, a criterion that the morpholine-substituted structure satisfies when the dissolved oxygen uptake curve shows a lag phase not exceeding 3 days.

Where solubility parameters dictate that the benzothiazole mesogenic core must exhibit a dipole moment between 2.8 and 3.2 D to achieve the desired dielectric anisotropy for twisted nematic (TN) and super-twisted nematic (STN) electro-optical cells, 5-methylbenzothiazole is converted to 2-(4-alkylphenyl)-5-methylbenzothiazole via a palladium-catalyzed Suzuki coupling. The cross-coupling employs Pd(PPh₃)₄ at a loading of 0.5 mol% relative to 5-methylbenzothiazole-2-boronic acid pinacol ester, with potassium carbonate (2.0 equiv.) in a degassed toluene/ethanol/water ternary mixture (volume ratio 5:1:1) at reflux for 8 hours. The boronate ester precursor is obtained by lithiation of 5-methylbenzothiazole with n-butyllithium at -78°C in anhydrous THF, followed by quenching with 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane at -70 to -65°C. After aqueous work-up and chromatography over silica gel (eluent: hexane/ethyl acetate 95:5 v/v), the 2-(4-alkylphenyl)-5-methylbenzothiazole intermediates are hydrogenated over 5% Pt/C at 0.3 MPa to saturate the alkyl chain to a trans-cyclohexyl unit when low birefringence is targeted. The nematic phase range of the homologous series with alkyl chain lengths C₃ to C₇ spans from 42°C to 118°C (clearing point), as determined by differential scanning calorimetry at a scan rate of 5°C/min. Voltage holding ratio (VHR) measurements at 25°C and 1 Hz frame rate, following IEC 61747-5-3:2020, demand that the mesogen exhibit a resistivity >5×10¹³ Ω·cm after silica gel filtration and vacuum degassing at 2 Pa for 24 hours to eliminate ionic impurities below the 10⁻⁸ mol/L threshold. Blends containing 8–12 wt% of this benzothiazole dopant in a base mixture of alkylcyanobiphenyls and alkylterphenyls reduce the threshold voltage (V₁₀) of a 90° TN cell from 1.85 V to 1.58 V while the optical response time (τ_on+τ_off) at ±3 V driving remains below 45 ms. The methyl substituent at the 5-position suppresses the formation of the smectic A phase that otherwise appears with unsubstituted benzothiazole derivatives at temperature ranges overlapping the nematic operating window, a critical processing requirement for injection-filling of 4–6 μm cell gaps on production lines operating with a fill-speed of 1.5–2.5 mm/s.

The cyanine dye chromophore 3-ethyl-2-[3-(3-ethyl-3H-benzothiazol-2-ylidene)-propenyl]-5-methylbenzothiazolium iodide, used as a spectral sensitizer in silver halide tabular-grain emulsions for green-light recording layers, is assembled by condensation of 5-methyl-2-methylthiobenzothiazole with 2-(2-acetanilidovinyl)-3-ethylbenzothiazolium iodide in acetic anhydride at 120°C in the presence of triethylamine (1.1 eq.). The key intermediate 5-methyl-2-methylthiobenzothiazole is prepared by S-alkylation of the corresponding 2-mercapto derivative with dimethyl sulfate at 55°C in aqueous sodium bicarbonate, the exothermicity requiring controlled reagent addition over 45 minutes to keep the temperature below 60°C. After the dye condensation, the reaction mass is drowned into ice water, and the precipitated dye is collected, washed with methanol until the filtrate is colorless, and vacuum-oven dried at 40°C. The absorption maximum in methanol is 512 ± 2 nm (log ε = 4.92), which shifts to 535 nm in the J-aggregate state adsorbed onto AgBrI (3 mol% iodide) tabular grains of 1.2 μm equivalent circular diameter. Coating formulations for negative-working graphic arts film employ a dye loading of 0.35–0.50 mmol per mole of silver halide to achieve a spectral sensitivity of 3.2×10⁻³ J/m² at 530 nm (exposure required for an optical density of 1.0 above fog), evaluated in a 21-step sensitometric wedge exposure through a 530 nm interference filter. The residual methylthiobenzothiazole precursor must be <0.05% area by HPLC (UV 254 nm) because it functions as a desensitizer on the grain surface, elevating the intrinsic fog level by displacing the cyanine dye from its adsorption sites. Manufacturing specifications for the finished dye, aligned with ISO 18921:2008 (silver image stability), require that a forced-aging test at 70°C and 80% RH for 7 days produces a change in optical density of dye-only extraction solvent of less than 0.03 units, confirming that the 5-methyl substitution imparts sufficient oxidative stability by blocking the benzothiazole ring position most susceptible to photo-Fenton discoloration.

When the benzothiazole core is incorporated as a heterocyclic substituent on the triazolopyrimidine scaffold of the fungicide 5-chloro-7-(4-methylpiperidin-1-yl)-6-(2,4,6-trifluorophenyl)-[1,2,4]triazolo[1,5-a]pyrimidine, 5-methylbenzothiazole-2-carbaldehyde functions as the key building block that introduces a hydrogen-bond-acceptor motif to the ATP-binding pocket of succinate dehydrogenase. The aldehyde is generated by Vilsmeier-Haack formylation of 5-methylbenzothiazole using POCl₃ and DMF at 80°C, with the mole ratio of POCl₃ to substrate set at 2.5:1 to drive complete conversion within 6 hours. After quenching onto crushed ice and neutralization to pH 7.0, the crude aldehyde is purified by vacuum distillation at 2.5–3.0 mm Hg (boiling range 148–152°C) and stored under nitrogen at –20°C to suppress auto-oxidation to the corresponding carboxylic acid, which would otherwise exceed a specification limit of 0.2 wt% as measured by acid-base titration. The subsequent Knoevenagel condensation with ethyl cyanoacetate in toluene with piperidine acetate catalyst at reflux, followed by cyclization with aminoguanidine bicarbonate, yields a benzothiazole-triazolopyrimidine intermediate in 65–70% overall yield after recrystallization from methyl tert-butyl ether. Pesticide registration under Regulation (EC) No 1107/2009 requires an impurity profile demonstrating that the 5-methylbenzothiazole-2-carbaldehyde intermediate contains less than 0.1 mg/kg of mutagenic benzothiazole-derived nitrosamines, as verified by LC-MS/MS with a method detection limit of 0.02 mg/kg. The commercial wettable powder formulation (500 g/kg active ingredient) is obtained by jet-milling the active substance with sodium lauryl sulfate (2.0 wt%) and precipitated silica (8.0 wt%) to a particle size distribution with D₉₀ <5 μm (laser diffraction, Malvern Mastersizer 3000), ensuring suspension stability of >90% after 30 minutes in a 2.0% aqueous dilution per CIPAC MT 184.

The amine-catalyzed condensation of 5-methylbenzothiazole-2-sulfonamide with 2-chloro-4,6-dimethoxy-1,3,5-triazine in a two-phase water/toluene system produces a sulfonylurea herbicide safener that upregulates glutathione S-transferase in maize. The sulfonamide intermediate is synthesized by chlorosulfonation of 5-methylbenzothiazole with chlorosulfonic acid at –5 to 0°C, followed by ammonolysis with 25% aqueous ammonia at 10–15°C. The addition rate of ammonia is controlled so that the pH of the reaction mixture does not exceed 10.5 to avoid ring-opening of the thiazole via nucleophilic attack at the 2-position. After filtration, the sulfonamide is recrystallized from isopropanol to a melting point of 184–186°C. In the triazine coupling step, the stoichiometry of sulfonamide to 2-chloro-4,6-dimethoxy-1,3,5-triazine is 1:1.1, with tetrabutylammonium bromide phase-transfer catalyst at 3 mol% and aqueous NaOH as acid scavenger to maintain the interphase pH at 8.0–8.5. The reaction completes in 12–14 hours at 30°C, after which the organic layer is separated, dried over sodium sulfate, concentrated, and the safener precipitated by addition of n-heptane. The safener is typically coated onto corn seed at a rate of 30–50 g a.i. per 100 kg seed in combination with a film-forming polymer binder (polyvinyl alcohol, hydrolysis degree 88%) to ensure adhesion and controlled release. Efficacy testing according to Council Directive 76/116/EEC methodology, as amended, demonstrates that the safener reduces the phytotoxicity of sulfonylurea herbicides such as nicosulfuron by >85% at the 2-leaf stage of maize, measured by chlorophyll fluorescence parameter Fv/Fm after 7 days post-application. The impurity 5-methylbenzothiazole released from incomplete ammonolysis must be kept below 0.5 wt% in the final safener, because it acts as a phytotoxin itself at concentrations above 15 g/ha foliar, a value determined by dose-response trial on Zea mays inbred line LH244.