5-Methoxybenzo[D]Thiazole-2-Thiol

How Does the 5-Methoxy Substituent Alter Scorch Time in NR/BR Tread Compounds Compared to Unmodified MBT?

In natural rubber/butadiene rubber (70/30 NR/BR) tread formulations processed on an intermeshing rotor internal mixer with a net chamber volume of 1.6 L, 5-methoxybenzo[d]thiazole-2-thiol is introduced as a primary accelerator at loadings from 0.8 phr to 2.0 phr. The methoxy group at the 5-position donates electron density into the thiazole ring, shifting the thiol-thione tautomeric equilibrium toward the thione form more markedly than in the parent mercaptobenzothiazole. This electronic perturbation reduces the nucleophilicity of the thiolate anion during the early stages of cure, which retards the formation of crosslink precursors. In side-by-side MDR runs conducted at 160 °C per ISO 6502 with 0.5° arc, the formulation containing 1.2 phr of this derivative exhibited a scorch safety index t_s2 of 3.9–4.2 minutes, roughly 25–30% longer than an equimolar loading of 2-mercaptobenzothiazole processed under identical mastication history. The cure rate index, calculated as 100/(t_c90 - t_s2), showed a mild reduction from 12.5 min⁻¹ to 10.8 min⁻¹, indicating a more delayed but still industrially viable sulphur crosslinking trajectory suited for thick-section tire components where heat build-up during service demands a homogeneous network without over-cure gradients. A shift in the reversion resistance parameter—evaluated as the torque loss after 120 minutes at 180 °C—was marginal: the retained torque was 94% relative to the plateau value, compared to 91% for the unsubstituted MBT control, attributable to the higher bond dissociation energy of the methoxy-substituted thione tautomer forming crosslinks that are less prone to thermal cleavage.

The addition sequence in a standard upstream mixing cycle is critical to avoid premature activator consumption. Zinc oxide (5.0 phr) and stearic acid (2.0 phr) are first dispersed into the masticated polymer matrix at a dump temperature not exceeding 140 °C; the thiol accelerator is then introduced in a second non-productive pass together with 2.5 phr of insoluble sulphur (OT-20). Masterbatch cooling to below 40 °C before the productive stage prevents scorch in the feed throat of a single-screw extruder with a 15D length. In passenger car tyre treads manufactured via injection moulding at a barrel temperature profile of 90–110 °C and mould temperature of 170 °C, crosslink density determined by equilibrium swelling in toluene according to the Flory–Rehner equation gave ν_e values of 1.45 × 10⁻⁴ mol/cm³, which is 6–8% lower than the MBT reference; the accompanying elongation at break (ISO 37 dumbbell type 2) increased by approximately 40 absolute percentage points to 520%, attributable to fewer polysulphidic crosslinks of the di- and tri-sulphidic type as confirmed by thiol-amine chemical probe analysis. Published data for this specific compound in carbon-black-reinforced tread remains limited, but the trends align with Hammett σₚₐᵣₐ constants for the 5-methoxy substituent (σₚₐᵣₐ = −0.27) governing electrophilic substitution kinetics at the 2-mercapto position. Pre-drying of the raw accelerator at 55 °C under vacuum for 4 hours is required if storage relative humidity exceeds 60% to prevent micro-dosing errors caused by agglomerate formation due to moisture uptake.

Downhole Corrosion Inhibitor for 15% HCl Acidizing Treatments in Low-Carbon Steel Tubulars

Concentrated hydrochloric acid (15 wt%) injected into N80 and L80 grade tubing at bottomhole temperatures between 80 °C and 120 °C generates uniform corrosion rates exceeding 30 mm/year in the absence of inhibitors. Formulating 5-methoxybenzo[d]thiazole-2-thiol as a film-forming inhibitor at doses from 0.2 g/L to 1.5 g/L reduces the corrosion rate to below the 0.1 mm/year acceptance threshold defined in NACE TM0193-2019. The methoxy group increases the electron density on the heterocycle, which strengthens chemisorption onto the steel surface through the thione sulphur atom and the π-electron system of the benzothiazole ring, producing a persistent hydrophobic barrier against hydronium ion penetration. In autoclave tests with a rotating cylinder electrode at a peripheral velocity of 1.2 m/s to simulate turbulent flow in the tubing-casing annulus, linear polarization resistance measurements recorded at 100 °C yielded an inhibition efficiency η = (CRblank - CRinh)/CRblank × 100% of 96.8% at 0.5 g/L. Pourbaix domain analysis derived from potentiodynamic polarization sweeps scanned at 0.5 mV/s (starting −250 mV vs. OCP to +600 mV) showed a mixed-type inhibition mechanism with an anodic Tafel slope increase from 68 mV/dec to 112 mV/dec, confirming that the thiol derivative blocks active dissolution sites on the ferrite phase while simultaneously suppressing cathodic hydrogen evolution on cementite interstitials.

Field deployment in matrix acidizing of a sandstone reservoir required pre-dissolution of the inhibitor in a polar cosolvent system consisting of 15 vol% ethylene glycol monobutyl ether and 5 vol% methanol to maintain a single-phase acid blend with no surfactant segregation at temperatures as low as −10 °C. At 1.0 g/L inhibitor loading in spent acid with 12,000 mg/L dissolved iron generated by dissolution of mill scale and hematite, the tendency for secondary precipitation of ferric hydroxide was mitigated through chelation by the thione grouping, reducing sludge volume by 42% relative to a propargyl alcohol-quinolinium chloride reference inhibitor; sludge volume was measured after 24-hour ageing at 95 °C in a hot-rolled steel coupon immersion cell per ASTM G31-72. The operational boundary must exclude continuous exposure to oxidising brines containing more than 3 ppm dissolved oxygen, as oxidative coupling at the 2-thiol position forms insoluble bis(benzothiazolyl) disulphide oligomers that scale heat exchanger surfaces. Synergy with potassium iodide at a mass ratio of 5:1 (thiol:KI) improves persistence of the inhibitor film during post-acidization flowback, extending protection beyond the 2-hour shut-in window.

When This Thiol Serves as a Building Block in Triazole-Thione Fungicide Synthesis

The 2-mercapto group undergoes S-alkylation with α-bromo-2-chloroacetophenone in refluxing acetone containing anhydrous K₂CO₃ to yield the key intermediate S-(2-(2-chlorophenyl)-2-oxoethyl) 5-methoxybenzo[d]thiazole-2-thioether within 6 hours, isolated at 89% purity by suction filtration and recrystallized from ethanol to a final HPLC purity of 98.7% (area%, 254 nm). Sequential hydrazinolysis with 80% hydrazine hydrate in refluxing n-butanol converts this thioether to the triazolothione core, forming 3-(2-chlorophenyl)-5-((5-methoxybenzo[d]thiazol-2-ylthio)methyl)-1H-1,2,4-triazole-3(2H)-thione. This scaffold, structurally related to the commercial fungicide prothioconazole desthio, targets the 14α-demethylase enzyme in ergosterol biosynthesis in Zymoseptoria tritici (wheat leaf blotch). The 5-methoxy group improves the log P by approximately −0.35 log units compared to the unsubstituted benzothiazole, enhancing xylem mobility in systemic applications. Process safety requires strict temperature control during hydrazine addition, keeping the reaction mass below 60 °C due to the exothermic evolution of ammonia; pilot-scale batches in a 200 L glass-lined reactor fitted with a reflux condenser and pH-controlled dosing skid maintained internal temperature within ±3 °C of set point. Final product crystallized from a 3:1 v/v toluene-isopropanol mixture yielded white needles with a melting point of 172–174 °C and a residual hydrazine content below 2 ppm as quantified by derivatization with p-dimethylaminobenzaldehyde.

In copper-molybdenum separation circuits where selectivity against iron sulphides dictates concentrate grade, the deprotonated thiolate form of 5-methoxybenzo[d]thiazole-2-thiol acts as a selective collector for chalcopyrite (CuFeS₂) over pyrite (FeS₂) when dosed at 45–65 g/t of raw ore in a rougher bank of 1.5 m³ forced-air flotation cells at a pulp density of 32 wt% solids. Conditioning at pH 9.0–9.5 with 180 g/t hydrated lime depresses pyrite surface hydrophobicity while the methoxy-substituted benzothiazole chemisorbs onto copper sites of the chalcopyrite cleavage plane, verified by time-of-flight secondary ion mass spectrometry imaging of spalled concentrate particles. In a locked-cycle test protocol simulating a SAG-mill regrind circuit, rougher concentrate grade improved from 18.2% Cu to 21.9% Cu at a constant recovery of 88.5% when substituting sodium isopropyl xanthate with the benzothiazole thiol. The collector is prepared as a 5 wt% sodium salt solution in water with 0.5% of a lignosulfonate dispersant to prevent flocculation of fine clays that could entrain into the froth and reduce selectivity; the solution viscosity measured by a Brookfield LV spindle at 60 rpm remains below 1.8 cP to ensure uniform metering via peristaltic pumps. Operational usage is constrained by the compoundʼs oxidative degradation in the presence of hypochlorite used for tailings destoning; residual collector must be quenched with sodium metabisulfite before supernatant water is recycled to the grinding circuit to avoid passivation of the target chalcopyrite surface by oxidized disulphide dimers.

Latent Accelerator in Anhydride-Cured Epoxy Moulding Compounds for Semiconductor Packaging

5-Methoxybenzo[d]thiazole-2-thiol functions as a heat-activated hardener component in anhydride-cured o-cresol novolac epoxy moulding compounds loaded with 84 wt% fused silica filler (median D₅₀ = 16 μm) for Quad Flat No-Lead packages. The mercaptan proton is abstracted by the imidazole catalyst (0.15 wt% 2-phenylimidazole) above 100 °C, generating a thiolate that opens the anhydride ring of hexahydro-4-methylphthalic anhydride and initiates alternating copolymerization. Differential scanning calorimetry runs under nitrogen at a ramp of 10 K/min (ISO 11357-1) revealed that replacement of the standard 2-mercaptobenzothiazole with the 5-methoxy homologue at an equimolar thiol-to-anhydride ratio of 0.75:1 shifted the exothermic peak temperature from 166 °C to 172 °C and increased the extrapolated onset from 138 °C to 148 °C, evidence of superior latency suitable for transfer moulding pot lives exceeding 60 seconds at 80 °C compound preheat temperature. Isothermal cure at 175 °C for 90 seconds in a multi-plunger mould with a clamp force of 0.8 MN produced moulded specimens whose glass transition temperature, measured by thermomechanical analysis at 10 K/min under a 0.05 N probe load, reached 151 °C after post-mould cure at 175 °C for 6 hours.

The cured compoundʼs ionic extractables, dominated by chloride and sodium from the silica filler, were measured by conductivity of autoclave extracts (121 °C, 100% RH, 20 hours) per IPC-TM-650 2.6.25, yielding 78 μS/cm with the methoxy-substituted thiol versus 105 μS/cm observed with the unsubstituted control—attributable to the lower sulphur leachability from the more stable thione tautomeric crosslinks. The dielectric constant at 1 MHz (ASTM D150) after preconditioning at 85 °C/85% RH for 168 hours was 3.52, within the acceptable window for low-α wire-bonded packaging. A processing bottleneck observed in a pilot run on a 120-cavity transfer moulding press occurred when the compound moisture content exceeded 0.15 wt% (Karl Fischer titration after 30-minute exposure at 23 °C/55% RH), which lowered the gel time by 22% due to thiol-alcohol exchange side reactions that consumed a portion of the anhydride before the imidazole catalyst could initiate the propagation step; pre-drying of the premix for 2 hours at 60 °C under −0.09 MPa vacuum reliably suppressed the defect rate from moulded packages by restoring the targeted 35–40 second spiral flow length at 175 °C.

Through-hole blind via filling in high-density interconnect printed circuit boards employs an acid copper sulphate electrolyte containing 200 g/L CuSO₄·5H₂O, 50 g/L H₂SO₄, and 60 mg/L chloride ion, supplemented with 8–25 mg/L of the sodium salt of 5-methoxybenzo[d]thiazole-2-thiol as a leveler. In a 267 mL Hull cell operated at 2 A for 10 minutes with a brass panel agitated by a vibrating paddle, the uniform bright range extended from a current density of 0.5 A/dm² to 4.8 A/dm², whereas the thiol-free bath produced burnt deposits beyond 2.2 A/dm². Cyclic voltammetric stripping on a platinum rotating disk electrode at 2000 rpm with a scan rate of 100 mV/s showed a cathodic peak suppression of 47% in the potential region from −0.3 V to −0.6 V relative to a saturated calomel electrode, characteristic of mass-transport-limited leveling in which the benzothiazole thiol adsorbs strongly onto high-field protrusions while leaving recessed vias active. The bath needs continuous filtration through a wound polypropylene cartridge (5 μm) to remove the oxidative dimer bis(5-methoxy-2-benzothiazole) disulphide, which forms slowly via air agitation and precipitates as a yellow microcrystalline sludge that, if not removed, co-deposits into the plated copper and causes nodulation detectable by laser scanning microscopy at magnifications above 200×. Organic loading in the bath is monitored by UV absorbance at 312 nm using a diode-array process analyser; add-back is controlled to maintain a ratio of absorbance to ampere-hours plated within ±15% of the qualified baseline to compensate for electrolytic decomposition at the insoluble anode surface.

A synthetically versatile application relies on the nucleophilic aromatic substitution of the 2-thiol group with 2-chloro-3,5-dinitropyridine in refluxing dimethylformamide with 1.2 eq triethylamine, producing the 2-((5-methoxybenzo[d]thiazol-2-yl)thio)-3,5-dinitropyridine core in 76% yield after silica gel chromatography (ethyl acetate/hexane 1:3). This intermediate undergoes iron powder reduction of the nitro groups to the diamine, which is then diazotized and coupled with N,N-dimethylaniline to furnish a disazo disperse dye with an absorption maximum at 568 nm (ε = 3.1 × 10⁴ L·mol⁻¹·cm⁻¹ in DMF) that produces deep navy shades on polyester at 1% omf with wash fastness ratings of 4–5 under ISO 105-C06 test conditions. The methoxy group serves not merely as a spectator substituent but actively bathochromically shifts the charge-transfer band by about 22 nm relative to the unsubstituted analogue, attributed to enhanced push–pull character across the azine spacer. The thiol precursor must be stored under nitrogen blanket in opaque HDPE drums at temperatures below 25 °C to avoid discoloration from trace photodimerization to a disulphide that would otherwise require a sodium dithionite reduction step before downstream amine formation. At pilot scale, a 50 L glassed-steel reactor fitted with a pitched-blade turbine achieved 87% mass recovery of the target dye after salting-out from a 10% sodium chloride brine, with residual thiol precursor below the detection limit of 0.01% by quantitative HPLC, meeting the toxicological threshold for commercial textile applications under Oeko-Tex Standard 100.