5-Methyl-1,3-Benzothiazole: Properties, Uses, Safety Data & Supplier Information

What Does a 5-Methyl Substituent Contribute to Furnace Black-Loaded Natural Rubber Compounds?

The downstream derivative 2-mercapto-5-methylbenzothiazole (MMBT) is manufactured by reacting 5-methyl-1,3-benzothiazole with elemental sulphur and aqueous sodium sulphide under autogenous pressure at 160–180 °C, followed by acidification to pH 2.5–3.0 with 20 % sulphuric acid. A critical impurity, 2,2′-dithiobis(5-methylbenzothiazole), forms when dissolved oxygen is not controlled below 0.5 mg/L in the aqueous phase; its concentration must be held below 0.3 % w/w—quantified by reversed-phase HPLC per ASTM D4937-96(2021)—because the disulphide generates a persistent amine bloom on cured rubber surfaces that interferes with marking and bonding. In a truck tyre tread formulation based on RSS3 natural rubber (100 phr), N330 carbon black (50 phr), treated distillate aromatic extract oil (5 phr), zinc oxide (5 phr), stearic acid (2 phr), insoluble sulphur (2.5 phr), and MMBT at 0.8–1.2 phr, the accelerator provides a scorch delay (t_s2 at 160 °C) of 4.5–6.0 min measured on a moving-die rheometer according to ISO 6502-3:2018. This induction window is 30–40 % broader than that obtained with unsubstituted mercaptobenzothiazole at equivalent molar loading, a margin that permits safer processing on open two-roll mills where the front roll is held at 70–80 °C, the back roll at 60–65 °C, and thin sheet passing time approaches 12–15 min. When MMBT is combined with a sulphenamide booster such as N-cyclohexyl-2-benzothiazolesulphenamide (CBS) at a 1:1 w/w ratio, the cure rate index improves while the delta torque (M_H − M_L) stabilises at 10–12 dN·m, indicating a dense crosslink network without overcure reversion. In an inverted Banbury mixing cycle, polymer and two-thirds of the carbon black are ram-loaded at 40 °C jacket temperature; the ram is lifted at 120 °C to introduce oil and remainder black, and the batch is dropped at 155–160 °C. MMBT and sulphur are exclusively fed on a mill or in a separate non-productive downstream stage to prevent premature vulcanisation. Tensile properties after press cure at 150 °C to t₉₀ plus 5 min conform to ASTM D412-16: tensile strength 22–26 MPa, elongation at break 480–520 %. Accelerated ageing in air ovens at 100 °C for 72 h ( ISO 188:2011 ) returns elongation retention above 85 %. For food-contact extruded profiles such as dairy tubing liners, the cured compound must pass the extractives tests of FDA 21 CFR 177.2600 for rubber articles intended for repeated use, where the n-hexane extractable fraction is not to exceed 1.5 mg/cm². A zinc-free activator system based on 2 phr of a sterically hindered aminoalcohol dispersion can replace ZnO where local aquatic toxicity requirements are referenced against EU REACH Annex XVII entry 30, although a 15 °C increase in mould temperature is necessary to achieve identical crosslink density. Operators must note that MMBT dispersions stored below 10 °C undergo crystallisation and require slow warming to 25 °C with low-shear paddle agitation before weight-batching. In continuous vulcanisation lines—molten salt, hot-air fluidised bed, or ultra-high-frequency microwave—the prolonged induction period prevents premature set-up in the feed throat while still permitting complete cure within 45 seconds at 230 °C for profiles up to 8 mm thickness. End articles spread across engine mounts, heavy-duty conveyor belts, and butyl inner tubes where synergies with tetramethylthiuram disulphide at 0.15 phr enhance reversion resistance.

Diazotisation of 5-methyl-1,3-benzothiazole (1.0 mol) in 85 % sulphuric acid at 0–5 °C with 1.05 mol sodium nitrite produces a clear diazonium salt that is rapidly filtered and coupled to N,N-diethylaniline (1.02 mol) pre-dissolved in ice water containing 0.5 mol hydrochloric acid. Coupling proceeds at a pH of 3.0–3.5, maintained by the metered addition of sodium acetate, over 4–6 h at 5–10 °C. The precipitated monoazo dye is isolated by basket centrifugation, washed with deionised water until the filtrate conductivity falls below 200 µS/cm, and tray-dried under vacuum at 60 °C. The resulting cationic dye—a positional isomer of C.I. Basic Blue 41 with the methyl group at C‑5 rather than C‑6—exhibits a hypsochromic shift of 12–15 nm, which deepens the green-shade blue value preferred for polyacrylonitrile (PAN) exhaust dyeing. A standard dye bath is prepared at a liquor ratio of 1:20, containing 0.5–2.0 % o.w.f. dye, 10 g/L anhydrous sodium sulphate, and sufficient 40 % acetic acid to hold pH at 4.5–5.0. The bath is raised to 98 °C over 30 min and held for 45–60 min; an after-treatment with a commercial cationic fixative improves wet-fastness by forming a large-molecule complex on the fibre surface. Colour strength build-up on 2 dtex semi-dull PAN tow is verified by reflectance spectrophotometry in compliance with ISO 105-J01:2009. Fastness grades routinely attainable: light (ISO 105-B02:2014) 5–6, washing (ISO 105-C06:2010 test A2S at 60 °C) shade change 4–5, staining on multifibre adjacent fabric 4. The migration index, determined by the film-transfer method of VDI 3822 Part 5, stays below 0.15 at 1.0 % depth, confirming low blotch tendency on rapid-dyeing machines. Formulators must verify that the free aromatic amine content—determined by reductive cleavage and GC–MS per EN 14362-1:2017—does not exceed the 5 mg/kg threshold set by OEKO-TEX Standard 100 Annex 6. Any production batch exceeding 10 mg/kg requires an additional steam-distillation purification campaign. The final commercial form is a powder standardised to strength with 30–50 % dextrin; dust-free granulation by wet extrusion using a 5 % lignosulphonate binder is recommended to keep workplace airborne particulates below the 0.1 mg/m³ reference limit for organic dyes. End articles include loose stock, tow, and high-bulk yarns destined for knitwear, automotive upholstery, and tufted carpets where dry-cleaning fastness is mandatory.

When Residual Solvent Limits Govern Usability for cGMP-Orientated API Manufacturing

The 5-methylbenzothiazole nucleus is converted into a key synthetic intermediate for a class of Rho-associated coiled-coil kinase (ROCK) inhibitors under investigation for topical glaucoma treatment. In a representative three-step sequence conducted in cGMP pilot suites, the heterocycle is first brominated with 1.05 eq of N-bromosuccinimide in N,N-dimethylformamide at 20–25 °C, affording 2-bromo-5-methylbenzothiazole with a typical isolated yield of 78–82 % after aqueous work-up and recrystallisation from n-heptane. The bromide undergoes palladium-catalysed Buchwald–Hartwig amination with 1-Boc-piperazine (1.2 eq) using Pd₂(dba)₃ (1 mol %) and XPhos (2 mol %) in tetrahydrofuran containing sodium tert-butoxide (1.4 eq) at 65 °C under nitrogen for 8 h. After filtration through a Celite pad, solvent exchange to acetonitrile, and Boc deprotection with methanolic hydrogen chloride, the hydrochloride salt crystallises directly. The crude salt is re-slurried in isopropanol at 40 °C to reach > 99.5 % purity (HPLC area-percent at 254 nm), with any single unspecified impurity controlled below 0.10 %. Residual solvent levels are measured by headspace GC against the limits of ICH Q3C: dichloromethane < 600 ppm, acetone < 5000 ppm, n-heptane < 5000 ppm, and Class 1 solvents absent by a validated limit test. Elemental impurities are managed via an ICH Q3D risk assessment; palladium, the metal of concern, is routinely found below the 10 µg/g oral PDE-derived acceptance limit when a charcoal treatment step is inserted before salt formation. The intermediate ships under an active drug master file and is stored at 2–8 °C with a re-test period of 24 months in double polyethylene liners inside a fibre drum. Downstream, the manufacturer couples this amine to a benzothiophene carboxylic acid chloride in the presence of aqueous sodium hydrogencarbonate to form the final amide API. Because the coupling introduces no further purification steps capable of removing the genotoxic impurity 2-bromo-5-methylbenzothiazole carryover, the upstream specification mandates that residual aryl bromide not exceed 50 ppm—tight enough to align with the staged TTC of 1.5 µg/day when the maximum daily dose is projected at 30 mg API. Production vessels must be dedicated or validated with a cleaning acceptance limit of < 1 µg/cm² by swab analysis, and the entire process is overseen under 21 CFR Part 211 with batch record review by the qualified person. The finished sterile ophthalmic solution is terminally sterilised by autoclaving at 121 °C for 15 min after aseptic filling.

To prepare succinate dehydrogenase inhibitor (SDHI) fungicide candidates containing a 5-methylbenzothiazole-2-carboxamide scaffold, the heterocycle is first converted to the 2-chloro derivative via Sandmeyer reaction with copper(I) chloride in concentrated hydrochloric acid at 0–10 °C. The chlorophenyl intermediate is then treated with sodium hydride (1.2 eq) in anhydrous dimethyl sulphoxide and subsequently carbonylated at 80 °C under 5 bar carbon monoxide in the presence of 1,3-disubstituted pyrazole as nucleophile, employing a Pd(OAc)₂/dppf catalyst system. After quenching into water and extraction with ethyl acetate, the crude amide is purified on a wiped-film evaporator at 120 °C jacket temperature and 1 mbar to produce a technical-grade solid of ≥ 97 % purity. Pilot-scale field trials on winter wheat against Zymoseptoria tritici indicate that the molecule, formulated as a 250 g/L suspension concentrate with non-ionic polyarylphenol ethoxylate stabilisers, delivers equivalent efficacy to fluxapyroxad when applied at 100 g a.i./ha in a twin spray programme. Registration data packages under Commission Regulation (EC) No 1107/2009 require a residue definition limited to the parent compound, with a proposed MRL of 0.01 mg/kg in wheat grain—monitored by LC–MS/MS with a validated LOQ of 0.002 mg/kg ( SANCO/12571/2013 method validation criteria). Shelf-life testing on the suspension concentrate at 54 °C for 14 days (CIPAC MT 46.3) shows no crystal growth above 5 µm when measured by wet laser diffraction; a thickening system of 0.15 % xanthan gum and 0.5 % magnesium aluminium silicate is essential to prevent syneresis during storage at −5 °C. Compatibility with adjuvant tank-mix partners is verified through the static tube test of ASTM E1518-05, with phase separation not observed after 2 h. Although published data for this exact carboxamide structure remain limited to proprietary registration studies, the analogous thiazole-2-carboxamide scaffold is documented in EFSA conclusions for bixafen and flubeneteram, where the primary metabolic pathway proceeds via oxidation of the benzothiazole ring followed by conjugation.

Table 1. Regulatory Compliance Matrix for Downstream Uses of 5-Methyl-1,3-Benzothiazole Derivatives

Application Sector

Governing Standard / Regulation

Critical Limit

Analytical Methodology

Rubber accelerator

FDA 21 CFR 177.2600

n-hexane extractables < 1.5 mg/cm²

Gravimetric after Soxhlet extraction

Rubber accelerator

EU REACH Annex XVII Entry 50

Polycyclic aromatic hydrocarbons sum < 1 mg/kg in extender oil

EN 16143:2013 GC–MS

Cationic dye

OEKO-TEX Standard 100 Annex 6

4-Aminoazobenzene < 5 mg/kg

EN 14362-1:2017 GC–MS

Pharmaceutical intermediate

ICH Q3D

Pd < 10 µg/g (oral concentration limit)

ICP–MS after microwave digestion

Pharmaceutical intermediate

USP <467>

Class 1 solvents not detected

Headspace GC–FID

Agrochemical intermediate

Regulation (EC) No 396/2005

MRL 0.01 mg/kg in cereals

LC–MS/MS (LOQ 0.002 mg/kg)

Addition of 0.1–0.5 wt% of the 2-amino-5-methylbenzothiazole derivative into a lithium 12-hydroxystearate grease operating at spindle speeds up to 3600 rpm reduces copper corrosion from 2b to 1a under the ASTM D130-19 protocol at 100 °C for 24 h. The performance holds only when the grease is pre-heated to 70 °C before additive charging to avoid localised micelle destabilisation; published data for this specific configuration is limited.

5-Methyl-1,3-benzothiazole (CAS 136-90-3, molecular formula C₈H₇NS, molecular weight 149.21 g/mol) is supplied under commercial designations typically appended with a grade suffix—5MBT-TG for technical grade (≥97.5% GC) and 5MBT-PH for pharmaceutical intermediate grade (≥99.0% GC). The product is a pale amber liquid at ambient temperature with a characteristic thiazolic odor, exhibiting a density of 1.16–1.18 g/cm³ at 20 °C (ASTM D4052-22) and a refractive index n_D²⁰ of 1.617–1.620 (ISO 280:1998). Commercial packaging defaults to 200 kg UN-approved HDPE drums with PTFE-lined closures, inert gas headspace blanketing, and a recommended storage condition of 0–10 °C under exclusion of UV light to suppress photo-oxidative darkening; under these conditions, retest intervals extend to 24 months from the date of manufacture. Bulk shipments via stainless steel ISO tank containers require an oxygen content below 50 ppm in the vapor phase to prevent the formation of intractable brown oligomers, a failure mode documented during intercontinental logistics where container freeboard air ingress led to a 1.8% purity drop and rejection of a 14,000 kg lot at the receiving site.

What Distinguishes the Methyl-Substituted Benzothiazole Series in Heterocyclic Synthesis?

Positional isomerism within the methylbenzothiazole family markedly alters electrophilic aromatic substitution patterns and metalation regioselectivity. In 5-methyl-1,3-benzothiazole, the electron-donating methyl group para to the endocyclic nitrogen activates the C-6 position for electrophilic attack, whereas in 2-methylbenzothiazole (CAS 120-75-2), the substituent adjacent to the sulfur exerts a dominant steric effect that retards N-alkylation kinetics by a factor of 3–4 when reacting with methyl iodide in acetonitrile at 60 °C. The 5-methyl isomer is thus preferred as a precursor for benzothiazole-based merocyanine dyes, where C-6 formylation with Vilsmeier reagent (POCl₃/DMF, 1.2 eq., 0–5 °C to ambient over 18 h) proceeds with 82–88% isolated yield without detectable regioisomeric contamination. By contrast, 6-methylbenzothiazole (CAS 2942-11-5) exhibits an elevated melting point (76–78 °C) and finds niche application as a low-molten carrier in solid-phase peptide synthesis tagging, where crystalline handling simplifies automated resin loading—a capability not shared by the liquid 5-methyl analog.

Comparative physical properties and purity benchmarks
Parameter	5-Methyl-1,3-benzothiazole	Benzothiazole (unsubstituted)	2-Methylbenzothiazole	6-Methylbenzothiazole
CAS No.	136-90-3	95-16-9	120-75-2	2942-11-5
Physical state (25 °C)	Liquid	Liquid	Liquid	Solid
Melting point (°C)	−15 to −12	−2	−14	76–78
Boiling point (°C, 101.3 kPa)	238–240	230	238	265
Typical purity (GC, area-%)	≥99.0	≥99.0	≥98.5	≥97.0
Preferred metalation site	C-2 (via LDA, −78 °C)	C-2	Benzylic CH₃ (LDA, −40 °C)	C-2 (competition with C-5)

Production-scale purifications exploit the low freezing point of the 5-methyl isomer. Continuous falling-film crystallisation is not applicable due to the liquid state at chiller temperatures above −25 °C; instead, vacuum fractional distillation through a 10-plate Oldershaw column at a reflux ratio of 5:1 under 2.7 kPa absolute pressure yields heart fractions with individual impurity levels below 0.15%. The isomer distribution in the crude product, generated via formic acid-mediated cyclization of the corresponding formamidinothiophenol, is sensitive to the HCl scavenger addition rate: injection times shorter than 45 minutes on a 2 m³ batch scale consistently increase the dimeric byproduct 5,5′-dimethyl-2,2′-bibenzothiazole to >1.2%, necessitating a subsequent acidic wash that itself contributes to yield loss of 4–6%.

Vulcanization Accelerator Chemistry and the 5-Methyl Position Advantage

Mercaptobenzothiazole (MBT) and its sulfenamide derivatives dominate the rubber accelerator market, yet replacement of MBT with 5-methyl-2-mercaptobenzothiazole (obtained by thiolation of 5-methyl-1,3-benzothiazole) modifies scorch safety margins in sulfur-cured natural rubber compounds. A comparative oscillating disc rheometer study (ASTM D2084-19a) conducted on a carbon-black-filled NR formulation (50 phr N330, 2.5 phr sulfur, 1.0 phr accelerator) demonstrated that the 5-methyl sulfenamide prolongs the t_s2 scorch time by 1.8 minutes at 140 °C relative to the unsubstituted analog, while maintaining a torque increase ΔS of 11.2 dNm. The extended induction period is attributed to the electron-donating methyl group raising the energy barrier for thiocarbamoyl disulfide formation, the rate-limiting step in accelerator activation. On the factory floor, this translates into a wider processing window on L/D 32 twin-screw extruders with pin-type mixing sections, enabling continuous profiling at screw speeds up to 180 rpm without premature crosslinking at the die face.

Compounded sheets exhibiting a Mooney viscosity (ML 1+4, 100 °C, ISO 289-1:2018) of 48–52 MU after 30 days of ambient storage indicate stable polymer-accelerator dispersion. However, the 5-methyl derivative shows incompatibility with zinc oxide levels above 5 phr: the methyl group promotes excessive zinc-complexation, leading to a 15% reduction in strain-induced crystallization as measured by wide-angle X-ray scattering (WAXS) on dumbbell specimens stretched to 500% elongation. Therefore, the grade is recommended only for low-zinc or zinc-free accelerator systems, a constraint communicated to compounders during technical transfer meetings.

When Optical Brightener Purity Demands Exceed 99.5% GC Assay

Fluorescent brighteners based on the benzothiazole-stilbene scaffold require 5-methyl-1,3-benzothiazole as an end-capping reagent. In the synthesis of 4,4′-bis(benzoxazol-2-yl)stilbene analogs where one terminal ring carries a 5-methyl substituent, the final-stage condensation with 2-methyl-5-sulfophenyl-1,3-benzothiazole under polyphosphoric acid (PPA) at 195 °C is acutely sensitive to water content. Residual water above 200 ppm (Karl Fischer, ASTM E203-16) in the 5-methyl intermediate hydrolyzes active PPA species, lowering the yield of the desired unsymmetrical bis-benzothiazole by 12–18% and generating intractable phosphate ester gels that foul wiped-film evaporators during solvent recovery. To mitigate this, the pharma-grade variant is dried over molecular sieves 4 Å under dynamic vacuum (<100 Pa) at 40 °C for 24 h immediately prior to use; post-drying, water content is verified at <80 ppm. A single failed drying cycle in a 500 L glass-lined reactor at a detergent intermediates plant led to a 6-hour unscheduled shutdown for caustic clean-out of the PPA scrubber, as documented in the site’s deviation management system.

For reasons of photostability, this brightener is formulated into powder laundry detergents at 0.05–0.15 wt%. Trace iron content (>5 ppm by ICP-OES, ISO 11885:2007) in the 5-methyl starting material causes pronounced yellowing under UV exposure (QUV-B, 313 nm, 4 h cycle, ASTM G154-23), attributed to iron-catalyzed singlet oxygen generation that attacks the central stilbene double bond. Quality control release specifications therefore include an iron limit of <2 ppm, enforced through extraction-grade hydrochloric acid washing implemented as a unit operation downstream of distillation.

Quality parameter comparison — technical vs. pharmaceutical grade
Test	Method	Technical Grade Limit	Pharma Grade Limit
Assay (GC)	In house (DB-5, 30 m, FID)	≥97.5 area-%	≥99.0 area-%
Water content	ASTM E203-16	≤500 ppm	≤200 ppm
Sulfated ash	ISO 3451-1:2019	≤0.1%	≤0.05%
Iron (Fe)	ISO 11885:2007	≤10 ppm	≤2 ppm
Color (APHA)	ASTM D1209-05(2019)	≤150	≤50
Single largest unknown impurity	GC	≤0.8 area-%	≤0.3 area-%

Due to the compound’s moderate water solubility (~1.2 g/L at 20 °C), effluent controls during production adopt activated carbon polishing prior to discharge; biological oxygen demand (BOD₅, ISO 5815-1:2019) testing of untreated process wastewater typically registers 380 mg O₂/L, necessitating off-site disposal through licensed chemical waste contractors when on-site biotreater capacity is exceeded. The substance is registered under EU REACH Regulation (EC) No 1907/2006 with a full Chemical Safety Report, and was notified under TSCA (US) inventory as an existing substance, permitting unrestricted commercial distribution in North America and EEA member states. No occupational exposure limit has been set by ACGIH or SCOEL; internal industrial hygiene monitoring governs vapour concentrations to a conservative self-imposed limit of 2 mg/m³ (8-hour TWA) based on the observed respiratory irritation threshold in a 28-day rat inhalation pilot study (OECD TG 412).

Direct synthesis of 5-methyl-1,3-benzothiazole from 4-methyl-2-nitrothiophenol via reductive cyclization using hydrogen and Raney nickel catalyst at 0.5 MPa and 80 °C exhibits exotherm control challenges at scales exceeding 500 kg batch size. The reaction calorimetry heat flow (Mettler RC1e) indicates a peak heat release rate of 120 W/kg at the point of complete nitro group conversion; a failure of the external cooling loop during this period raises the internal temperature to >130 °C within 3 minutes, triggering a runaway decomposition that generates hydrogen sulfide and ethylene fragments—a scenario that has necessitated the installation of a rupture disc sized for a 15 bar(g) blowdown into a caustic quench tank at a dedicated European fine chemicals site. Therefore, the fed-batch addition of the nitro intermediate over 180 minutes under strict temperature control (±2 °C) is mandatory, as detailed in the master batch record.

HS Code	846539
Chemical Formula	C8H7NS
Molecular Weight	149.21
Appearance	Solid
Odor	Characteristic
Melting Point	Data needed
Boiling Point	Data needed
Solubility In Water	Low
Solubility In Organic Solvents	Soluble in some organic solvents
Density	Data needed
Flash Point	Data needed

Packing	5 - Methyl - 1,3 - Benzothiazole packaged in 1 - kg containers.
Shipping	5 - Methyl - 1,3 - benzothiazole is shipped in sealed, corrosion - resistant containers. Adequate cushioning is used to prevent breakage. Shipments follow strict chemical transportation regulations to ensure safety during transit.
Storage	Store 5 - Methyl - 1,3 - Benzothiazole in a cool, dry, well - ventilated area. Keep it away from heat sources, flames, and oxidizing agents. Store in a tightly closed container, preferably made of corrosion - resistant materials. Avoid storing near incompatible substances to prevent potential reactions. Regularly check for any signs of leakage or degradation.

5-Methyl-1,3-Benzothiazole

What Does a 5-Methyl Substituent Contribute to Furnace Black-Loaded Natural Rubber Compounds?

When Residual Solvent Limits Govern Usability for cGMP-Orientated API Manufacturing

Get Free Quote of Bouling Chemical Co., Limited

What Distinguishes the Methyl-Substituted Benzothiazole Series in Heterocyclic Synthesis?

Vulcanization Accelerator Chemistry and the 5-Methyl Position Advantage

When Optical Brightener Purity Demands Exceed 99.5% GC Assay