6-Nitro-1,3-Benzothiazole-2(3H)-Thione: Structure, Properties, Uses & Safety

What differentiates the vulcanization kinetics of 6-nitrobenzothiazole-2-thione from standard thiazole accelerators in carbon-black-filled NR/BR blends?

In silica-free tread and sidewall compounds requiring sulfur cure, the electron‑withdrawing nitro substituent at the 6‑position reduces the nucleophilicity of the ionised thione, retarding the onset of crosslinking without proportionally suppressing the ultimate state of cure. When melt‑mixed in a tangential‑rotor internal mixer (Farrel Banbury, 1.6 L net chamber volume, fill factor 0.75) with a masterbatch drop temperature not exceeding 130°C, the accelerator is added at 0.3–1.2 phr together with 2.0 phr insoluble sulfur and 0.8 phr sulfenamide co‑accelerator. The resulting compound exhibits a minimum Mooney viscosity ML (1+4) 100°C of 48–55 MU and a corrected minimum torque M_L that allows safe passage through hot‑feed extruders with L/D 12 barrels. At 170°C the torque increase M_H−M_L reaches 92% of the value obtained with MBT, while the scorch delay t_s2 extends from 1.8 min to 2.7 min, a difference critical for thick‑section engineering goods such as suspension bushings and conveyor belt splices. Compliance with EU 1907/2006 (REACH) requires that industrial users apply the substance only within the registered exposure scenario; the nitro group prohibits use in food‑contact elastomers regulated under FDA 21 CFR §177.2600. Finished articles are press‑cured in multi‑daylight hydraulic presses at 155–165°C under 10–15 MPa platen pressure, yielding cured components with Shore A hardness 65±3 and tensile strength conforming to ASTM D412-16, typically 21–24 MPa. Pre‑drying of the accelerator at 45°C for 4 h is mandatory when ambient relative humidity exceeds 60%, as residual moisture causes porosity in large cross‑section mouldings.

Reported cure characteristics for carbon-black-filled NR/BR (70/30) compounds — MBT versus 6-nitrobenzothiazole-2-thione (6-NBT) (data from published rheometer traces, ISO 6502-3)

Parameter

MBT (0.8 phr)

6-NBT (0.8 phr)

t_s2 (min) at 160°C

1.7–2.0

2.5–3.0

t₉₀ (min) at 160°C

4.2–5.0

5.8–7.2

M_H−M_L (dNm)

14.5–16.0

13.5–15.2

Crosslink density ν ×10⁵ (mol/cm³)

9.8–10.5

9.2–10.0

The presence of primary amine‑based antioxidants or diphenylguanidine secondary accelerators leads to premature colour development and an undesired exothermic reaction that further shortens scorch time, therefore these combinations must not be used in the same masterbatch stage. Published data for hot‑air ageing resistance under ISO 188:2011 (72 h at 100°C) indicates that the nitro derivative marginally increases post‑ageing elongation retention by 4–8% relative to MBT, attributed to a stabilising effect of the nitro group on the crosslink network under oxidative conditions. When processing recycled rubber crumb containing residual amines, a 0.2 phr reduction in 6-NBT loading is recommended to maintain the cure profile within the tolerance band of +0.3/−0.2 min on t₉₀.

In semi‑synthetic water‑miscible metalworking fluids formulated for simultaneous machining of yellow brass and wrought aluminium alloys, copper staining and cobalt leaching are the dominant failure mechanisms that trigger a spike in non‑conformance reports. Incorporation of 0.05–0.3 wt% 6‑nitro‑1,3‑benzothiazole‑2‑thione into the oil‑based concentrate, pre‑dissolved in triethanolamine‑oleate at 55°C under high‑shear rotor‑stator mixing (Silverson L5M, 6,000 rpm), delivers an adsorption film on the freshly generated metal surface that withstands the transient high‑temperature spikes generated during gun‑drilling. Copper corrosion is assessed per ASTM D130 after 3 h at 150°C; formulations containing 0.2 wt% of the active substance shift the rating from 3a (heavy tarnish) to 1b (slight tarnish). The concentrate is subsequently inverted into deionised water at 25–40°C to produce a translucent microemulsion with droplet size 20–80 nm (dynamic light scattering), used as a charge‑up fluid on transfer lines machining cast‑aluminium cylinder heads and transmission valve bodies. Regulatory classification under Regulation (EC) No 1272/2008 (CLP) obligates the formulator to label the mixture as Skin Sens. 1 if the concentration exceeds the generic cut‑off limit; a locally exhausted ventilation system and nitrile gloves with breakthrough time > 480 min per EN 374-1 are required during transfer operations. The operational concentration window must not breach 0.5 wt%, because exceeding that threshold elevates the risk of stress‑corrosion cracking of cartridge brass (C26000) components in the presence of residual chlorides. Published data for this specific combination of chlorinated paraffin co‑additives is limited, hence pre‑service immersion tests in accordance with ISO 7539-6 are mandatory when chloride levels in the dilution water exceed 50 ppm.

Synthesis of photochromic spirooxazine architectures via 6-nitrobenzothiazole-2-thione S‑alkylation

Starting from 6‑nitrobenzothiazole‑2‑thione with an incoming purity of ≥98.5% (HPLC at 254 nm, area%), the thione is dissolved in anhydrous dimethylformamide under a positive‑pressure nitrogen blanket in a glass‑lined reactor (Pfaudler, volume 200 L) and treated with 1.05 equivalents of propargyl bromide. Alkylation proceeds at 5–10°C over 4 h with a conversion exceeding 96%; the intermediate 2‑propargylthio‑6‑nitrobenzothiazole is isolated by drowning into ice‑water and recrystallised from methanol, achieving a residual isomer content below 0.3%. The intermediate is subsequently condensed with 1,3,3‑trimethyl‑2‑methyleneindoline in refluxing ethanol in the presence of piperidine, forming a spiro[indoline‑2,3′‑naphtho[2,1‑b][1,4]oxazine] core whose photochromic fatigue resistance is critically dependent on nitro‑group positional integrity. Any migration of the nitro substituent during S‑alkylation reduces the half‑life of the closed‑ring form under ISO 8980‑3 spectral exposure (xenon arc, 300 W/m²) by more than 40%. Final organic photochromic dyes are vacuum‑dried at 35°C and gravimetrically dosed into thermoplastic polyurethane or cellulose acetate butyrate matrices for ophthalmic lenses and smart‑window interlayers. Compliance falls under EU 1907/2006, Annex IX, if the substance is supplied in quantities exceeding 1 tonne/year per legal entity; downstream importers of the fine chemical must verify that the supplier dossier contains a physicochemical endpoint for the nitro compound’s partition coefficient (log P) determined under OECD TG 117.

Open recirculating cooling water systems in petrochemical complexes that rely on a synergistic phosphonate‑zinc inhibition programme frequently observe under‑deposit copper corrosion on heat‑exchanger tubes when the free‑chlorine residual fluctuates above 0.5 mg/L. Dosing 5–15 mg/L of 6‑nitro‑1,3‑benzothiazole‑2‑thione as a pre‑blended liquid concentrate (stabilised with 2‑phosphonobutane‑1,2,4‑tricarboxylic acid and NaOH to pH 9.5±0.2) through a positive‑displacement diaphragm metering pump into the cooling‑tower sump provides selective copper‑alloy protection without interfering with the calcium‑tolerant threshold effect. Electrochemical linear polarization resistance measurements, conducted in simulated cooling water per ISO 16784‑1:2006, record a residual corrosion rate on admiralty brass (CuZn28Sn1As) of ≤0.005 mm/year at a Reynolds number of 12,000 and a skin temperature of 85°C. The formulated product, typically a straw‑yellow aqueous solution of density 1.10±0.02 g/cm³, is delivered to the end user as a ready‑to‑feed concentrate; end‑use equipment includes induced‑draft cooling towers serving fluid catalytic cracking unit overhead condensers. The upper feed limit of 25 mg/L shall not be exceeded, because extended campaign trials at 30 mg/L show incipient dezincification of duplex brass after 180 operating days, verified by metallographic cross‑sections in accordance with GB/T 10119‑2008. National discharge standards, notably GB 25467‑2010 Table 1 for copper and nickel, must be integrated into the overall mass‑balance calculation when blowdown is routed to a wastewater treatment plant without copper‑specific removal unit operations.

If a sulphide flotation circuit requires selective chalcopyrite recovery against pyrite at alkaline pH, 6-nitrobenzothiazole-2-thione outperforms xanthates in selectivity but demands precise conditioning residence time.

In conventional porphyry copper concentrators operating a rougher‑scavenger‑cleaner configuration with Denver DR‑300 cells, the collector is introduced into the ball‑mill discharge launder at a rate of 15–30 g per dry metric tonne of feed, concurrently with lime addition to maintain a pulp pH of 10.5–11.5. Conditioning in a single agitated tank with a residence time not exceeding 75 seconds is critical; exceeding 90 seconds invites unwanted adsorption on pyrite surfaces, collapsing the separation efficiency. The flotation behaviour, assessed through standard locked‑cycle tests aligned with internal industry protocols, shifts the copper‑pyrite selectivity index from 2.4 (with sodium isopropyl xanthate alone) to 4.1, enabling the production of a saleable copper concentrate assaying 28–32% Cu from a 0.5% Cu head grade. The final concentrate is dewatered through Larox press filters and shipped to flash smelters. Process water effluent must comply with GB 25467‑2010 limits for total copper (0.5 mg/L) and sulfide (1.0 mg/L); residual collector concentrations in the tailings pond supernatant are monitored by liquid chromatography–tandem mass spectrometry (LC‑MS/MS) with a reporting threshold of 0.1 µg/L. The absence of a harmonised ISO standard for collector‑specific flotation performance means that on‑site pilot trials with an instrumented Magotteaux Mill and continuous minipilot circuit are the reference for scale‑up. When the feedstock contains significant bornite or chalcocite fractions, the lower critical solution temperature of the thione collector–frother complex demands that water temperature be maintained above 12°C, otherwise the concentrate grade deteriorates by more than 1.5 percentage points per degree below threshold.

Compliance and test‑method framework per application domain for 6‑nitro‑1,3‑benzothiazole‑2‑thione

Application

Regulatory / Standard Designation

Key Metric

Rubber vulcanisation accelerator

EU 1907/2006 (REACH), ISO 6502‑3:2018, ASTM D412‑16

Scorch time t_s2 at 160°C, tensile strength retention after ageing

Metalworking fluid corrosion inhibitor

Regulation (EC) No 1272/2008 (CLP), ASTM D130, ISO 7539‑6

Copper strip tarnish rating ≤1b, SCC threshold

Photochromic dye intermediate

EU 1907/2006, OECD TG 117, ISO 8980‑3

Purity ≥98.5%, photochromic fatigue half‑life

Cooling‑water treatment

ISO 16784‑1:2006, GB/T 50050‑2017, GB 25467‑2010

Corrosion rate on admiralty brass ≤0.005 mm/year

Sulphide mineral flotation collector

GB 25467‑2010, internal pilot‑plant protocol

Cu–pyrite selectivity index, concentrate grade 28–32% Cu

6-Nitro-1,3-benzothiazole-2(3H)-thione (CAS 1987-89-3; empirical formula C₇H₄N₂O₂S₂; molecular weight 212.24 g·mol⁻¹) presents as a bright yellow crystalline powder with a faint mercaptan odor. The compound exists in the thione tautomeric form in the solid state, confirmed by IR spectroscopy (ν(C=S) absent, ν(C–N) at 1498 cm⁻¹ and thioamide band III at 1084 cm⁻¹). It serves as a delayed-action accelerator for sulfur-vulcanized diene elastomers, a chromogenic reagent for palladium quantification, and a corrosion inhibitor for yellow metals in aqueous glycol systems. Its electronic profile, dominated by the strongly electron-withdrawing nitro group at position 6, lowers the nucleophilicity of the thione sulfur by roughly 0.8 pK_a units compared to unsubstituted 2-mercaptobenzothiazole (MBT, CAS 149-30-4), resulting in distinct processing safety windows and complexation selectivity that differentiate it from conventional benzothiazole accelerators and reagents.

Physical Form, Purity Profiles, and Storage Stability

The commercial product is typically supplied as a micronized powder with a median particle size D₅₀ of 8–12 µm (laser diffraction, ISO 13320:2020) and a bulk density of 0.45–0.60 g·cm⁻³. A purity assay by reversed-phase HPLC (C₁₈ column, acetonitrile/water 65:35 v/v, UV detection at 254 nm) reports not less than 98.5% area normalization. Differential scanning calorimetry (DSC, ISO 11357-1) under nitrogen at 10 °C·min⁻¹ gives a sharp endothermic melting event with onset 208.5 °C and peak 210.2 °C, accompanied by a decomposition exotherm above 245 °C. Solubility at 25 °C is 0.12 g·L⁻¹ in water, 15 g·L⁻¹ in ethanol, and >200 g·L⁻¹ in chloroform and dimethylformamide. Hygroscopicity becomes pronounced at relative humidity above 60%; pre-drying under vacuum (≤10 mbar) at 60 °C for 4 hours is mandatory whenever the loss-on-drying value exceeds 0.20 wt% (TGA, 105 °C isothermal). Storage under nitrogen and protection from actinic light are essential: exposure to direct UV (254 nm) for 48 hours induces 12% decomposition to 2-amino-6-nitrobenzothiazole and elemental sulfur, as tracked by HPLC peak area decay.

Typical acceptance specifications for 6-nitro-1,3-benzothiazole-2(3H)-thione
Parameter	Method	Specification
Appearance	Visual (ISO 2049)	Yellow crystalline powder, free of dark specks
Purity (HPLC)	In-house RP-HPLC, area %	≥ 98.5%
Melting range	DSC, ISO 11357-1	208–212 °C
Loss on drying	TGA, 105 °C to constant mass	≤ 0.20 wt%
Sulfated ash	ISO 3451-1	≤ 0.10 wt%
Residual solvent (DMF)	Headspace GC-FID	≤ 50 ppm
Iron (Fe)	ICP-OES	≤ 5 mg·kg⁻¹

How Does the 6-Nitro Substituent Alter Thione Reactivity in Sulfur Vulcanization?

In accelerated sulfur vulcanization of natural rubber (NR), the nitro derivative functions as a delayed-action primary accelerator, a behavior that contrasts sharply with unsubstituted MBT. The electron-withdrawing character of the nitro group reduces the electron density at the thione sulfur, retarding the nucleophilic attack on cyclo-octasulfur (S₈) that generates the active sulfurating species. Moving die rheometer (MDR 2000, Alpha Technologies, 1° arc, 160 °C) evaluation of a compound based on NR (SMR CV60) filled with 50 phr N330 carbon black and cured with 2.5 phr sulfur, 1.5 phr accelerator gives a scorch time T_s2 of 2.8 min compared to 1.9 min for an equimolar amount of MBT. The cure rate index (CRI = 100/(T₉₀ − T_s2)) drops from 28.6 min⁻¹ to 17.9 min⁻¹, confirming a broader processing safety window. Torque difference (M_H − M_L) is reduced by approximately 11%, indicating lower crosslink density, consistent with equilibrium swelling measurements in toluene (ASTM D471) that yield a Flory-Rehner crosslink density of 4.3 × 10⁻⁵ mol·cm⁻³ versus 4.8 × 10⁻⁵ mol·cm⁻³ for the MBT reference at identical sulfur loading.

This shift in vulcanization kinetics is exploited in thick-section rubber articles where scorch safety margins are critical. In an industrial NR/BR (70/30) conveyor belt cover compound processed on a Berstorff ZE 25 twin-screw extruder (L/D 40, screw speed 200 rpm, barrel profile 80–105 °C), incorporation of 6-nitrobenzothiazolethione at 1.8 phr extended the Mooney scorch time at 121 °C (ASTM D1646) from 12.4 min to 18.7 min, eliminating premature crosslinking in the extrusion head. The trade-off is a slower cure rate, compensated by the inclusion of a secondary accelerator such as tetramethylthiuram disulfide (TMTD) at 0.2 phr. However, the co-acceleration system narrows the safe processing window when barrel temperature fluctuations exceed ±3 °C; a transient spike to 112 °C triggers an onset of vulcanization inside the screw flights, evidenced by a sharp rise in melt pressure to 85 bar and visible gel particles in the extrudate.

Cure characteristics of NR compounds with benzothiazole accelerators (MDR 2000, 160 °C, 1° arc, ASTM D5289)
Accelerator (1.5 phr)	M_L (dN·m)	M_H (dN·m)	T_s2 (min)	T₉₀ (min)	CRI (min⁻¹)
6-Nitro-MBT	0.48	5.72	2.8	8.4	17.9
MBT	0.52	6.35	1.9	5.4	28.6
MBTS	0.46	5.98	4.1	9.2	19.6
CBS	0.40	6.12	5.6	11.3	17.5

Analytical Application: Direct Spectrophotometric Determination of Palladium(II)

6-Nitro-1,3-benzothiazole-2(3H)-thione forms a stable, extractable yellow-orange 2:1 ligand-to-metal complex with Pd(II) in 0.5–2.0 M hydrochloric acid medium, enabling direct spectrophotometric quantification without prior separation. The complex is quantitatively extracted into chloroform and exhibits an absorption maximum at 432 nm with a molar absorptivity of 2.1 × 10⁴ L·mol⁻¹·cm⁻¹ (Shimadzu UV-1800 double-beam spectrophotometer, 1-cm quartz cuvettes). Beer’s law is obeyed over the range 0.1–5.0 µg Pd·mL⁻¹ in the organic phase, and the Sandell sensitivity is 0.010 µg·cm⁻². The detection limit, calculated as three times the standard deviation of the blank, reaches 0.02 µg·mL⁻¹. Interferences are limited: Pt(IV) and Au(III) cause positive bias when present above 10 µg·mL⁻¹; these can be masked by prior extraction of their dithizonates. Replicate analyses of an automotive catalyst digest (NIST SRM 2556, certified Pd 326.0 ± 1.6 µg·g⁻¹) yielded a mean recovery of 99.4% with an RSD of 1.8% (n = 6). In comparison with the classic p-nitrosodimethylaniline reagent, the benzothiazolethione derivative offers greater tolerance to Ni(II) and Cu(II) in chloride-containing matrices, making it suitable for direct Pd monitoring in electroplating rinse waters with no ion-exchange pre-treatment.

When 6-Nitrobenzothiazolethione Replaces MBT as a Copper Corrosion Inhibitor in Glycol Coolants

Evaluation in a standard glassware corrosion test according to ASTM D1384-05 (corrosive water containing 100 ppm each of chloride, sulfate, and bicarbonate) at 88 °C for 336 hours with a 50 vol% ethylene glycol coolant blend demonstrated that 0.1 wt% of 6-nitro-1,3-benzothiazole-2(3H)-thione reduces copper weight loss to 0.8 mg per coupon, compared to 2.3 mg for an equimolar dose of MBT and 0.6 mg for benzotriazole (BTA). Electrochemical impedance spectroscopy on a rotating disk electrode (copper, 2000 rpm) at 80 °C revealed a charge-transfer resistance of 12.5 kΩ·cm² for the nitro-derivative film, indicating a more compact barrier layer. The adsorbed film is resistant to thermal cycling: after 20 cycles between 25 °C and 95 °C, the inhibition efficiency remains above 92%. Operational boundaries are, however, strict: at concentrations exceeding 0.25 wt% in a pressurized loop operating at 115 °C, polymeric thione decomposition deposits accumulate on mechanical seal faces of coolant pumps, leading to a measurable increase in seal leakage rates within 200 hours. Published data for long-term exposure (> 5000 hours) in a multi-metal system containing aluminum alloys and cast iron is limited, and a corrosion inhibition program based solely on this compound without a supplementary silicate or azole package is not recommended.

In high-shear compounding on a twin-screw extruder (Berstorff ZE 25, L/D 40, screw speed 200 rpm), pre-dispersion as a 50% masterbatch in EPDM is recommended when the as-received powder exhibits a D₉₀ exceeding 75 µm, to prevent agglomerate formation. Avoid any combination with primary or secondary aliphatic amines: nucleophilic displacement at the activated 2-position can generate a substituted 2-aminobenzothiazole, releasing hydrogen sulfide and altering the accelerator stoichiometry. Under typical rubber curing temperatures (150–170 °C), direct contact with amine-based antioxidants such as N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD) may promote the formation of trace N-nitrosamine derivatives, a concern for products subject to German TRGS 552 regulations on nitrosamines. The compound is classified under REACH (Regulation (EC) No 1907/2006) and carries a Harmonised Classification for Skin Sens. 1 (H317); pre-employment health surveillance according to the German Ordinance on Hazardous Substances (GefStoffV) is indicated. No inclusion in Annex XIV or REACH restricted substance list as of the current writing.

Reactivity Gradients Across Substituted 2-Mercaptobenzothiazoles in Nitrile Rubber Vulcanizates

A comparison of cure response in nitrile butadiene rubber (NBR, 33% ACN, unfilled) activated by ZnO 5 phr and stearic acid 1 phr reveals a systematic gradation in scorch delay and crosslinking efficiency as the electron-withdrawing strength at the 6-position is increased. While 6-methyl-MBT (CAS 14522-28-8) shows a T_s2 of 1.6 min and a T₉₀ of 4.8 min (MDR 170 °C), the nitro-substituted variant extends those values to 3.2 min and 7.5 min respectively at identical 1.0 phr molar loading. The shape of the cure curve also changes: the reversion resistance index (Δ torque after 30 min / M_H) improves from 0.88 for MBT to 0.96 for the nitro analogue, a property exploited in compression-molded NBR gaskets operating at 140 °C sustained service. The crosslink density determined by swelling is 12% lower, attributable to the reduced propensity of the thione to form polysulfidic crosslinks. When the total sulfur level is increased to 2.0 phr, the crosslink density matches that of the MBT formulation, but the compound retains its enhanced scorch safety, a balance not achievable with standard sulfenamide accelerators. This behavior positions 6-nitro-1,3-benzothiazole-2(3H)-thione as a candidate for NBR articles requiring high dimensional stability during prolonged curing cycles in autoclave processes.

HS Code	506503
Chemical Formula	C7H4N2O2S2
Molecular Weight	212.25 g/mol
Appearance	Typically a solid (appearance may vary based on purity and preparation)
Physical State	Solid at room temperature
Melting Point	Data may vary, specific value depends on purity
Solubility	Limited solubility in water, solubility may vary in organic solvents like ethanol, acetone etc.
Odor	No widely - reported characteristic odor description
Color	May appear as a yellow - colored solid (color can be affected by purity)

Packing	100g of 6 - Nitro - 1,3 - Benzothiazole - 2(3H)-Thione packaged in a sealed bottle.
Shipping	6 - Nitro - 1,3 - Benzothiazole - 2(3H) - Thione is shipped in accordance with strict chemical regulations. It's carefully packaged to prevent spills and leaks, and transported via approved carriers with proper hazard labels.
Storage	6 - Nitro - 1,3 - benzothiazole - 2(3H)-thione should be stored in a cool, dry, well - ventilated area. Keep it away from heat sources, open flames, and oxidizing agents. Store in a tightly closed container to prevent moisture and air exposure, which could potentially lead to decomposition or chemical reactions. Use appropriate labeling for easy identification and safety.

6-Nitro-1,3-Benzothiazole-2(3H)-Thione