2-(Prop-2-Yn-1-Ylsulfanyl)-1,3-Benzothiazole

CAS 42293-76-4, molecular formula C10H7NS2, molecular weight 205.30 g·mol⁻¹. This heterocyclic thioether, classified as a substituted 2-mercaptobenzothiazole, is supplied commercially as a crystalline solid of 98 % purity (GC, area normalization) under product codes such as BZT-ALK-98 or PG-BT-S. The compound melts in the range 65–68 °C and exhibits a density of approximately 1.32 g·cm⁻³ at 20 °C. Its solubility profile reveals high affinity for aprotic organic media—solubility in tetrahydrofuran exceeding 250 g·L⁻¹ at 25 °C—while aqueous solubility remains below 0.08 g·L⁻¹, a partition that dictates downstream solvent selection in both formulation and synthesis. Direct application of heat or prolonged storage above 40 °C in the presence of air leads to gradual discoloration and the formation of sulfoxide by‑products (detectable by HPLC at relative retention time 1.35). Consequently, the material is packaged under argon in aluminium‑laminated foil bags and should be re‑sealed under inert gas after each use. Moisture absorption is not thermodynamically favoured; however, pre‑drying over silica gel for 4 h at 30 °C is recommended when ambient relative humidity exceeds 75 %, primarily to prevent agglomeration during precision dispensing in masterbatch preparation.

Propargyl Substitution Extends Processing Safety without Sacrificing Crosslink Density

In sulfur‑vulcanised natural rubber (NR) and styrene‑butadiene rubber (SBR) compounds, the substitution of the mercapto proton of 2‑mercaptobenzothiazole (MBT) with a propargyl group fundamentally alters the accelerator’s reactivity profile. Data generated on a 1.8‑L internal mixer (Werner & Pfleiderer GK 1.5E, intermeshing rotors, fill factor 0.72, dump temperature 125–135 °C) and subsequently monitored with an MDR 2000 rheometer (0.5° arc, 160 °C, ASTM D5289-17) indicate that at equimolar sulfur loading (2.0 phr sulfur, 0.8 phr accelerator in an NR/BR 70/30 blend), the scorch time t_s2 extends by 35–45 % relative to MBT, while the cure rate index (CRI, defined as 100/(t₉₀ – t_s2)) drops from 8.5 min⁻¹ to 6.2 min⁻¹. Despite this delayed onset of crosslinking, the maximum torque (M_H – M_L) remains within 94 % of the MBT reference, and tensile strength (ASTM D412-16, die C) recorded on a Zwick Z010 universal tester after press‑curing at 150 °C for t₉₀ returns values of 24.1 MPa versus 24.9 MPa for the MBT control. This behaviour is attributed to the thioether linkage stabilising the benzothiazole‑sulfur complex and slowing the release of active sulfurating species; the terminal alkyne remains largely inert under these curing conditions, as confirmed by Raman spectroscopy showing the ν(C≡C) band at 2120 cm⁻¹ persisting in the vulcanizate. The practical ramification on the shop floor is a wider processing window. Injection moulding trials conducted with a 180‑tonne clamp force machine (Engel victory 180, screw L/D 22, melt temperature 95 °C) for a rubber‑to‑metal bonded bushing compound revealed that the compound containing this propargyl derivative tolerated a 12‑second deliberate hold‑time extension without scorch‑induced flow defects, whereas the MBT‑based compound exhibited premature gel particles at the gate after only 7 seconds. This operational latitude translates to reduced scrap rates in multi‑cavity tools where thermal equilibration lag is unavoidable. However, the compound requires 4–6 % higher addition of zinc oxide (ZnO) to fully activate the curative complex, a compensation that can be accommodated within standard 5 phr ZnO formulations without exceeding zinc limits set by certain automotive OEM specifications.

A Comparative Survey of Benzothiazole Accelerator Architectures

Accelerator	CAS	Scorch Safety ts2 at 160 °C (min)	MH–ML (dN·m)	Typical Loading (phr)	Distinctive Feature
MBT (2‑Mercaptobenzothiazole)	149-30-4	1.8	9.4	0.6–1.0	Rapid cure onset, low cost, limited scorch safety
MBTS (Dibenzothiazole disulfide)	120-78-5	3.2	9.1	0.8–1.2	Moderate delay, blooming tendency
CBS (N‑Cyclohexyl‑2‑benzothiazolesulfenamide)	95-33-0	5.5	8.7	0.8–1.5	Sulfenamide retarder, amine release
2‑(Prop‑2‑yn‑1‑ylsulfanyl)‑1,3‑benzothiazole	42293-76-4	2.8–3.4	8.8	0.6–1.0	Alkyne handle for post‑functionalisation, moderate scorch delay, no free amine

Rheometer data obtained per ASTM D5289-17 with 0.5° arc, 160 °C, compound base: NR (SMR CV60) 70, BR (Nd‑BR 40) 30, N330 carbon black 45 phr, sulfur 2.0 phr, ZnO 5 phr, stearic acid 2 phr. Values are averages of three independent batches; coefficient of variation does not exceed 6 %.

The absence of a sulfenamide N–S bond eliminates the risk of amine‑induced nitrosamine formation during curing, a growing concern under European tyre labelling regulations. The alkyne group, while not participating in sulfur crosslinking, provides a covalent anchoring site for subsequent chemical modification—a property absent in all conventional benzothiazole accelerators. This duality positions the compound at the intersection of classical rubber compounding and functional elastomer design. In environments where copper corrosion must be arrested during chemical cleaning, the molecule adopts an entirely different role. The benzothiazole ring adsorbs onto cuprous oxide surfaces through the endocyclic nitrogen and exocyclic sulfur, while the propargyl moiety undergoes chemisorption via the triple bond, forming a polymeric inhibitive film. Electrochemical verification using a standard three‑electrode cell (saturated calomel reference, platinum counter, copper working electrode) in aerated 1.0 M H₂SO₄ at 298 K, per ASTM G59-97 on a Gamry Interface 1010 potentiostat, demonstrated that at a concentration of 1.0 × 10⁻³ M the charge transfer resistance (R_ct) extracted from Nyquist plots rose from 140 Ω·cm² (blank) to 3,120 Ω·cm², corresponding to an inhibition efficiency of 95.5 %. Tafel extrapolation gave a corrosion current density of 4.7 µA·cm⁻², compared to 107 µA·cm⁻² for the uninhibited electrolyte. Immersion tests conducted over 168 h in accordance with ASTM G31-21 showed weight‑loss reduction by 92 % at 200 mg·L⁻¹. The inhibitor is particularly effective in the temperature range 30–60 °C; above 70 °C the film begins to desorb, causing a drop in efficiency to 78 %, so its deployment is recommended for pickling baths operating at moderate temperature. Unlike propargyl alcohol, which volatilises and imparts a sharp odour, the benzothiazole derivative has a vapour pressure below 0.01 Pa at 20 °C, significantly reducing toxic inhalatory exposure for line operators. Still, local exhaust ventilation is mandated because the compound is classified as a skin sensitiser (GHS Category 1A) and aquatic chronic category 2 hazard, consistent with other benzothiazoles.

What Purity Specifications Are Demanded for Copper‑Catalysed Alkyne‑Azide Cycloaddition?

The terminal alkyne enables Huisgen 1,3‑dipolar cycloaddition with azides under copper(I) catalysis (CuAAC). For this application, purity must exceed 99 % (HPLC, 254 nm) to avoid side reactions from MBT impurities (frequently present at 0.3–0.8 % in the 98 % grade), which can poison the copper catalyst and lead to incomplete conversion. A product specification designed for bioconjugation labelling therefore includes a dedicated assay by quantitative ¹H NMR integrating the acetylenic proton at δ 2.25 ppm (CDCl₃) against an internal standard, with an acceptance criterion of ≥ 99.0 %. Metal content is controlled to < 10 ppm Fe, < 5 ppm Cu, and < 2 ppm Pd to prevent unintended cyclisation prior to use. In a typical reaction with benzyl azide (1.0 eq), CuSO₄·5 H₂O (5 mol%), sodium ascorbate (10 mol%), in H₂O/t‑BuOH (1:1) at ambient temperature, the triazole product forms in 95 % isolated yield after 2 h. The reaction has been successfully scaled to 500 g in a jacketed glass reactor with overhead stirring, with no exotherm detected beyond 2 °C from baseline, rendering the process amenable to kilo‑lab synthesis without specialised calorimetry. Because the benzothiazole ring exhibits a fluorescence quantum yield of 0.04 in the triazole conjugate (excitation 310 nm), the derivative is used to introduce a spectroscopic label into polymers and bioconjugates. The thioether bridge, however, imposes a stability limitation: the conjugate must be shielded from direct UV‑B radiation (280–315 nm) during long‑term storage to prevent photolytic C–S bond cleavage. Dark storage at 4 °C under argon is therefore stipulated; under these conditions, less than 2 % decomposition is observed over 12 months by HPLC.

Standard / Regulation	Scope	Status / Compliance Threshold
REACH (EC) 1907/2006	Registration, Evaluation, Authorisation	Pre‑registered; >1 t/a requires full dossier with exposure scenarios
TSCA (15 U.S.C. §2601)	Chemical Substance Inventory	Listed on Inventory; PMN required for new uses
IEC 62321‑7‑2:2017	Determination of restricted substances	Not directly restricted; benzothiazole content must be declared if >0.1 % w/w in consumer articles per REACH Annex XVII
FDA 21 CFR 175.300	Resinous and polymeric coatings for food contact	Not presently listed; extractive testing mandatory for incidental food contact
GHS Classification (EC) 1272/2008	Harmonised hazard communication	Skin Sens. 1A (H317), Aquatic Chronic 2 (H411)

Storage incompatibilities merit explicit enumeration. Contact with concentrated nitric acid or peroxides leads to rapid exothermic decomposition and formation of benzothiazole sulfonic acids; any cleaning or reaction quenching operations must avoid these oxidants. The compound also reacts vigorously with sodium hydride or organolithium reagents at the acetylenic C–H, generating acetylides that can initiate polymerisation if not properly solvated—a consideration when employing the alkyne in deprotonative metalation sequences. When designing masterbatches for rubber, the accelerator must not be pre‑blended with elemental sulfur and MBTS in the absence of ZnO, because the combination produces a metastable complex that self‑heats and can clump inside silos, blocking pneumatic conveyance lines (25 mm diameter tubing affected in one documented site incident). Instead, a split addition protocol with ZnO added in the first mixing pass and the accelerator with sulfur in the second pass is employed. The product is shipped in fibre drums with polyethylene liners, net weight 25 kg, labelled with GHS pictograms GHS07 (exclamation mark) and GHS09 (environment). A Safety Data Sheet dated to the latest revision of Regulation (EC) No 2020/878 should accompany every consignment. Because an official occupational exposure limit has not been promulgated, a derived no‑effect level (DNEL) of 1.5 mg·m⁻³ for inhalation and 0.2 mg·kg⁻¹ bw·day⁻¹ for dermal exposure is applied based on read‑across from 2‑mercaptobenzothiazole data, as recommended by ECHA’s guidance R.8. Shop‑floor air monitoring using OSHA Method 96 with HPLC‑UV detection allows quantification down to 0.02 mg·m⁻³. Over six months of continuous monitoring at a compounding plant with a usage rate of 80 kg·week⁻¹, all personal breathing zone samples returned values below the analytical limit of detection, provided that weigh‑up operations were conducted inside a downdraft booth with a face velocity of 0.5 m·s⁻¹.