2-Benzothiazolesulfinicacid(9Ci)

2-Benzothiazolesulfinic acid (9CI), CAS 4731-64-2, is a bench-stable organic sulfinic acid derivative characterized by a benzothiazole heterocycle substituted at the 2-position with a sulfinic acid (–SO₂H) group. The molecular formula C₇H₅NO₂S₂ corresponds to a molar mass of 199.25 g·mol⁻¹. Commercial supply typically presents the compound as a white to pale-yellow crystalline powder with a purity of ≥98.0% (HPLC, area normalization at 254 nm) and a loss on drying not exceeding 0.5% (105 °C, 2 h, gravimetric). Unlike heavy-metal-catalysed intermediates for rubber vulcanization—most notably 2-mercaptobenzothiazole (MBT)—this sulfinic acid contains sulfur in the +4 oxidation state, imparting a distinct reactivity profile that circumvents the odorous thiol volatility and autoxidation pathways associated with mercapto analogues. The product is supplied in 25 kg HDPE drums with double PE liners under nitrogen and labelled in accordance with EU REACH Annex II, meeting transport classification UN 3261, 8, PG III when the free acid is handled. Its current position in the fine-chemical portfolio intermediates between laboratory building blocks and production-scale raw material for sulfonamide antibiotics, agrochemical safeners, and photographic emulsion stabilisers. ---

Typical commercial specification for 2-benzothiazolesulfinic acid, free acid form

Parameter	Specification	Test method
Assay (C₇H₅NO₂S₂)	≥ 98.0%	HPLC (C18, 254 nm, area%)
Water (Karl Fischer)	≤ 0.5%	Ph. Eur. 2.5.12, coulometric
2,2’-Dithiobis(benzothiazole)	≤ 0.2%	HPLC as above, retention-time reference
2-Benzothiazolesulfonic acid	≤ 0.5%	Ion chromatography, conductivity detection
Sulfated ash	≤ 0.1%	Ph. Eur. 2.4.14
Heavy metals (as Pb)	≤ 10 ppm	ICP-OES after microwave digestion
pH (1% aqueous suspension)	2.0 – 3.0	Potentiometric, 25 °C

When the sulfinic group outperforms the thiol in pharmaceutical coupling The benzothiazole-2-sulfinic acid scaffold enables direct installation of the pharmacophoric benzothiazolesulfonamide unit via oxidative coupling with amines under mild conditions. In process chemistry, this route avoids the handling of volatile 2-mercaptobenzothiazole and its attendant sulphurous off-notes, which frequently mandate scrubbed ventilation and personal-monitoring regimes for exposure to airborne thiols (TLV-TWA 0.5 ppm). Using the sodium salt of 2-benzothiazolesulfinic acid generated in situ by neutralization with aqueous NaOH to pH 7.2–7.5, copper(I) iodide-catalysed coupling with primary or secondary aryl amines proceeds in DMF/H₂O mixtures at 60 °C over 12–18 h. Isolated yields of the corresponding sulfonamides regularly exceed 85% after extractive workup and trituration. The procedure has been demonstrated at pilot scale in a 200 L glass-lined reactor equipped with a retreat-blade impeller running at 110 rpm; off-gas monitoring for residual SO₂ was maintained below 1 ppm using a wet caustic scrubber. Because the sulfinic acid participates as a four-electron donor in the catalytic cycle, the reaction does not require external oxidants that would otherwise generate halogenated by-products, thereby streamlining ICH M7 impurity control for genotoxic residuals. Commercial synthesis of the active pharmaceutical ingredient sulfabenzamide and certain COX-2 inhibitor candidates has adopted the sulfinic acid route specifically because the mercapto analogue promoted unwanted formation of the disulfide during amidation. This operational selectivity, combined with a shelf-stable solid intermediate amenable to air-freight, makes 2-benzothiazolesulfinic acid a preferred building block in heterocyclic sulfonamide libraries. Application in dye intermediates and photographic additives In colour-developer formulations for silver halide photography, the sodium salt of 2-benzothiazolesulfinic acid is added at 0.1–0.5 wt% as an antioxidant preservative that retards aerial oxidation of p-phenylenediamine derivatives without forming coloured condensation products. Accelerated aging tests at 60 °C and 90% RH over 14 days have demonstrated colour density stability within ± 2% of initial values when the sulfinate is present, outperforming conventional sulfite-only systems at equivalent molar concentrations. In azoic dye synthesis, the compound functions as a latent sulfonylating agent that can be oxidised in situ to the reactive sulfonyl chloride, thereby eliminating the need to isolate the moisture-sensitive sulfonyl halide. What limits the shelf life of 2-benzothiazolesulfinic acid in humid environments? The free acid is inherently hygroscopic and undergoes acid-catalysed disproportionation in the presence of moisture, producing 2-benzothiazolesulfonic acid and benzothiazole-2-thiosulfonate. Extended storage at relative humidity exceeding 60% and temperatures above 25 °C leads to a measurable decline in purity: accelerated stability studies (ICH Q1A conditions, 40 °C / 75% RH, open dish) recorded a loss of assay of ~7% after 3 months, with concomitant increase in the sulfonic acid impurity to 4.2%. To preserve quality, the product is re-tested at 12-month intervals when stored in unopened original packaging under nitrogen at ≤ 25 °C. Site practice on multi-ton production campaigns requires dry nitrogen blanketing of day bins, with inline dew-point sensors tripping an alarm at −30 °C dew point. Alkaline sodium salt solutions stored in stainless steel 304L hold tanks at pH 7.5 ± 0.2 exhibit far greater stability, where disproportionation half-life exceeds 30 days at ambient temperature, making the salt the preferred form for continuous downstream processing. The sulfinic–sulfonic disproportionation equilibrium under production-scale conditions The pivotal process conflict in large-scale handling of 2-benzothiazolesulfinic acid resides in the sensitive disproportionation equilibrium that converts two molecules of sulfinic acid into one molecule of sulfonic acid and one equivalent of the corresponding thiosulfonate. In a 4,000 L glass-lined neutralisation vessel, where the free acid is transformed to its sodium salt with 30% aqueous NaOH, the exotherm is controlled by jacket cooling to maintain a bulk temperature of 15–20 °C and a dosing rate such that the local pH never falls below 6.0. At pH 4.5—a condition inadvertently reached when caustic addition was interrupted during a batch—disproportionation half-life was determined via Raman spectroscopy to be approximately 2 h at 25 °C, resulting in the rapid precipitation of a sticky, filter-clogging benzothiazole-2-thiosulfonate gel. Recovery from such a deviation required a vessel clean-out cycle of 16 h and incurred a yield loss of 18% relative to the campaign average. To rigidly enforce a processing window, in-line pH probes with automatic feedback to the caustic dosing pump maintain the setpoint at pH 7.3 ± 0.3. The addition of 50 ppm tetrasodium EDTA is standard practice to sequester trace metal ions (Fe, Cu) that catalyse radical-mediated side reactions, including the oxidative scission that releases SO₂. Temperature excursions above 35 °C during salt crystallisation have been correlated with increased formation of 2,2’-dibenzothiazole disulfide, quantified by HPLC at levels up to 1.8% in final product lots when a rupture-disc vent event momentarily stripped cooling. Thus the safe operating envelope for the neutralisation step is documented as pH 7.0–7.6, jacket inlet ≤ 5 °C, and maximum batch temperature 25 °C, with continuous nitrogen sweep. Generating 2-Benzothiazolesulfonyl Chloride: A High-Purity Route to Electrophilic Sulfonylation For electrophilic sulfonylation of nucleophilic substrates, 2-benzothiazolesulfinic acid is directly converted to the sulfonyl chloride using chlorine gas in aqueous hydrochloric acid at 0–5 °C. In a dedicated 500 L PTFE-lined reactor, the sulfinic acid is suspended in 2 N HCl and sparged with chlorine at a rate of 0.8–1.2 kg·h⁻¹ until complete dissolution is observed and the oxidant potential, monitored by ORP electrode, exceeds 850 mV vs Ag/AgCl. The resulting 2-benzothiazolesulfonyl chloride precipitates as a white crystalline solid and is isolated by filtration under a nitrogen blanket, washed with ice-cold deionised water, and dried in a vacuum shelf dryer at 30 °C and 5 mbar to a water content ≤ 0.1%. Yields are typically 92–95% with a purity of 99.0% (GC). This sulfonyl chloride serves as the starting material for the synthesis of benzothiazole-2-sulfonamides, which have been evaluated as selective endothelin receptor antagonists. The oxidative chlorination route eliminates the need to handle pre-formed chlorosulfonic acid, reduces acid-waste volume by 40% compared to direct sulfonation pathways, and ensures that residual inorganic chloride can be scrubbed to ≤ 5 ppm via a water-wash step, thereby preventing corrosion in subsequent stainless-steel equipment used for amidation. ---

Divergent functional reactivity of benzothiazole derivatives in industrial intermediate applications

Property	2-Benzothiazolesulfinic acid	2-Mercaptobenzothiazole	2-Benzothiazolesulfonic acid
Functional group	–SO₂H	–SH	–SO₃H
Sulfur oxidation state	+4	−2	+6
pKₐ (approx., H₂O)	1.0–2.0 (free acid)	~7.0 (thiol)	< −1 (strong acid)
Solubility in water (g·L⁻¹ at 25 °C)	~15 (free acid), salt > 200	0.12	> 500
Primary industrial application	Sulfonamide pharmaceutical intermediate, photographic antioxidant	Vulcanisation accelerator, corrosion inhibitor	Electroplating brightener, acid catalyst
Key incompatibility	Moisture at pH < 5 leads to rapid disproportionation	Strong oxidisers, metals under heat	Sensitive amines may form genotoxic sulfonate esters