1-(3'-Sulfophenyl)-3-methyl-5-pyrazolone

1-(3′-Sulfophenyl)-3-methyl-5-pyrazolone is a substituted 5-pyrazolone derivative assigned CAS 119-18-6, molecular formula C₁₀H₁₀N₂O₄S, and relative molecular mass 254.26 g/mol. The compound is manufactured by condensation of 3-sulfophenylhydrazine with an acetoacetic ester, followed by alkaline ring closure; the sodium salt grade is prepared by neutralisation and drying. The free acid is recovered as a white to pale yellow crystalline powder, while the sodium salt, relative molecular mass 276.24 g/mol, is isolated as a pale yellow to light beige powder. The substance functions primarily as an azo-coupling component in water-soluble acid dye chromophores and is also used as a fine chemical intermediate. Its sulfonate group permits azo coupling in aqueous alkaline media and reduces the need for methanol or acetone co-solvent; this is distinct from 1-phenyl-3-methyl-5-pyrazolone, CAS 89-25-8, which requires co-solvent or strong alkali for comparable processing. The product is not interchangeable with 1-(4′-sulfophenyl)-3-methyl-5-pyrazolone, CAS 89-36-1; the two isomers share the same empirical formula but differ in substitution geometry, coupling pH profile, and downstream colouristic behaviour.

Two commercial model designations are encountered in purchasing documents: a dye-intermediate grade with an assay acceptance limit of not less than 98.0% high-performance liquid chromatography area, and a refined grade intended for analytical or pharmaceutical intermediate use with lower residual by-product limits. Because model codes are supplier-specific, the robust ordering descriptor is the full chemical name plus CAS 119-18-6. When direct aqueous processing is required, the sodium salt is specified as sodium 1-(3-sulfophenyl)-3-methyl-5-pyrazolon-5-olate, or by adding the phrase “sodium salt of CAS 119-18-6”. This prevents receipt of the free acid in formulations where neutral pH or higher initial solubility is required.

What process-control variables are most sensitive when this pyrazolone is used as an azo coupling component?

Process-control data from production-scale campaigns in 5,000 L glass-lined coupling vessels indicate that batch-to-batch tinctorial strength variation is more closely associated with pH drift than with coupler assay alone. The diazonium salt is prepared in a separate closed reactor at 0–5°C using sodium nitrite and hydrochloric acid; excess nitrite is destroyed with sulfamic acid before transfer. The pyrazolone is predissolved in dilute sodium hydroxide or sodium carbonate and charged to the coupling vessel. Coupling is exothermic and is maintained at 0–10°C by brine cooling. The reaction pH is held between 8.5 and 10.5 by metered addition of 20% sodium carbonate solution. Excursions above 11 are associated with increased hydrolysis by-products and reduced colour strength, while pH values below 8 slow the coupling rate and may leave unreacted diazonium salt that decomposes to tar. Endpoint confirmation is performed by thin-layer chromatography or by HPLC with UV detection at 254 nm.

The 3-sulfophenyl isomer exhibits a distinct coupling pH profile relative to the 4-sulfophenyl isomer because the meta-sulfonate group changes electron density at the pyrazolone 4-position through inductive withdrawal without the same resonance contribution of the para isomer. This electronic difference can shift the end point and the final hue; therefore a dye house qualification protocol should not assume isomer interchangeability based on molecular formula alone. High-performance liquid chromatography using a reversed-phase C₁₈ column with a phosphoric acid/acetonitrile gradient can resolve the two isomers when the laboratory co-injects reference standards for both CAS numbers.

In equipment terms, the diazotisation vessel is closed and fitted with fume extraction, while the coupling vessel is glass-lined carbon steel or 316L stainless steel. The isolated azo product or dye intermediate is collected on a plate-and-frame filter press or Nutsche filter, washed with chilled water or dilute sodium chloride solution, and dried under vacuum. Residual nitrite, sulfonate by-products, and inorganic salts in the filtrate are controlled before discharge under local permits. Pilot-scale solids-transfer studies show that the free acid can form adherent dust layers on rotary valve surfaces when relative humidity exceeds 60%. A 100 L conical dryer with vacuum and a 40°C jacket set point reduces caking during subsequent bag discharge. For dosing into aqueous coupling vessels, a pre-dispersed slurry at 10–15% solids reduces dust exposure and improves feed accuracy relative to direct powder addition.

Representative physicochemical specification for the free acid technical grade

The following acceptance limits are representative of commercial free acid technical-grade material. The sodium salt grade differs in water content, pH, and ash profile and is assayed on an anhydrous basis.

The free acid is hygroscopic under relative humidity above 60%; pre-drying at 40–50°C under vacuum may be required before gravimetric lot adjustments. The sodium salt may retain up to 5.0% water and is assayed on an anhydrous basis. Because the sulfonic acid group generates a corrosive dust, closed transfer and local exhaust ventilation are required. Avoid storage with strong oxidizers, nitrites in acidic media, and copper-containing alloys during prolonged exposure, as these conditions can promote oxidative or nitrosation side reactions. Under CLP Regulation (EC) No 1272/2008, the free acid may be classified as a skin and eye irritant based on acidic pH; the sodium salt may not carry the same pH-related classification. REACH registration status and harmonised classification should be confirmed with the supplier for the specific product model before use.

When a meta-sulfonated phenyl ring is selected instead of the para-sulfophenyl isomer in dye synthesis

Selection of the 3′-sulfophenyl derivative over the 4′-sulfophenyl isomer is not a simple substitution. The meta sulfonate changes the polarity, aggregation behaviour, and chromatographic retention of the free coupler and of the resulting azo dyestuff. In dye synthesis, these differences influence tinctorial strength, shade direction, and effluent adsorption characteristics. The 4-isomer is used widely in several acid yellow azo dye families; the 3-isomer is selected where a modified hue, altered solubility in hard water, or different membrane filtration behaviour is required. The 3-isomer also differs from the non-sulfonated 1-phenyl-3-methyl-5-pyrazolone in that the latter is mainly used in pharmaceutical pyrazolone synthesis and has low water solubility, requiring solvent-based coupling or stronger alkali.

1-(3′-Sulfophenyl)-3-methyl-5-pyrazolone should not be treated as a direct drop-in replacement for the 4-sulfophenyl isomer without requalification of coupling endpoint, salt content, and shade. The high polarity of the sulfonate group reduces solvent extraction efficiency from acidic aqueous solutions; preparative chromatographic isolation may require ion-pair reagents or pH-buffered mobile phases. The substance is incompatible with strong reducing agents and with nitrous acid under acidic storage conditions. For lots stored longer than 24 months, retest of water content, HPLC purity, and pH of a 10 g/L suspension is recommended. Published data for this specific configuration is limited; no formal accelerated stability study covering all tropical storage conditions is available in the public literature.