1-(4'-Sulfophenyl)-3-methyl-5-pyrazolone

1-(4′-Sulfophenyl)-3-methyl-5-pyrazolone is supplied as an off-white to pale-yellow crystalline powder with the molecular formula C₁₀H₁₀N₂O₄S and a molar mass of 254.26 g mol^-1. The compound is identified by CAS Registry Number 89-36-1 and appears in technical literature under synonyms including 3-methyl-1-(4-sulfophenyl)-2-pyrazolin-5-one and 4,5-dihydro-5-oxo-1-(4-sulfophenyl)-3-methyl-1H-pyrazole. Commercial lots may be isolated as the free sulfonic acid or the sodium salt; the form affects the pH of a 1% aqueous suspension, which is commonly reported in the range 2.5 to 4.0. Bulk density values in supplier documentation are typically 0.45 g cm^-3 to 0.60 g cm^-3. The material is freely soluble in alkaline water, moderately soluble in hot water, slightly soluble in ethanol, and essentially insoluble in toluene and n-hexane. No harmonized ISO model number has been assigned to this substance. Instead, supplier-specific grade designations are used: technical grade is usually controlled at ≥98.0% HPLC area, purified grade at ≥99.0%, and analytical reagent grade at comparable assay limits with lower transition-metal residue specifications.

The sulfonic acid substituent is fully ionized under neutral and alkaline conditions, giving the molecule anionic character and high aqueous solubility. This property distinguishes the product from less polar pyrazolones and determines its dominant use as a water-soluble azo coupling component. Storage of unopened containers is generally specified below 30 °C and 60% relative humidity because the powder is hygroscopic. Opened material should be re-dried under vacuum at 60 °C to 80 °C if water content rises above 1.0% w/w.

What Is the Effect of the 4′-Sulfophenyl Group on Coupling Reactivity and Solubility?

The structural departure from 1-phenyl-3-methyl-5-pyrazolone is the introduction of a sulfonic acid group in the para position of the N-1 phenyl ring. This modification does not eliminate the pyrazolone tautomerism, but it sharply changes partition behaviour, aqueous solubility, and the anionic character of both the coupler and the resulting azo dyestuffs. In aqueous alkaline coupling media, the sulfonate is ionized and the coupler is present predominantly as the sodium sulfonate salt. This allows a homogeneous reaction with diazonium salts, whereas the non-sulfonated analogue usually requires aqueous-organic solvent mixtures to achieve comparable solubility. The active C-4 methylene site remains susceptible to electrophilic attack by the diazonium ion, and enolate formation is favoured at pH 7.5 to 9.0.

The electronic effect of the sulfophenyl group is moderate. The sulfonate is electron-withdrawing by induction, but its mesomeric influence is attenuated by the anionic charge. The coupler is therefore slightly less reactive than the corresponding 4-nitrophenyl analogue under identical pH, but the practical advantage is an aqueous coupling process without added solvent. Published kinetic data for this exact substitution pattern are limited; process development routinely relies on comparative coupling trials rather than ab initio prediction. The difference in hydrophilicity also changes the chromatographic behaviour of the sulfonated derivative. On reversed-phase C18 columns, the permanent negative charge produces early elution and may require an ion-pair mobile phase to achieve adequate retention, whereas the uncharged phenyl analogue is retained more strongly.

Representative commercial specifications are shown in the table below. These values are typical of technical data sheets and are not a legally binding specification; each lot should be confirmed against the certificate of analysis supplied by the manufacturer.

Production of the sulfonated pyrazolone is generally conducted under ISO 9001:2015-certified quality management, and supply within the European Union requires registration under REACH Regulation (EC) 1907/2006. Because the substance is an intermediate and not a finished consumer article, no food-contact or pharmaceutical monograph designation should be assumed without explicit supplier confirmation.

When the Sulfophenyl Pyrazolone Is Used as an Azo Coupling Component in Acid Dye Manufacture

A representative reaction sequence begins with dissolution of 1-(4′-sulfophenyl)-3-methyl-5-pyrazolone in demineralised water at 20 °C to 30 °C, followed by pH adjustment to 7.5 to 9.0 with sodium carbonate or sodium bicarbonate. A diazonium salt prepared from a substituted aromatic amine is added below the liquid surface while the pH is held within ±0.5 pH units by automatic dosing. Coupling is maintained at 5 °C to 15 °C in a glass-lined stirred reactor. Higher temperatures favour diazonium decomposition and generate coloured by-products that shift the final dye shade and reduce brightness. The endpoint is monitored by spot test with an alkaline reactive coupling component; absence of free diazonium ion is confirmed by a negative colour response. The resulting monoazo dye is precipitated by salting out, isolated in a plate-and-frame filter press, washed with cold demineralised water, and dried under vacuum at 60 °C to 80 °C.

This aqueous coupling route is the principal operational difference from less polar pyrazolones. The sulfonate function keeps the coupler in solution during the reaction and contributes to the water solubility and anionic levelling behaviour of the finished acid dye. In production-scale campaigns, batch-to-batch shade variation is controlled by maintaining identical molar ratios, controlled addition times between 60 min and 120 min, and agitation power per unit volume in the range 30 W m^-3 to 50 W m^-3 in pilot-scale vessels. Nitrogen blanketing is recommended because oxidative coupling at the C-4 position can generate dark impurities when air is entrained. Iron and copper concentrations in process water should be maintained below 0.1 mg L^-1 because transition-metal ions complex with pyrazolone intermediates and dull the final dye shade.

Published data for continuous-flow processing of this specific pyrazolone are limited. Most current manufacturing campaigns remain batch operations because the coupling pH window is narrow and the diazonium feed must be tightly matched to enolate availability. Deviation above pH 9.5 can hydrolyse the diazonium salt to the corresponding phenol, while deviation below pH 7.0 slows enolate formation and leaves unreacted coupler in the final dyestuff.

Although the compound is primarily a dye intermediate, some analytical laboratories investigate its use as a ligand for transition-metal ions and as a derivatization agent for carbonyl compounds. Published validations for this exact sulfonated derivative are less extensive than those for 1-phenyl-3-methyl-5-pyrazolone, which is widely used for carbohydrate tagging in HPLC. The sulfonated analogue gives earlier elution on reversed-phase C18 columns because of its permanent negative charge; this property can be useful when the target analyte must be separated from neutral matrix components, but it also reduces retention and may require an ion-pairing mobile phase. This is an analytical formulation constraint, not a defect of the coupler, but it explains why the non-sulfonated compound remains preferred for many chromatographic derivatization methods.

Comparative Product Differentiation Against 1-Phenyl-3-Methyl-5-Pyrazolone and Sulfonamide Analogues

Three structural analogues are frequently considered when a pyrazolone coupler is required. The sulfophenyl derivative is differentiated by its permanent anionic charge in aqueous dyeing; the unsubstituted phenyl derivative is a neutral molecule requiring organic solvent; the sulfonamide analogue has weaker acid character and different hydrogen-bonding behaviour. The table below summarizes the distinctions that affect formulation and process choice.

The choice of analogue in dye manufacture is rarely based on colour strength alone. Water solubility, anti-agglomeration during salting-out, electrolyte tolerance, and migration behaviour on polyamide fibres all influence the selection. The sulfophenyl derivative is preferred where the dye must be applied from an aqueous bath without dispersing agents and where residual organic solvent is unacceptable. The non-sulfonated phenyl derivative remains the standard derivatization reagent for reducing carbohydrates because its hydrophobic retention improves chromatographic resolution. Where moderate water solubility and reduced anionic charge are required, the sulfonamide analogue may be evaluated, but its pH-dependent ionisation must be considered separately.

Storage Stability and Process Boundaries for Sulfonated Pyrazolone Powders

Stability testing under ICH Q1A(R2) is not typically applied to dye intermediates unless a pharmaceutical use is specified. Suppliers commonly assign a retest or shelf life of 24 months when the product is stored in the original sealed packaging at 25 °C and 60% relative humidity. The powder is hygroscopic and should be re-dried at 60 °C to 80 °C under vacuum if water content exceeds 1.0%. Packaging generally comprises a double low-density polyethylene liner inside a fibreboard drum; stainless steel or glass-lined handling equipment is preferred because prolonged contact with carbon steel can release iron that complexes with the pyrazolone and shifts the colour of the final dye.

The substance is incompatible with strong oxidizing agents, hot strong acids, and nitrosating agents in acidic media. Contact with nitrous acid can generate undesired nitrosated pyrazolone derivatives, which are chemically distinct and may alter the coupling behaviour. Dust control and local exhaust ventilation are required where airborne concentrations exceed national occupational exposure limits. Particulate respirators conforming to EN 143 with P2 filters are recommended for dust-generating transfers. Skin and eye contact should be managed by standard chemical hygiene practices, and process operators should consult the safety data sheet for disposal limits and spill-control measures specific to sulfonated pyrazolone salts.