1,8-Dihydroxy-4,5-dinitroanthraquinone

Chemical identity for the compound supplied under CAS 81-55-0 is 1,8-dihydroxy-4,5-dinitroanthracene-9,10-dione, abbreviated in purchase specifications as DHDNAQ. The molecular formula is C14H6N2O8, giving a nominal molar mass of 330.21 g/mol on the anhydrous basis. The molecule contains two peri-hydroxy groups at positions 1 and 8 and two nitro groups at positions 4 and 5 of the anthraquinone nucleus. This substitution pattern is deliberately distinct from 1,8-dihydroxyanthraquinone and 1,5-dinitroanthraquinone because the nitro groups withdraw electron density from the aromatic system while the peri-hydroxy groups form intramolecular hydrogen bonds with the quinone carbonyl oxygens. Commercial material is supplied as a yellow-orange to brown crystalline powder. Supplier model codes remain proprietary, but two usual commercial forms are industrial grade and purified analytical grade. The industrial grade is specified as a reduction substrate; the purified grade carries lower residual isomer content and a certified purity statement.

Because the molecule contains both acidic phenolic hydroxyl groups and reducible nitro groups, quality control cannot rely on melting point alone. A combination of chromatographic purity, moisture, and inorganic residue is used to define the release envelope. The compound decomposes near its melting transition, and published capillary melting ranges vary with supplier and heating rate; a fixed single melting point is not used as the primary acceptance criterion.

What Release Parameters Govern Industrial Acceptance?

Industrial acceptance is generally conditioned on three release properties: chromatographic purity, moisture, and inorganic residue. Isomeric nitroanthraquinone impurities are not detectable by visual inspection and can persist into downstream dye structures; ash and moisture affect dosing accuracy in continuous reduction plants and can shift the pH of an aqueous slurry. The following framework release limits are representative of industrial-grade material. Customer-specific specifications may tighten moisture to ≤ 0.30% when the downstream reactor is moisture-sensitive.

For method transfer between laboratories, the HPLC system should use a reversed-phase C18 column with a pH-stable mobile phase containing phosphoric acid or formic acid. Detection at 254 nm is preferred because the nitro and quinone chromophores absorb strongly in that region. Area normalization should be checked against a mass-balance method if non-UV-absorbing impurities are suspected. Karl Fischer moisture determination should use a desolvation oven if the sample is not fully soluble in the titration solvent, because incomplete dissolution can lead to low water recovery and false acceptance. Residue on ignition should be run in a ventilated furnace with a controlled temperature ramp to avoid spattering. Inter-laboratory reproducibility for chromatographic purity is typically improved when samples are dissolved in dimethylformamide at a fixed concentration and filtered through a 0.45 µm PTFE membrane immediately before injection.

In continuous reduction plants the moisture content has a disproportionate effect on output. A moisture level above 0.50% can reduce the feed rate through rotary valves and create sticky dust layers on the suction manifold. Water entry usually occurs during open-hopper transfer or from condensation in vacuum dryers. Operators measuring the pH of a 1% aqueous slurry use the result as an indirect check for residual acid from nitration; values below 4.0 can indicate insufficient washing and should trigger an additional conductivity check. Residual acid is usually acetic or sulfuric acid carried over from the isolation step, and it can interfere with the subsequent reduction stoichiometry by consuming alkalinity before the nitro groups are reduced.

Downstream conversion usually removes the nitro groups by reduction to 1,8-dihydroxy-4,5-diaminoanthraquinone. In a glass-lined batch reactor the reduction is not a single-step transformation; the nitro groups pass through nitroso and hydroxylamine intermediates that can undergo condensation if local mixing is poor. The most common cause of off-color product is not the bulk jacket temperature but the localized concentration of sodium sulfide or sodium hydrogen sulfide at the feed point. The reducing agent should be dosed below the liquid surface through a dip pipe, with the slurry maintained between 8.5 and 9.5 pH using sodium carbonate. The temperature rise should be limited to ≤ 3 K/min during the main reduction exotherm; published kinetic data for this specific substrate are limited, so the maximum safe dosing rate must be established by a reaction calorimeter. After reduction, the wet cake of 1,8-dihydroxy-4,5-diaminoanthraquinone is oxygen-sensitive and should be kept under nitrogen until the next reaction step to prevent quinone-imine polymer formation and increased filtration pressure.

Thermal stability studies on nitroanthraquinones indicate that decomposition onset is strongly affected by residual acid and metal salts. Iron and copper ions at milligram-per-kilogram levels can lower the decomposition exotherm and broaden the DSC trace. For this reason, stainless steel contact during drying should be minimized when the material is still wet with acidic mother liquor; glass-lined or PTFE-lined equipment is preferred for isolation and washing. The dried solid is more stable and can be handled in stainless steel equipment provided that moisture is controlled and residence time at elevated temperature is short. Differential scanning calorimetry should be run under nitrogen, not air, if the objective is to distinguish decomposition from oxidation.

When the 4,5-Dinitro Substitution Is Compared with 1,5- or 1,8-Isomers

The difference between DHDNAQ and structurally related anthraquinone intermediates is not merely the position of the nitro groups. 1,8-Dihydroxyanthraquinone lacks the nitro functions and therefore does not provide the same reduction route to a 4,5-diamino intermediate. 1,5-Dinitroanthraquinone lacks the peri-hydroxy network, so its solubility and hydrogen-bonding behavior differ, and it does not offer the same intramolecular stabilization of the quinone carbonyls. The reduced product, 1,8-dihydroxy-4,5-diaminoanthraquinone, is the actual diazo or condensation component in many dye sequences; the nitro intermediate is purchased because the nitro groups protect the amine positions during earlier isolation and purification steps. These distinctions are summarized below.

Batch-to-batch variance in industrial-grade material is commonly traced to incomplete removal of the 1,8-dihydroxy-4-nitroanthraquinone intermediate or to over-nitration by-products. The chromatographic profile at 280 nm can reveal these impurities even when the 254 nm area ratio remains within specification. Purchasers using the material for high-value dye intermediates may request a secondary purity report at 280 nm or an impurity profile by liquid chromatography-mass spectrometry. The absence of this profile does not make the product unsuitable, but it shifts the control burden to the downstream reduction step.

Anthraquinone dyes prepared from the reduced dihydroxy-diamino intermediate are used in solvent dye, disperse dye, and vat dye applications. In polyester coloration, the final dye is typically wet-milled to a particle size of ≤ 1 µm D90 by ISO 13320 laser diffraction before formulation. A twin-screw extruder with 40:1 L/D can disperse the final colorant into polyethylene terephthalate at typical loadings of 1.0–2.0 wt%; however, DHDNAQ itself is not normally used as a direct polymer additive because the nitro groups can behave as thermal and photochemical quenching sites. In polycarbonate or engineering thermoplastic masterbatch trials, residual unreduced nitro content above the analytical limit should be rejected because it can shift melt viscosity and contribute to yellowing under high-temperature processing. Published data for this specific polymer-additive configuration is limited, and plant trials are recommended before scale-up.

The product also differs from mononitroanthraquinone-based intermediates in reduction selectivity. Because the two nitro groups are structurally equivalent and positioned on opposite outer rings, the desired 4,5-diamino product can be formed without creating a mixed nitro-amino intermediate as the sole end point. In comparison, mononitroanthraquinone feedstocks require a different protection strategy. This symmetry is one reason DHDNAQ is specified when a symmetrical diaminoanthraquinone chromophore is required.

Dust Control and Transfer Operations in Multi-Purpose Plants

Dry powder transfer of DHDNAQ requires engineered dust control because the dried solid can generate respirable fractions during milling and bag discharge. A typical transfer line includes a bag dump station with local exhaust ventilation, a rotary valve, and a vacuum receiver under nitrogen. Grounding and bonding are specified because the powder can accumulate static charge in non-conductive flexible hoses. The material should not be transferred with strong reducing agents or powdered metals in the same equipment. Separate washdown water should be collected for nitroaromatic waste treatment. Prolonged contact with concentrated caustic at temperatures above 80°C can generate colored decomposition products and should be avoided.

Dissolution of DHDNAQ in dimethylformamide or dimethyl sulfoxide should be performed at 60–70°C with stirring. Rapid addition to hot solvent can produce a temporary viscosity increase and incomplete wetting of the powder. In a plant environment, the preferred charging sequence is to add the powder to a heel of solvent under agitation, then raise the temperature. This sequence reduces dust generation and prevents crust formation on the vessel wall. If the solution is to be held before reduction, it should be blanketed with nitrogen and kept under mild agitation because the nitro groups can react slowly with reducing impurities present in recycled solvent.

Reaction calorimetry is recommended before scaling up the reduction because the heat of reaction is not directly predictable from the standard enthalpy of formation of the isolated nitro compound. Adiabatic temperature rise, time to maximum rate, and maximum pressure should be measured for the specific solvent and reducing agent system. A conservative engineering limit for the addition exotherm is ≤ 5 K/min at the addition point; above this value, localized decomposition can outrun the condenser and generate nitrogen oxide vapors. Published data for this exact reduction configuration are limited, so no universal value should be substituted for a site-specific thermal risk assessment.

Storage conditions are defined by the nitroaromatic character of the compound. Closed containers should be kept below 40°C, away from direct sunlight, and under an inert headspace if the material has been dried below 0.30% moisture. Avoid contact with strong reducing agents, powdered aluminum or zinc, and concentrated mineral acids at elevated temperature. Regulatory classification should be verified against the current ECHA C&L inventory and the supplier safety data sheet before purchase because nitroaromatic substances may be subject to specific workplace exposure limits and waste controls. Analytical-grade purchases should state whether the material is intended for quantitative calibration because industrial-grade material may contain 1–2% of related anthraquinone isomers that are acceptable for synthesis but not for reference use.