Abstract

Quinizarine Green SS, systematically named 1,4-bis(4-methylanilino)anthracene-9,10-dione (CAS Registry Number: 128-80-3), represents an anthraquinone-based synthetic dye with the molecular formula C₂₈H₂₂N₂O₂. This compound manifests as a black crystalline powder with a melting point range of 220-221°C and demonstrates insolubility in aqueous media but significant solubility in polar organic solvents. The molecular structure features a planar anthraquinone core system substituted with two p-toluidine groups at the 1,4-positions, creating an extended π-conjugated system responsible for its intense green coloration. Quinizarine Green SS finds primary application as Solvent Green 3 in cosmetic formulations, pharmaceutical colorants, and specialized pyrotechnic compositions. The compound exhibits characteristic electronic absorption maxima in the visible spectrum between 600-700 nanometers, rendering it effective for coloring applications requiring green hues.

Introduction

Quinizarine Green SS belongs to the anthraquinone dye class, a significant category of synthetic colorants characterized by their structural stability and diverse coloration properties. First synthesized in the early 20th century through the condensation of quinizarin with aromatic amines, this compound has established itself as an important industrial colorant under the designation Solvent Green 3 (Color Index: 61565). The molecular architecture combines the electron-accepting anthraquinone system with electron-donating diaminotoluene substituents, creating a push-pull electronic configuration that governs its optical characteristics. Industrial production exceeds several metric tons annually worldwide, primarily serving the cosmetics industry where it is approved as D&C Green No. 6 for external applications. The compound's thermal stability and solubility profile make it particularly valuable for applications requiring coloration of non-polar matrices.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

X-ray crystallographic analysis confirms that Quinizarine Green SS adopts a predominantly planar molecular configuration with minimal deviation from coplanarity. The anthraquinone core system exhibits bond lengths characteristic of quinoid systems: carbonyl bonds measure 1.22 ± 0.02 Å, while the connecting carbon-carbon bonds in the central ring system range from 1.40-1.45 Å. The p-toluidine substituents orient at angles of approximately 5-10° relative to the anthraquinone plane, minimizing steric interactions while maintaining effective π-conjugation. Nitrogen atoms in the amine linkages display sp² hybridization with bond angles of 120° ± 2° around the nitrogen centers. The electronic structure features highest occupied molecular orbitals localized predominantly on the amine substituents and lowest unoccupied molecular orbitals concentrated on the anthraquinone acceptor system, creating a charge-transfer transition responsible for the compound's coloration.

Chemical Bonding and Intermolecular Forces

Covalent bonding in Quinizarine Green SS follows predictable patterns for conjugated aromatic systems. Carbon-carbon bonds in the anthraquinone core demonstrate bond orders between 1.5-2.0, with the shortest bonds (1.36-1.38 Å) occurring between positions 9,10 and the carbonyl carbons. The C-N bonds connecting the amine substituents to the anthraquinone system measure 1.42 ± 0.02 Å, indicating partial double-bond character due to resonance with the aromatic system. Intermolecular forces include substantial π-π stacking interactions between anthraquinone systems with interplanar distances of 3.4-3.6 Å in the crystalline state. Hydrogen bonding occurs between amine protons and carbonyl oxygen atoms with N-H···O distances of 2.02 ± 0.05 Å and angles of 165-170°. The molecular dipole moment measures 4.2 ± 0.3 Debye, oriented along the molecular axis connecting the two amine substituents.

Physical Properties

Phase Behavior and Thermodynamic Properties

Quinizarine Green SS presents as a black crystalline powder with metallic luster under microscopic examination. The compound undergoes sharp melting at 220-221°C with enthalpy of fusion measuring 38.5 ± 1.2 kJ·mol⁻¹. Crystallographic analysis reveals a monoclinic crystal system with space group P2₁/c and unit cell parameters a = 14.32 Å, b = 7.85 Å, c = 15.47 Å, and β = 112.5°. Density calculations based on crystallographic data yield 1.32 ± 0.02 g·cm⁻³ at 25°C. The compound sublimes appreciably above 180°C under reduced pressure (0.1 mmHg) with sublimation enthalpy of 92.3 ± 2.5 kJ·mol⁻¹. Thermal gravimetric analysis demonstrates decomposition commencing at 320°C under nitrogen atmosphere. Solubility parameters include complete dissolution in chloroform (125 g·L⁻¹), dimethylformamide (98 g·L⁻¹), and dimethyl sulfoxide (86 g·L⁻¹) at 25°C, with negligible solubility in water (<0.01 g·L⁻¹).

Spectroscopic Characteristics

Infrared spectroscopy reveals characteristic vibrational modes including carbonyl stretching at 1665 cm⁻¹, C-H aromatic stretching at 3050-3100 cm⁻¹, and N-H stretching at 3380 cm⁻¹. The fingerprint region between 1400-1600 cm⁻¹ shows multiple aromatic C=C stretching vibrations consistent with substituted anthraquinone systems. Nuclear magnetic resonance spectroscopy demonstrates proton signals at δ 8.2-8.4 ppm (anthraquinone protons), δ 7.1-7.3 ppm (aromatic protons from toluidine groups), δ 6.8-7.0 ppm (protons adjacent to amine groups), and δ 2.3 ppm (methyl protons). Carbon-13 NMR shows carbonyl carbons at δ 183-185 ppm, aromatic carbons between δ 120-150 ppm, and methyl carbons at δ 20.5 ppm. Electronic absorption spectroscopy in chloroform solution exhibits maxima at 640 nm (ε = 15,200 M⁻¹·cm⁻¹) and 580 nm (ε = 12,800 M⁻¹·cm⁻¹) with shoulder absorption at 410 nm. Mass spectral analysis shows molecular ion peak at m/z 418.1682 (calculated for C₂₈H₂₂N₂O₂: 418.1681) with major fragments at m/z 403, 240, and 212 corresponding to sequential loss of methyl groups and cleavage of the amine-anthraquinone bonds.

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

Quinizarine Green SS demonstrates moderate chemical stability under ambient conditions but undergoes photochemical degradation upon prolonged UV exposure. The compound exhibits resistance to hydrolysis across pH ranges 3-9, with decomposition occurring under strongly acidic (pH < 2) or basic (pH > 10) conditions. Acid-catalyzed hydrolysis proceeds via protonation of carbonyl oxygen atoms followed by nucleophilic attack at the 9,10-positions, with rate constant k = 3.2 × 10⁻⁴ L·mol⁻¹·s⁻¹ at 25°C in 1M HCl. Oxidation with hydrogen peroxide or chromic acid results in degradation of the anthraquinone system, while reduction with sodium dithionite produces the leuco form which reoxidizes upon aerial exposure. The compound participates in electrophilic substitution reactions preferentially at the 5,8-positions of the anthraquinone system, with bromination occurring at these positions with second-order rate constant k₂ = 12.5 M⁻¹·s⁻¹ in dichloromethane at 25°C.

Acid-Base and Redox Properties

The amine functionalities in Quinizarine Green SS exhibit basic character with pKₐ values of 3.2 and 4.5 for the conjugate acids, determined by spectrophotometric titration in aqueous-organic mixed solvents. Protonation occurs preferentially at the amine nitrogen atoms rather than carbonyl oxygen atoms. The compound demonstrates reversible redox behavior with reduction potential E₁/2 = -0.75 V vs. SCE for the anthraquinone system in acetonitrile solution. Cyclic voltammetry shows quasi-reversible one-electron reduction waves with peak separation ΔEₚ = 85 mV at scan rate 100 mV·s⁻¹. The compound maintains stability across oxidation-reduction cycles with less than 5% decomposition after 50 cycles. In strongly oxidizing environments (potassium permanganate, ceric ammonium nitrate), complete degradation of the aromatic system occurs within minutes at room temperature.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

The primary synthetic route to Quinizarine Green SS involves condensation of quinizarin (1,4-dihydroxyanthraquinone) with p-toluidine in polar aprotic solvents. Typical laboratory preparation employs refluxing quinizarin (2.40 g, 10 mmol) with p-toluidine (4.28 g, 40 mmol) in nitrobenzene (50 mL) at 210°C for 8-12 hours under nitrogen atmosphere. The reaction proceeds via nucleophilic aromatic substitution where the hydroxy groups of quinizarin act as leaving groups. After completion, the mixture cools to room temperature and the product precipitates upon addition of methanol. Purification involves column chromatography on silica gel using chloroform:hexane (3:1) as eluent, yielding dark green crystals upon evaporation. Typical isolated yields range from 65-75%. Alternative synthetic approaches include Ullmann-type coupling reactions using copper catalysts or microwave-assisted synthesis which reduces reaction time to 45-60 minutes with comparable yields.

Industrial Production Methods

Industrial production of Quinizarine Green SS employs continuous flow reactors operating at 200-220°C with residence times of 4-6 hours. The process utilizes quinizarin and p-toluidine in approximately 1:2.2 molar ratio dissolved in o-dichlorobenzene as solvent at 15-20% concentration. Catalytic amounts of p-toluenesulfonic acid (0.5-1.0 mol%) accelerate the condensation reaction. The reaction mixture passes through multiple heated zones with precise temperature control to maximize conversion while minimizing decomposition. Product isolation involves cooling and filtration, followed by washing with methanol to remove unreacted starting materials. Final purification employs recrystallization from xylene or toluene, achieving technical grade purity of 95-98%. Production waste streams contain primarily solvent residues and small amounts of unreacted amines, which are recovered through distillation and recycled. Global production estimates range from 50-100 metric tons annually across major manufacturing regions.

Analytical Methods and Characterization

Identification and Quantification

High-performance liquid chromatography provides the most reliable method for identification and quantification of Quinizarine Green SS. Reverse-phase systems employing C18 columns with methanol:water (85:15) mobile phase at 1.0 mL·min⁻¹ flow rate achieve baseline separation with retention time 6.8 ± 0.2 minutes. Detection utilizes photodiode array detection monitoring absorbance at 640 nm. The method demonstrates linear response from 0.1-100 μg·mL⁻¹ with limit of detection 0.05 μg·mL⁻¹ and limit of quantification 0.15 μg·mL⁻¹. Thin-layer chromatography on silica gel plates with toluene:ethyl acetate (4:1) development yields Rf value 0.45 with characteristic green spot visible under visible light. Spectrophotometric quantification employs the absorption maximum at 640 nm (ε = 15,200 M⁻¹·cm⁻¹ in chloroform) with Beer's Law validity range 1×10⁻⁶ to 1×10⁻⁴ M.

Applications and Uses

Industrial and Commercial Applications

Quinizarine Green SS serves primarily as a colorant in cosmetic products, particularly those requiring oil-soluble green dyes. Approved as D&C Green No. 6 for externally applied cosmetics in various jurisdictions, the compound colors products including lipsticks, nail polishes, soaps, and bath oils at concentrations typically ranging from 0.01-0.1% by weight. The printing industry utilizes this dye in specialty inks for security applications and packaging where solvent resistance is required. Pyrotechnic formulations incorporate Quinizarine Green SS in colored smoke compositions at 10-25% concentration, producing green smoke upon combustion. The compound's insolubility in water makes it particularly valuable for applications requiring coloration of hydrocarbon-based systems including lubricating greases, waxes, and polishes. Industrial consumption patterns show approximately 60% of production destined for cosmetic applications, 25% for pyrotechnics, and 15% for miscellaneous industrial coloring applications.

Historical Development and Discovery

The development of Quinizarine Green SS emerged from early 20th century research on anthraquinone dyes conducted by German chemists at Bayer AG. Initial synthesis reported in patent literature circa 1925 described the condensation of quinizarin with various aromatic amines to produce green colorants for the growing synthetic dye industry. Commercial production commenced in the 1930s as manufacturers sought stable, lightfast green dyes for the automotive and cosmetic industries. Structural characterization progressed through the 1950s-1960s using emerging spectroscopic techniques, with complete X-ray crystallographic analysis confirming the molecular structure and hydrogen bonding patterns in 1978. Regulatory approval for cosmetic use followed extensive toxicological testing in the 1970s, resulting in inclusion in the D&C Green category. Manufacturing processes have evolved from batch operations to continuous flow systems with improved yield and reduced environmental impact.

Conclusion

Quinizarine Green SS represents a structurally well-characterized anthraquinone dye with significant industrial applications primarily in cosmetic coloration and specialized pyrotechnics. The compound's molecular architecture features a planar anthraquinone system substituted with electron-donating p-toluidine groups, creating an extended π-conjugated system responsible for its intense green coloration and distinctive spectroscopic properties. Its stability in non-polar media and resistance to fading under normal storage conditions make it particularly valuable for applications requiring long-term color fastness. Current manufacturing processes achieve high purity material suitable for sensitive applications including cosmetic products. Future research directions may explore modifications to the molecular structure to enhance solubility characteristics or develop analogues with improved environmental profile while maintaining the desirable coloration properties that have established this compound as an important industrial colorant.