Abstract

Oil Blue 35, systematically named 1,4-bis(butylamino)anthracene-9,10-dione, represents a significant anthraquinone-based dye compound with the molecular formula C₂₂H₂₆N₂O₂ and CAS Registry Number 17354-14-2. This synthetic organic compound exhibits a characteristic blue coloration and demonstrates exceptional solubility in non-polar organic solvents including hydrocarbons, alcohols, fats, and waxes. The compound manifests a melting point range of 104-105°C and displays photostability under normal lighting conditions. Its chemical structure features an anthraquinone core with two butylamino substituents at the 1 and 4 positions, creating a conjugated π-electron system responsible for its intense blue color. Industrial applications span solvent coloring, fuel marking, ink formulation, and specialized microscopy staining techniques. The compound undergoes distinctive color changes under acidic conditions, transforming from blue to a characteristic dirty green hue when exposed to 5% hydrochloric acid solutions.

Introduction

Oil Blue 35 belongs to the anthraquinone dye class, a category of synthetic organic colorants characterized by their anthracene-9,10-dione backbone structure. As a member of the aminoanthraquinone subgroup, this compound demonstrates the characteristic stability and intense coloration typical of anthraquinone derivatives. The compound's systematic IUPAC nomenclature identifies it as 1,4-bis(butylamino)anthracene-9,10-dione, reflecting its symmetric substitution pattern. Commercial designations include Solvent Blue 35, Blue 2N, Blue B, and Oil Blue B, with the Color Index designation CI 61554 providing standardized classification within the dye industry.

The compound's development emerged from systematic investigations into anthraquinone derivatives during the early 20th century, as chemists sought stable, lightfast colorants for industrial applications. The strategic placement of amino groups at the 1 and 4 positions of the anthraquinone framework produces a bathochromic shift resulting in the deep blue coloration, while the butyl chains enhance solubility in non-aqueous media. This molecular design represents a deliberate optimization for solvent-based applications where traditional water-soluble dyes prove inadequate.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

The molecular architecture of Oil Blue 35 centers on an anthraquinone core system comprising three fused six-membered rings arranged in a linear configuration. X-ray crystallographic analysis of related aminoanthraquinones reveals a nearly planar central anthracene system with slight puckering of the terminal rings. The carbonyl groups at positions 9 and 10 adopt typical ketone geometry with bond angles of approximately 120° around the carbon atoms. The butylamino substituents at positions 1 and 4 extend outward from the planar core, adopting staggered conformations that minimize steric interactions.

Electronic structure analysis indicates significant π-electron delocalization throughout the conjugated system. The highest occupied molecular orbital (HOMO) primarily resides on the amino-substituted rings, while the lowest unoccupied molecular orbital (LUMO) shows greater density on the carbonyl-containing ring system. This electronic distribution facilitates charge-transfer transitions responsible for the compound's intense visible absorption. The HOMO-LUMO gap measures approximately 2.3 electronvolts, corresponding to absorption in the orange-red region of the visible spectrum and resulting in complementary blue coloration.

Chemical Bonding and Intermolecular Forces

Covalent bonding in Oil Blue 35 follows established patterns for conjugated organic systems. The anthraquinone framework features alternating single and double bonds with bond lengths intermediate between typical single and double bonds, indicating significant electron delocalization. Carbon-carbon bond lengths in the central ring measure approximately 1.40 angstroms, while carbonyl bond lengths measure 1.22 angstroms, consistent with polarized double bond character. The carbon-nitrogen bonds linking the butylamino groups to the anthraquinone core measure approximately 1.38 angstroms, indicating partial double bond character due to resonance with the quinoid system.

Intermolecular interactions dominate the compound's solid-state behavior and solubility characteristics. London dispersion forces between hydrocarbon portions provide the primary cohesive energy in crystalline forms. The carbonyl groups participate in dipole-dipole interactions, while the amino groups can engage in weak hydrogen bonding with suitable partners. The molecular dipole moment measures approximately 4.2 Debye, oriented along the long molecular axis from the amino groups toward the carbonyl oxygen atoms. This polarity contributes to the compound's solubility in moderately polar organic solvents while maintaining insolubility in aqueous systems.

Physical Properties

Phase Behavior and Thermodynamic Properties

Oil Blue 35 presents as a crystalline solid with a characteristic blue appearance under standard conditions. The compound exhibits polymorphism, with at least two crystalline forms identified. The stable α-form displays a melting point of 104-105°C, while a metastable β-form melts at 98-100°C. The heat of fusion for the α-polymorph measures 38.5 kilojoules per mole, with entropy of fusion of 102 joules per mole per kelvin. The density of crystalline material measures 1.25 grams per cubic centimeter at 25°C.

The compound demonstrates limited volatility at ambient temperatures, with vapor pressure below 10^-5 pascals at 25°C. Sublimation occurs at reduced pressures above 150°C, yielding crystalline deposits suitable for purification. The heat capacity of the solid phase follows typical organic compound behavior, measuring 450 joules per mole per kelvin at 25°C. Thermal decomposition commences above 280°C, with initial breakdown involving the butyl side chains followed by fragmentation of the anthraquinone core.

Spectroscopic Characteristics

Ultraviolet-visible spectroscopy reveals strong absorption maxima at 640 nanometers (molar absorptivity ε = 15,200 liters per mole per centimeter) and 590 nanometers (ε = 12,800 liters per mole per centimeter) in toluene solution, corresponding to π-π* transitions of the conjugated system. The spectrum exhibits solvatochromism, with bathochromic shifts observed in more polar solvents. Infrared spectroscopy shows characteristic carbonyl stretching vibrations at 1665 centimeters^-1 and 1678 centimeters^-1, along with N-H stretching at 3380 centimeters^-1 and aromatic C-H stretches between 3050-3100 centimeters^-1.

Proton nuclear magnetic resonance spectroscopy in deuterated chloroform displays aromatic proton signals between 7.5-8.2 parts per million, with the peri-protons adjacent to carbonyl groups appearing downfield. The butyl chain protons resonate as multiplets: methylene groups adjacent to nitrogen at 3.4 parts per million, inner methylenes at 1.5-1.7 parts per million, and terminal methyl groups at 0.9-1.0 parts per million. Carbon-13 NMR shows carbonyl carbons at 182-184 parts per million, aromatic carbons between 120-140 parts per million, and aliphatic carbons from 14-45 parts per million.

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

Oil Blue 35 demonstrates characteristic reactivity patterns of aminoanthraquinone systems. The electron-rich amino groups activate the aromatic system toward electrophilic substitution, particularly at positions ortho to the amino substituents. Reactions with strong electrophiles including nitronium ions and acyl chlorides proceed at measurable rates under controlled conditions. The second-order rate constant for nitration at 25°C measures 2.3 × 10^-3 liters per mole per second in acetic anhydride medium.

The carbonyl groups undergo typical ketone reactions, though steric hindrance from the fused ring system moderates reactivity. Reduction with sodium borohydride proceeds slowly at room temperature, requiring elevated temperatures of 60-70°C for complete conversion to the hydroxy derivatives. Oxidation reactions primarily affect the amino groups, with hydrogen peroxide converting them to nitroso derivatives under acidic conditions. The compound exhibits remarkable stability toward visible light irradiation, with quantum yield for photodegradation measuring less than 10^-5 in deaerated solutions.

Acid-Base and Redox Properties

The amino groups in Oil Blue 35 exhibit basic character with pK_a values of approximately 3.2 and 3.8 for protonation in aqueous ethanol solutions. The difference in basicity between the two amino groups arises from electronic effects following protonation of the first amino group. Protonation occurs preferentially at the amino group para to a carbonyl, resulting in a bathochromic shift that produces the characteristic dirty green color observed in acidic media. The color change is reversible upon neutralization, demonstrating the compound's utility as a pH indicator in non-aqueous systems.

Redox behavior involves both the carbonyl groups and the aromatic system. Cyclic voltammetry in acetonitrile reveals two reduction waves at -0.85 volts and -1.25 volts versus the saturated calomel electrode, corresponding to sequential one-electron reductions of the carbonyl groups. An oxidation wave appears at +0.95 volts, attributed to oxidation of the amino groups. The compound demonstrates electrochemical reversibility for the first reduction step, with diffusion coefficient measuring 6.7 × 10^-6 square centimeters per second in dimethylformamide.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

The most efficient laboratory synthesis of Oil Blue 35 involves nucleophilic substitution of 1,4-dichloroanthraquinone with butylamine. The reaction proceeds in refluxing ethanol or toluene with potassium carbonate as base, typically achieving yields of 85-90% after 6-8 hours at 80°C. The mechanism follows a standard aromatic nucleophilic substitution pathway, facilitated by the electron-withdrawing carbonyl groups ortho to the chlorine substituents. Reaction monitoring by thin-layer chromatography shows complete consumption of starting material within 4 hours under optimized conditions.

Purification typically involves recrystallization from ethanol or toluene, yielding analytically pure material with melting point consistent with literature values. Alternative synthetic routes include the reaction of 1,4-dihydroxyanthraquinone with butylamine in the presence of catalytic boric acid at elevated temperatures (150-160°C), though this method produces lower yields of 65-70% due to competing decomposition pathways. The leuco form of the dye, generated by reduction with sodium dithionite, can be oxidized to regenerate the quinoid structure, providing an alternative purification strategy.

Industrial Production Methods

Industrial production scales the laboratory synthesis using continuous flow reactors for improved efficiency and consistency. The process typically employs toluene as solvent with excess butylamine (2.5-3.0 molar equivalents) and potassium carbonate base at 90-100°C under pressure (3-4 atmospheres). Reaction times reduce to 2-3 hours with complete conversion, followed by distillation to recover excess amine and solvent. The crude product undergoes purification through acid-base treatment to remove inorganic salts and unreacted starting materials.

Final product specification requires minimum purity of 98% by high-performance liquid chromatography, with specific limits on residual solvents and inorganic impurities. Production economics favor the dichloroanthraquinone route despite higher raw material costs due to superior yields and reduced waste streams. Global production estimates approximate 200-300 metric tons annually, with major manufacturing facilities located in Europe and Asia. Environmental considerations focus on solvent recovery and amine recycling, with modern facilities achieving greater than 95% recovery rates for both materials.

Analytical Methods and Characterization

Identification and Quantification

Standard identification employs thin-layer chromatography on silica gel with toluene:ethyl acetate (4:1) mobile phase, exhibiting R_f value of 0.45 under standardized conditions. High-performance liquid chromatography utilizing C18 reverse-phase columns with methanol:water (85:15) mobile phase provides retention times of 6.8-7.2 minutes at flow rates of 1.0 milliliter per minute. Ultraviolet-visible detection at 640 nanometers offers detection limits of 0.1 micrograms per milliliter and quantification limits of 0.5 micrograms per milliliter.

Spectroscopic confirmation requires matching of infrared spectra with characteristic carbonyl and amine absorptions, complemented by proton NMR showing the distinctive pattern of aromatic protons and butyl chain signals. Mass spectrometric analysis via electron impact ionization shows molecular ion at m/z 350 with characteristic fragmentation pattern including loss of butyl groups (m/z 279 and 264) and sequential loss of carbonyl groups. These analytical techniques provide unambiguous identification and permit differentiation from structurally similar anthraquinone dyes.

Applications and Uses

Industrial and Commercial Applications

Oil Blue 35 serves primarily as a colorant for non-aqueous systems across multiple industries. In the petroleum sector, it functions as a marker dye for hydrocarbon fuels, particularly heating oils and industrial solvents, at concentrations of 10-50 parts per million. The plastics industry employs the compound for coloring polystyrene, polycarbonate, and acrylic resins, with typical incorporation rates of 0.01-0.05% by weight. Printing ink formulations utilize the dye for specialty applications requiring solubility in organic solvents and resistance to fading.

The compound finds significant application in metalworking fluids and industrial lubricants, where it provides coloration for identification purposes without affecting performance properties. Wax and polish manufacturers incorporate the dye at concentrations of 0.1-0.5% to produce blue-colored products. The excellent solvatochromic properties enable use as an indicator dye in non-aqueous titration systems, particularly for acid-base determinations in organic solvents. These diverse applications leverage the compound's exceptional stability, intense coloration, and compatibility with hydrophobic media.

Historical Development and Discovery

The development of Oil Blue 35 emerged from systematic research on anthraquinone derivatives during the early twentieth century. German chemists at Bayer AG first reported aminoanthraquinone dyes around 1920, seeking alternatives to triphenylmethane dyes that exhibited superior lightfastness. The specific 1,4-disubstituted pattern was recognized by the 1930s as particularly effective for producing blue shades with good solubility characteristics.

Commercial production commenced in the 1950s as demand grew for solvent-soluble dyes in expanding petroleum and plastics industries. The compound received its Color Index designation in 1956, standardizing its identification across industrial applications. Manufacturing processes evolved from batch operations to continuous processes during the 1970s, improving consistency and reducing production costs. Recent decades have seen refinement of purification methods and increased attention to environmental aspects of production, though the fundamental chemistry remains unchanged since its initial development.

Conclusion

Oil Blue 35 represents a well-characterized anthraquinone derivative with specific structural features that confer valuable properties for industrial applications. The symmetric 1,4-disubstitution pattern with butylamino groups produces intense blue coloration combined with excellent solubility in non-polar media. The compound demonstrates remarkable stability under various conditions, reversible acid-base chromism, and well-understood spectroscopic characteristics. These properties ensure continued utility across diverse applications including fuel marking, polymer coloration, and specialty ink formulations. Future research directions may explore modified derivatives with enhanced environmental compatibility or tailored solubility characteristics while maintaining the fundamental anthraquinone architecture that provides the essential coloristic properties.