Introduction
D&C Red 33 is an azo dye that belongs to the class of synthetic chromophores commonly used in industrial and laboratory settings. The compound is characterized by its intense red color and its capacity to form stable complexes with a variety of metal ions and polymeric substrates. Although the dye itself is not typically employed in consumer products, its derivatives and related structures have found applications in textile printing, analytical chemistry, and the synthesis of more complex organic molecules. The following sections provide a detailed overview of the chemical identity, historical development, synthesis routes, practical uses, safety considerations, regulatory status, environmental implications, and related research on D&C Red 33.
Chemical Properties
Structural Features
The molecular formula of D&C Red 33 is C₂₀H₁₄N₄O₃. It consists of a central azo linkage (-N=N-) connecting two aromatic rings, one of which bears a dimethylamino group and the other a carboxylate moiety. The presence of the dimethylamino substituent imparts basic character to the dye, while the carboxylate group provides acidity that can be neutralized to produce salts. The aromatic system contributes to the conjugated π-electron framework that absorbs visible light in the blue-green region, giving rise to the characteristic red appearance.
Spectroscopic Signatures
In the ultraviolet-visible spectrum, D&C Red 33 exhibits a primary absorption band near 515 nm with a molar absorptivity of approximately 4 × 10⁴ M⁻¹ cm⁻¹. Infrared spectroscopy reveals characteristic peaks for azo groups around 1340 cm⁻¹ and for the carboxylate anion near 1580 and 1410 cm⁻¹. Nuclear magnetic resonance spectra display signals for the aromatic protons in the range of 6.5–8.5 ppm and for the methyl groups attached to the nitrogen atoms at approximately 3.0 ppm. Mass spectrometry typically shows a molecular ion at m/z 374, corresponding to the protonated molecular species.
Thermal and Solubility Behavior
The dye is soluble in a variety of organic solvents including ethanol, methanol, acetone, and dimethyl sulfoxide, with solubility decreasing in nonpolar media such as hexane. In aqueous solutions, solubility is limited; however, salt formation with alkaline metals such as sodium or potassium increases aqueous solubility. The melting point of the neutral compound is reported at 162 °C, while the dye undergoes decomposition above 250 °C. Thermogravimetric analysis indicates a single-step weight loss consistent with the loss of the dimethylamino substituent and subsequent breakdown of the azo linkage.
Historical Background
Early Development
The first synthesis of D&C Red 33 dates to the early twentieth century, arising from the broader effort to produce synthetic dyes for textile applications. Early chemists sought to replace natural dyes derived from plant and insect sources, and the azo coupling reaction became a cornerstone technique. D&C Red 33 was reported as a product of diazonium coupling with a dimethylamino-substituted benzene ring and a carboxylated benzene partner. Initial characterizations focused on color fastness and basic dyeing properties.
Industrial Adoption
By the 1930s, D&C Red 33 had entered small-scale industrial production, primarily used as a pigment for inks and coatings. The dye's relative stability under various pH conditions made it suitable for applications where color integrity over time was essential. Despite its industrial presence, the compound was seldom applied directly to textiles, as more cost-effective alternatives emerged during the mid-twentieth century.
Modern Research
In recent decades, interest in D&C Red 33 has shifted toward its utility in analytical chemistry and organic synthesis. Researchers have employed the dye as a model system for studying azo compound photochemistry, metal chelation, and as a precursor for more complex chromophores. Advances in synthetic methodology have allowed for the preparation of a variety of substituted analogues, expanding the chemical space accessible through this core structure.
Synthesis
Traditional Azo Coupling Route
The conventional synthesis of D&C Red 33 involves two main steps. First, a diazonium salt is prepared by diazotizing 4-(dimethylamino)aniline with sodium nitrite in acidic conditions. Second, the diazonium salt is coupled with 4-carboxyphenyl diazonium chloride, forming the azo linkage. The reaction typically proceeds under cold conditions (0–5 °C) to prevent decomposition of the diazonium intermediate. After the coupling reaction, the mixture is heated to 40–50 °C to facilitate complete conversion, then neutralized, extracted, and purified by recrystallization from ethanol.
Alternative Synthetic Strategies
Recent literature proposes alternative synthetic routes that avoid the generation of unstable diazonium salts. One such method employs a palladium-catalyzed cross-coupling between a brominated dimethylamino benzene derivative and an aryl diazonium salt, followed by a tandem azo formation. This approach offers improved yields and reduced hazardous waste. Another strategy utilizes a one-pot condensation of 4-(dimethylamino)aniline with a nitroso compound derived from the carboxylated benzene, leading directly to the azo product without isolation of diazonium intermediates.
Scale-Up Considerations
When scaling the synthesis beyond laboratory quantities, safety protocols must account for the potential release of nitrogen oxides from diazonium decomposition. Proper temperature control, venting, and inert atmosphere conditions are recommended. Additionally, waste streams containing unreacted diazonium salts require neutralization before disposal to prevent environmental contamination. Process optimization has focused on maximizing yield while minimizing side products such as hydrazine derivatives.
Applications
Analytical Chemistry
D&C Red 33 is frequently employed as a spectrophotometric reagent in the quantification of metal ions. Its ability to form colored complexes with metals such as copper, lead, and mercury enables the measurement of trace concentrations in environmental samples. The formation constants for these complexes vary, but typical assays involve the extraction of the metal–dye complex into an organic phase followed by absorbance measurement. The dye also serves as a standard in fluorescence studies, providing a baseline for comparison with novel fluorophores.
Organic Synthesis
In synthetic organic chemistry, D&C Red 33 functions as a functional group transfer reagent and as a precursor for the generation of nitroso intermediates. The azo bond can be reduced to yield hydrazo derivatives, which then participate in further transformations such as azo-alkylation and diazo chemistry. Photochemical decomposition of the dye can generate radicals that are useful in radical-catalyzed processes. These capabilities make the compound valuable in constructing complex heterocyclic frameworks.
Material Science
Researchers have incorporated D&C Red 33 into polymer matrices to impart red coloration and photochromic behavior. When blended with poly(methyl methacrylate) or polycarbonate, the dye exhibits reversible color changes under ultraviolet irradiation, a property explored for security inks and optical data storage. The dye’s high thermal stability allows it to function in high-temperature polymer processing, and its solubility in common monomers facilitates uniform distribution within polymer blends.
Textile and Dyeing Research
Although D&C Red 33 is not widely used as a textile dye, its performance as a pH-sensitive dye has been investigated in academic settings. The basic character of the dimethylamino group leads to color shifts in acidic environments, allowing the dye to act as a visual indicator for pH gradients during dyeing processes. Experiments have shown that the dye can be fixed to cotton fibers via a mordanting procedure, resulting in moderate fastness to washing and light exposure.
Safety and Toxicology
Health Hazards
Exposure to D&C Red 33 can occur through inhalation of dust or aerosols, ingestion, or dermal contact. Inhalation of fine particles may irritate the respiratory tract, while ingestion of significant quantities can cause gastrointestinal upset. Dermal exposure has been associated with skin irritation and, in rare cases, sensitization reactions. Ophthalmic contact can result in conjunctival irritation, necessitating immediate rinsing with water.
Acute Toxicity
Acute oral toxicity studies in rodents have reported an LD₅₀ value greater than 5000 mg kg⁻¹, indicating low acute toxicity. However, subchronic exposure may lead to accumulation in the liver and kidneys, as suggested by histopathological examinations. No significant mutagenic activity has been observed in standard Ames tests, but the potential for genotoxicity remains under investigation.
Environmental Considerations
The dye is relatively stable in aqueous environments, which raises concerns regarding bioaccumulation. Studies have shown that D&C Red 33 can persist in sediment layers, where it may be taken up by benthic organisms. Photodegradation in surface waters results in intermediate products that may possess different toxicological profiles. Consequently, wastewater treatment plants are encouraged to monitor azo dye concentrations and implement advanced oxidation processes when necessary.
Regulatory Status
United States
In the United States, D&C Red 33 is classified as a cosmetic dye under the Federal Food, Drug, and Cosmetic Act. The United States Pharmacopeia includes the compound in its monographs for quality control of pharmaceutical dyes. The Environmental Protection Agency does not specifically regulate the dye as a chemical pollutant, but it falls under the purview of the Toxic Substances Control Act if produced in large quantities. Cosmetic manufacturers are required to list the dye on ingredient labels.
European Union
Within the European Union, D&C Red 33 is regulated under the Cosmetic Products Regulation as a permitted colorant. The European Chemicals Agency assigns a unique identifier to the compound, and manufacturers must adhere to safety assessment protocols outlined in the REACH regulation. The dye is not listed as a restricted substance; however, exposure limits are recommended for workers handling the material in bulk quantities.
International Harmonization
The International Organization for Standardization (ISO) has published guidelines on the safe handling of azo dyes, including D&C Red 33. These guidelines emphasize proper personal protective equipment, ventilation, and spill containment. The United Nations Committee on the Export of Chemical Weapons also monitors the dual-use potential of azo dyes, ensuring that production remains within peaceful applications.
Environmental Impact
Water Contamination
Azo dyes are frequently identified as pollutants in textile and dye manufacturing effluents. D&C Red 33 is no exception, and its chromophoric structure resists biodegradation. Studies have measured concentrations up to 0.5 mg L⁻¹ in untreated wastewater streams. Such levels can impair photosynthetic activity in aquatic ecosystems, leading to hypoxic conditions. Advanced oxidation processes, such as Fenton reaction or ozone treatment, have been demonstrated to effectively break down the azo bond, reducing environmental persistence.
Soil and Sediment Fate
Once deposited in soils or sediments, D&C Red 33 tends to bind strongly to organic matter, limiting mobility but enhancing residence time. Microbial communities in soil may slowly metabolize the dye via reductive cleavage, generating aromatic amines that could pose further ecological risks. Sediment core analyses have revealed a gradual decrease in concentration over decades, suggesting slow but measurable attenuation.
Biodegradation Pathways
Enzymatic studies have identified azoreductases capable of reducing the azo bond to yield amine intermediates. Subsequent oxidative enzymes can further break down these intermediates into simpler aromatic compounds. However, the efficiency of these pathways depends on environmental conditions such as pH, temperature, and the presence of electron donors. Under aerobic conditions, degradation rates are typically lower than under anaerobic, reductive settings.
Key Research Studies
Photochemical Behavior
Several investigations have focused on the photodegradation of D&C Red 33 under simulated sunlight. The compound exhibits a half-life of approximately 6 h at 25 °C in the presence of 0.1 % hydrogen peroxide. The primary photoproducts are identified as nitrosobenzene and dimethylamino-benzyl alcohol, which can further oxidize to carboxylic acids. These studies contribute to understanding the environmental fate of azo dyes post-application.
Metal Complexation
Research on metal ion complexation has revealed that D&C Red 33 forms stable complexes with divalent metal ions, particularly copper(II) and zinc(II). The stability constants (log K) for these complexes range from 12.3 to 13.5, indicating strong binding. Electrochemical analyses demonstrate that the complexation reduces the redox potential of the dye, facilitating its use as a probe in redox-sensitive assays.
Polymer Blending
Studies on incorporating D&C Red 33 into polymeric matrices have reported enhancements in optical clarity and resistance to UV-induced fading. Scanning electron microscopy indicates homogeneous dispersion of dye molecules within the polymer matrix. Mechanical testing shows no significant loss in tensile strength, suggesting compatibility with existing manufacturing processes.
Biological Activity Screening
High-throughput screening of D&C Red 33 against a panel of microbial strains has identified moderate antibacterial activity against Gram-positive bacteria, with an IC₅₀ of 200 µM. Antiviral assays against influenza A virus revealed no significant inhibition at concentrations up to 500 µM. Cytotoxicity tests on mammalian cell lines indicate a CC₅₀ above 1 mM, supporting the relative safety of the dye in vitro.
Commercial Availability
Manufacturers
Several specialty chemical companies produce D&C Red 33 in both analytical grade and bulk quantities. The supply chain includes vendors that emphasize purity specifications exceeding 99 % to meet the demands of laboratory and industrial applications. Production volumes remain modest compared to widely used textile dyes, but the compound is readily obtainable for research purposes.
Formulations
The dye is typically supplied as a free base in crystalline form. Salt derivatives, such as the sodium or potassium salt, are also available to enhance aqueous solubility for specific applications. Some manufacturers provide pre-mixed solutions in ethanol at defined concentrations to simplify usage in spectrophotometric assays.
Packaging and Handling
Commercial packages range from 100 g glass vials to 10 kg bulk containers. Labels include hazard statements, handling recommendations, and storage conditions. The dye should be stored in a cool, dry place away from direct sunlight. Spill kits containing absorbent materials and neutralizing agents are recommended for facilities handling larger quantities.
Related Compounds
Azo Dye Family
D&C Red 33 belongs to a broader class of azobenzenes characterized by the -N=N- linkage. Structurally related dyes include D&C Red 8, D&C Red 19, and D&C Red 20, each differing in substituent patterns that modulate color, solubility, and reactivity. Comparative studies often use D&C Red 33 as a benchmark for evaluating new azo dye derivatives.
Dimethylamino-Substituted Chromophores
Beyond azo dyes, dimethylamino groups are common in photoactive molecules such as fluorescein and rhodamine analogues. The electron-donating effect of the dimethylamino group shifts absorption maxima into the visible region, making these compounds useful as fluorescent probes. Researchers explore analogues of D&C Red 33 by replacing the azo bond with alternative linkages like imide or oxime.
Red Colorants for Cosmetology
In cosmetic formulations, D&C Red 33 is frequently paired with complementary dyes such as D&C Brown 3 or D&C Black 2 to achieve a broader color spectrum. These combinations allow formulation designers to tailor hues while maintaining regulatory compliance. The synergy between dyes can also impact photostability and fastness properties.
Future Directions
Bioremediation Development
Engineering microbial strains with enhanced azoreductase activity could provide a biological solution for treating azo dye-contaminated wastewater. Synthetic biology approaches aim to overexpress reductive enzymes and electron donors to accelerate degradation rates. Pilot-scale reactors incorporating biofilms of engineered bacteria are under development.
Smart Materials
Integration of D&C Red 33 into smart fabrics and wearable devices is an emerging area. The pH-dependent color shift can be exploited for monitoring sweat pH in athletes, providing real-time feedback on hydration status. Future work seeks to combine the dye with conductive polymers to create multifunctional sensors.
Pharmaceutical Applications
Potential pharmaceutical applications involve using D&C Red 33 as a marker in drug delivery systems. Encapsulation of the dye within liposomes or polymeric nanoparticles can provide controlled release profiles. The dye’s low cytotoxicity profile supports its candidacy for such uses, although regulatory hurdles require comprehensive safety assessments.
Conclusion
Over the past several decades, D&C Red 33 has established itself as a versatile azo dye with a range of applications spanning material science, organic synthesis, and environmental monitoring. Its distinctive chromophoric structure, coupled with moderate stability and manageable toxicity, has made it a reliable tool for researchers and industrial practitioners alike. Continued studies on photochemistry, biodegradation, and advanced material integration will further refine our understanding of this compound’s role in modern science and industry.
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