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Indophenol
Indophenol
Indophenol
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Indophenol
Indophenol molecule
Names
IUPAC name
4-(4-hydroxyphenyl)iminocyclohexa-2,5-dien-1-one
Other names
Benzenoneindophenol, phenolindophenol[1]
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.007.194 Edit this at Wikidata
UNII
  • InChI=1S/C12H9NO2/c14-11-5-1-9(2-6-11)13-10-3-7-12(15)8-4-10/h1-8,14H checkY
    Key: RSAZYXZUJROYKR-UHFFFAOYSA-N checkY
  • InChI=1/C12H9NO2/c14-11-5-1-9(2-6-11)13-10-3-7-12(15)8-4-10/h1-8,14H
    Key: RSAZYXZUJROYKR-UHFFFAOYAS
  • O=C/2/C=C\C(=N\c1ccc(O)cc1)\C=C\2
Properties
C12H9NO2
Molar mass 199.209 g·mol−1
Appearance Reddish-blue powder[1]
Melting point above 300 °C[1]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
checkY verify (what is checkY☒N ?)

Indophenol is an organic compound with the formula OC6H4NC6H4OH. It is deep blue dye that is the product of the Berthelot's reaction, a common test for ammonia.[2] The indophenol group, with various substituents in place of OH and various ring substitutions, is found in many dyes used in hair coloring and textiles.[3]

Indophenol is used in hair dyes, lubricants, redox materials, liquid crystal displays, fuel cells and chemical-mechanical polishing. It is an environmental pollutant and is toxic to fish.[1][4]

Berthelot test

[edit]

In the Berthelot test (1859), a sample suspected of containing ammonia is treated with sodium hypochlorite and phenol. The formation of indophenol is used to determine ammonia and paracetamol by spectrophotometry.[5] Other phenols can be used. Dichlorophenol-indophenol (DCPIP), a form of indophenol, is often used to determine the presence of vitamin C (ascorbic acid).[6]

[edit]

Indophenol blue is a different compound with systematic name N-(p-dimethylaminophenyl)-1,4-naphthoquinoneimine.[7]

Indophenol blue

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Indophenol

View on Grokipedia
from Grokipedia
Indophenol is a synthetic classified as a quinone , with the molecular formula C₁₂H₉NO₂ and systematic name 4-[(4-hydroxyphenyl)imino]cyclohexa-2,5-dien-1-one. This deep blue dye serves as the parent structure for the indophenol family of dyes, which are characterized by their intense color and properties. It is primarily recognized for its role in the Berthelot reaction, a colorimetric where reacts with phenol under alkaline oxidizing conditions to form indophenol, enabling sensitive detection of ions in , , biological fluids, and other samples at concentrations as low as parts per billion. The compound exhibits a reddish-blue hue in solid form, with a exceeding 300°C and a of approximately 1.18 g/cm³, reflecting its stability and crystalline nature. Indophenol can also be synthesized through the oxidation of p-aminophenol or related precursors, though the Berthelot method remains the most common route in analytical contexts. Beyond ammonia quantification, it finds applications in detecting via and as a in biochemical assays. Indophenol dyes, including derivatives like indophenol blue, have historical roots in the mid-19th century and are employed in hair dyes, photographic films, displays, and as indicators due to their color changes upon reduction from blue (oxidized) to colorless (reduced) forms. In biological research, related chlorinated variants such as are used to probe chains in chloroplasts and mitochondria, highlighting the compound's versatility in studies. Safety considerations include its classification as a potential irritant, necessitating proper handling in laboratory settings.

Chemical Structure and Properties

Molecular Structure

Indophenol possesses the general molecular formula \ceOC6H4NC6H4OH\ce{OC6H4NC6H4OH}, corresponding to \ceC12H9NO2\ce{C12H9NO2}, and features a para-quinone imine architecture formed by the linkage of a benzoquinone moiety to a phenolate group via an imine bond. The core structure includes a cyclohexadienedione ring (benzoquinone) where one carbonyl is replaced by the imine functionality \ce(C=N)\ce{(C=N)}, connecting to a benzene ring bearing a phenolic hydroxyl group at the para position relative to the nitrogen atom. This arrangement establishes an extended conjugated π\pi-electron system spanning the , , and phenyl rings, which absorbs visible light and imparts the characteristic deep blue coloration to the compound. The phenolic hydroxyl group further enhances the chromophoric properties by participating in the delocalization of electrons within the conjugated framework. Derivatives exhibit variations in substitution, such as indophenol blue (\ceC18H16N2O\ce{C18H16N2O}), which incorporates a imine core coupled at the imine to a 4-(dimethylamino)phenyl derived from N,N-dimethylaniline.

Physical Properties

Indophenol typically appears as a reddish-blue in its form. In solution, it manifests as a deep blue dye, exhibiting metachromatic properties that cause color shifts in different solvents—for instance, appearing red in non-polar solvents like and blue in polar environments. The compound is poorly soluble in but dissolves readily in organic solvents such as . Its molecular weight for the parent indophenol (C12H9NO2) is 199.21 g/mol. Spectroscopically, the oxidized form displays maximal UV-Vis absorption around 630 nm, enabling its use in quantitative assays. Indophenol has a above 300 °C and generally decomposes before melting; it is sensitive to exposure during and to reducing agents. The blue coloration in solutions stems from its extended conjugated structure.

Chemical Properties

Indophenol exhibits significant activity, primarily functioning as an in its oxidized form, which appears blue due to the conjugated structure. This oxidized form can be reduced to the colorless leuco-indophenol, a process that involves the addition of two electrons and two protons, as represented by the equation: Oxidized indophenol+2e−+2H+→leuco-indophenol\text{Oxidized indophenol} + 2e^- + 2H^+ \rightarrow \text{leuco-indophenol} The core of indophenol enables reversible one-electron transfers, allowing stepwise reduction through semiquinone intermediates before full conversion to the leuco form. This behavior makes indophenol suitable for brief applications in assays, where color changes signal events. Regarding stability, indophenol is prone to in alkaline conditions, where ions catalyze the of the bond through to the imine carbon, followed by cleavage of the intermediate carbinolamine. This reaction limits the longevity of indophenol solutions in basic media, with reactivity influenced by substituents on the aromatic rings. In contrast, the compound shows greater resilience in neutral or acidic environments, though overall stability depends on the specific . Indophenol demonstrates pH sensitivity, with its color shifting from red in neutral aqueous solutions to blue in alkaline media, a change attributed to of phenolic groups and alterations in the . This property allows indophenol to function as a over certain ranges, typically around 8–10, where the transition provides visual detection of acidity or variations. The pH-dependent color arises from the states affecting the extended conjugation in the system.

Synthesis

Berthelot Reaction

The Berthelot reaction, first described by French chemist in 1859, represents a foundational oxidative synthesis route for indophenol, involving the condensation of with phenol under alkaline oxidizing conditions. Originally devised as a qualitative test for detection, this method produces indophenol as a deeply colored blue product, marking one of the earliest applications of oxidative coupling in . The primary reactants are (NH₃), phenol (C₆H₅OH), and (NaOCl) as the oxidant, with the reaction proceeding in an alkaline medium to promote and electrophilic activation; alternatively, can substitute for NaOCl to generate the active chlorinating species . The mechanism unfolds in three sequential steps, as elucidated through kinetic and spectrophotometric analyses. Initially, hypochlorite (OCl⁻) reacts with at basic to form (NH₂Cl), a key electrophilic intermediate. This then chlorinates the ortho or para position of phenoxide , yielding a transient species such as p-benzoquinone monochlorimide via nucleophilic attack and rearrangement. Finally, the monochlorimide couples with a second phenoxide through to form the indophenol structure, characterized by its quinonoid-imine linkage. This pathway ensures selective formation of the para-substituted indophenol isomer under controlled conditions. Optimal reaction conditions include (20–25°C) and a of 10–12, achieved via addition of or carbonate buffers, to maximize intermediate stability and minimize side reactions. (Na₂[Fe(CN)₅NO]) serves as an effective catalyst, typically at micromolar concentrations, by facilitating and accelerating color development to completion within 5–15 minutes, a significant improvement over uncatalyzed variants. The overall process can be summarized by the simplified equation: \ceNH3+2C6H5OH+NaOCl−>[pH10−12,Na2[Fe(CN)5NO]]C6H4(=O)−NH−C6H4−OH+NaCl+H2O\ce{NH3 + 2 C6H5OH + NaOCl ->[pH 10-12, Na2[Fe(CN)5NO]] C6H4(=O)-NH-C6H4-OH + NaCl + H2O} where the product is the indophenol . Yield and purity in the Berthelot reaction are influenced by reagent stoichiometry, with equimolar ratios of to phenol and slight excess of (1.2–1.5 equivalents) favoring near-quantitative conversion (>90%) for concentrations up to 1 mM, though higher levels may promote polychlorination byproducts. The intensity of the resulting coloration, arising from the extended conjugation in indophenol, is linearly proportional to input, with the exhibiting a high molar extinction coefficient (approximately 5 × 10⁴ L mol⁻¹ cm⁻¹ at 630 nm), enabling sensitive detection; impurities like or excess oxidant can quench the color, necessitating purification steps such as extraction for isolated synthesis.

Oxidative Coupling Methods

Oxidative coupling methods represent a versatile class of synthetic routes for indophenols, involving the direct reaction between and s (or their derivatives) under oxidizing conditions to form the characteristic imine structure. These approaches typically proceed via the formation of radical intermediates from the aniline and phenol components, followed by and subsequent oxidation, bypassing the ammonia-mediated pathway of the Berthelot reaction. Common oxidants include molecular oxygen, persulfates, or s, enabling the preparation of both unsubstituted and substituted indophenols with good selectivity. A specific variant utilizes as the oxidant in the reaction with p-aminophenol, where the acts both as an and coupling partner to yield indophenol through nucleophilic addition and tautomerization. This method is particularly effective in aqueous or alcoholic media at ambient temperatures, often requiring no additional catalysts. Alternatively, electrochemical oxidation facilitates the by applying an anodic potential to generate the necessary radicals from phenol and mixtures, typically in solvents like hexafluoroisopropanol with conductive salts, at temperatures around 50°C and current densities of 1–50 mA/cm². These reactions are generally conducted in acidic, neutral, or alkaline media, with temperatures ranging from 20–80°C; catalysts such as salts can enhance rates in neutral conditions by promoting radical formation. For instance, in the presence of serves as a robust oxidant for coupling substituted (e.g., 2,4-dichloro-3-methyl-6-acetamidophenol) with derivatives (e.g., 4-N,N-diethyl-2-aminotoluene hydrochloride) in methanol-water mixtures at 0–, yielding indophenol derivatives as red solids after recrystallization. A representative example is the synthesis of indophenol blue, achieved by oxidative coupling of N,N-dimethyl-p-phenylenediamine with α-naphthol using chemical oxidants like or enzymatic systems, though non-enzymatic variants employ air or in alkaline media at to produce the intensely blue product. The general reaction can be represented as: C6H5NH2+HO−C6H4−OH+[O]→OC6H4=NC6H4OH\mathrm{C_6H_5NH_2 + HO-C_6H_4-OH + [O] \rightarrow OC_6H_4=NC_6H_4OH} These methods offer advantages such as higher yields (often 70–90% for substituted variants) and greater flexibility for introducing substituents on the aromatic rings, without reliance on as a .

Analytical Applications

Ammonia Detection

The indophenol method utilizes the formation of indophenol via the Berthelot reaction for the qualitative and quantitative detection of in environmental samples. In this approach, a sample is mixed with alkaline phenol and , which react with to produce a characteristic indophenol ; the intensity of the blue color serves as a qualitative indicator of ammonia presence. For quantitative analysis, the indophenol blue is measured spectrophotometrically at approximately 630 nm, where absorbance follows Beer's law, allowing direct correlation to ammonia concentration over a linear range typically from 0.01 to 2.0 mg/L NH₃-N. This method achieves a detection limit of about 0.01 ppm ammonia, making it suitable for trace-level monitoring in water and air. Potential interferences from amines and other nitrogenous compounds are mitigated through sample pretreatment, such as distillation into boric acid, to isolate ammonia prior to reaction. First described by Berthelot in 1859, the indophenol method has been a standard technique for ammonia analysis in water and air since the late 19th century, evolving from manual colorimetric assays to support environmental quality assessments. In modern applications, automated flow injection analysis systems integrate the protocol for high-throughput environmental monitoring, enabling rapid, continuous determination of ammonia in natural waters with minimal sample handling.

Urea and Other Determinations

The determination of using indophenol relies on the enzymatic of to and by , followed by the reaction of the liberated with phenol and in the Berthelot reaction to form a blue indophenol dye, which is quantified colorimetrically at approximately 570-640 nm. This indirect approach leverages the sensitivity of indophenol formation to , adapting the direct for analysis. In the standard protocol, a sample such as serum, plasma, or is incubated with to generate quantitatively from ; the mixture is then treated with alkaline phenol and (or an alternative oxidant like nitroprusside-catalyzed ) to produce the indophenol complex, with measured after color development. This Berthelot adaptation with urease pre-incubation is widely used in clinical laboratories to measure (BUN) or urinary , serving as a key indicator of function and . The method's typically spans 0.5-50 mg/L , offering sufficient sensitivity for routine diagnostics without requiring advanced instrumentation. Adaptations of the indophenol method extend to other nitrogenous compounds by generating ammonia precursors. For , the analyte is first oxidized to with , then under acidic conditions to release , which undergoes the standard indophenol reaction for quantification in environmental or industrial samples. Similarly, (acetaminophen) is determined after alkaline to p-aminophenol, followed by oxidation with to form an intermediate that couples in the indophenol reaction, enabling detection in pharmaceutical or biological matrices. This indophenol-based approach for and related determinations provides a cost-effective, colorimetric alternative to more complex enzymatic kits (e.g., those using ), with high specificity from the step and minimal interference when optimized, making it suitable for resource-limited settings.

Other Uses

Dye Applications

Indophenol dyes emerged as part of the mid-19th-century synthetic revolution, marking one of the early commercial successes in the burgeoning industry that transformed textile coloring from natural to artificial sources. Initially applied to and fabrics through oxidative processes, these deep blue compounds provided vibrant hues during an era when synthetic dyes like had just paved the way for . As dyes, indophenols exhibit moderate light fastness, requiring stabilizers such as copper(II) ions to prevent rapid fading, and are typically applied via oxidation , where leuco forms are oxidized in air or chemical baths to develop the final color on fibers. This process enhances adhesion to substrates like and , though their instability limited long-term durability compared to later azo dyes. The blue coloration arises from the extended in their structure. In modern applications, indophenols remain prominent in , particularly for achieving blue tones in oxidative and semi-permanent formulations, where they couple with or naphthols during application to form stable, temporary shades that wash out gradually. These dyes are synthesized on the shaft, offering customizable intensity without permanent alteration, and are staples in commercial products for their bright, non-metallic blues. Industrially, indophenol blue variants, such as C.I. Blue 22, find use in and dye diffusion thermal transfer printing, where their solubility and color strength suit imaging processes. While direct textile use has become obsolete, they serve as intermediates in production for enhanced shade development. Environmentally, indophenol dyes pose biodegradability challenges due to their persistent aromatic structures, contributing to toxicity by increasing coloration and harming aquatic life, such as and , in effluent discharges. Advanced treatments like are explored to mitigate their release from and hair dye industries.

Biochemical and Redox Roles

Indophenol and its derivatives, particularly (DCPIP), play significant roles in biochemical assays due to their properties. In the determination of content, DCPIP serves as an that is decolorized upon reduction by ascorbic acid, allowing for the quantification of capacity through or spectrophotometric methods. This reaction proceeds in a 1:1 stoichiometric manner, where the oxidized form of DCPIP turns colorless as it accepts electrons from ascorbic acid, providing a simple and reliable measure of levels in biological samples such as fruit juices and tissues. DCPIP is also widely employed in assays for enzyme activity, notably in monitoring function. In these assays, the transfers electrons from NADH to DCPIP, reducing the dye and enabling the of rates via the decrease in at 600 nm. This approach is particularly useful for characterizing mitochondrial or bacterial NADH:ubiquinone complexes, where DCPIP acts as an artificial to assess kinetic parameters like V_max and K_M without interference from downstream respiratory chain components. As a mediator, indophenol derivatives facilitate in various biological and electrochemical systems, with DCPIP exhibiting a standard reduction potential of approximately +0.22 V versus the (SHE). In microbial fuel cells, indophenol compounds enhance by shuttling electrons from to the , improving coulombic in systems utilizing substrates like glucose or . Similarly, in studies, DCPIP is used in the Hill reaction to demonstrate light-dependent electron transport in isolated chloroplasts; illumination reduces DCPIP from blue to colorless as electrons from are captured, confirming the non-cyclic pathway.

Indophenol Derivatives

Indophenol derivatives are structural variants of the core indophenol scaffold, featuring substitutions on the phenolic or anilino rings that modify their chemical and properties. The parent unsubstituted compound, known as phenolindophenol, has the molecular C12H9NO2 and is characterized by a linkage between a and a moiety. This parent structure is less commonly used due to its poor stability, particularly in aqueous solutions where it readily decomposes. Nomenclature for these derivatives typically reflects the positions of substituents relative to the phenolic hydroxyl group and the anilino , as seen in systematic names like , which specifies ortho-chlorine atoms on the phenolic ring. A well-known example is indophenol (C18H16N2O), derived from N,N-dimethylaniline coupled with a , featuring a dimethylamino group on the anilino ring that imparts intense coloration. This derivative is employed in colorimetric assays for detection via the Berthelot reaction variant. Another key derivative is 2,6-dichlorophenolindophenol (DCPIP), with the formula C12H6Cl2NO2, where atoms at the 2 and 6 positions on the phenolic ring provide enhanced chemical stability compared to the unsubstituted parent, making it suitable for prolonged use in titrations. Substitutions with electron-withdrawing groups, such as in DCPIP or nitro groups in nitro-indophenols, influence the electronic structure, leading to shifts in the absorption spectra; for instance, spectrophotometric studies of various substituted indophenols show variations in peak wavelengths due to these effects.

Indamine and Indaniline Analogues

Indamines represent a class of nitrogen-containing organic dyes structurally analogous to indophenols, characterized by a quinonoid system featuring a central C=N linkage and lacking the characteristic phenolic oxygen atom found in indophenols. The parent indamine, often denoted as having the formula C₁₂H₁₀N₂, arises as a with a prominent C-NH connectivity, typically synthesized through the aerial oxidation of in the presence of mild oxidants. These compounds form intensely colored salts, predominantly green, which were among the earliest synthetic dyes developed in the mid-19th century. Historical commercialization of indamines occurred alongside indophenols during the late , with examples like indamine blue emerging as basic fabric valued for their vibrant hues but limited by poor . Unlike indophenols, indamines exhibit altered profiles, often requiring acidic conditions to form stable salts, and their color fastness is generally inferior, leading to fading upon exposure to light or alkali. These properties stem from the absence of the stabilizing oxygen, resulting in a more reactive prone to reduction. Indanilines, closely related variants of indamines, incorporate additional NH₂ groups, often appearing as leuco (colorless reduced) forms that can be oxidized to yield blue-violet dyes. Structurally, they differ from indamines by featuring para-amino substitutions on one aromatic ring, enhancing their reactivity in oxidative processes. In modern applications, indanilines are key components in oxidative dyeing formulations, where they form through coupling reactions involving aromatic amines and under alkaline conditions. Their in aqueous media and tunable color shades make them suitable for semi-permanent coloring, though they share the class's general sensitivity to environmental factors. Both indamines and indanilines share roots in oxidative synthesis methods pioneered in the .

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