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4-Aminophenol
4-Aminophenol
4-Aminophenol
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4-Aminophenol
Skeletal formula of 4-aminophenol
Space-filling model of the 4-aminophenol molecule
Names
Preferred IUPAC name
4-Aminophenol[1]
Other names
  • p-Aminophenol
  • para-Aminophenol
Identifiers
3D model (JSmol)
385836
ChEBI
ChEMBL
ChemSpider
ECHA InfoCard 100.004.198 Edit this at Wikidata
EC Number
  • 204-616-2
2926
KEGG
MeSH Aminophenols
UNII
UN number 2512
  • InChI=1S/C6H7NO/c7-5-1-3-6(8)4-2-5/h1-4,8H,7H2 checkY
    Key: PLIKAWJENQZMHA-UHFFFAOYSA-N checkY
  • InChI=1/C6H7NO/c7-5-1-3-6(8)4-2-5/h1-4,8H,7H2
    Key: PLIKAWJENQZMHA-UHFFFAOYAD
  • Oc1ccc(N)cc1
  • c1cc(ccc1N)O
Properties
C6H7NO
Molar mass 109.128 g·mol−1
Appearance Colorless to reddish-yellow crystals
Density 1.13 g/cm3
Melting point 187.5 °C (369.5 °F; 460.6 K)
Boiling point 284 °C (543 °F; 557 K)
1.5 g/100 mL
Solubility
log P 0.04
Acidity (pKa)
  • 5.48 (amino; H2O)
  • 10.30 (phenol; H2O)[2]
Structure
orthorhombic
Thermochemistry
−190.6 kJ/mol
Hazards
GHS labelling:
GHS07: Exclamation markGHS08: Health hazardGHS09: Environmental hazard
Warning
H302, H332, H341, H410
P201, P202, P261, P264, P270, P271, P273, P281, P301+P312, P304+P312, P304+P340, P308+P313, P312, P330, P391, P405, P501
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g. chloroformFlammability 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g. canola oilInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
2
1
0
Flash point 195 °C (383 °F; 468 K) (cc)
Lethal dose or concentration (LD, LC):
671 mg/kg
Related compounds
Related aminophenols
 2-Aminophenol
3-Aminophenol
Related compounds
 Aniline
Phenol
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 ?)

4-Aminophenol (or para-aminophenol or p-aminophenol) is an organic compound with the formula H2NC6H4OH. It is a metabolite of acetaminophen which the body converts to N-arachidonoylphenolamine and this compound is responsible for all or part of acetaminophen's analgesic action[3] and anticonvulsant effects.[4].

Commercially available as a white powder,[5] it is commonly used as a developer for black-and-white film, marketed under the name Rodinal.

Reflecting its slightly hydrophilic character, the white powder is moderately soluble in alcohols and can be recrystallized from hot water. In the presence of a base, it oxidizes readily. The methylated derivatives N-methylaminophenol and N,N-dimethylaminophenol are of commercial value.

The compound is one of three isomeric aminophenols, the other two being 2-aminophenol and 3-aminophenol.

Preparation

[edit]

4-Aminophenol can be prepared by several routes. One route is hydrogenation of 4-nitrophenol over Raney nickel.[6]

HOC6H4NO2 + 3 H2 → HOC6H4NH2 + 2 H2O

The nitrophenol can also be reduced by iron or by stannous chloride.[7][8]

4-Aminophenol can be produced by reduction of nitrobenzene via the intermediate phenylhydroxylamine, which spontaneously rearranges to 4-aminophenol.[9][6]

C6H5NHOH → HOC6H4NH2

Uses

[edit]

4-Aminophenol is a building block used in organic chemistry. Prominently, it is the final intermediate in the industrial synthesis of paracetamol. Treating 4-aminophenol with acetic anhydride gives paracetamol:[10][11][12]

It is a precursor to amodiaquine, mesalazine, AM404, parapropamol, B-86810 & B-87836 (cf. WO 2001042204 ).

4-Aminophenol converts readily to the diazonium salt.[13]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

4-Aminophenol

View on Grokipedia
from Grokipedia
4-Aminophenol, also known as p-aminophenol or 4-hydroxyaniline, is an with the molecular formula C₆H₇NO, characterized by a ring bearing an amino group and a hydroxyl group in the para position. It exists as a white to light brown crystalline solid that darkens upon exposure to air and light, and it plays a crucial role as a chemical intermediate in various industrial applications, including the synthesis of pharmaceuticals, dyes, and photographic chemicals. The compound has a molecular weight of 109.13 g/mol, a melting point of 187.5 °C, and it decomposes at approximately 284 °C under standard pressure without reaching a boiling point. It exhibits moderate solubility in water (1.5 g/100 mL at 20 °C), slight solubility in ethanol, and insolubility in non-polar solvents like benzene. Industrially, 4-aminophenol is primarily produced by the iron-acid reduction of p-nitrophenol, a process that yields the compound in high purity for downstream applications. In the pharmaceutical sector, 4-aminophenol serves as a key precursor for acetaminophen (), one of the most widely used analgesics and antipyretics globally. It is also utilized in the manufacture of dyes and pigments for textiles, plastics, inks, , furs, and feathers, where it contributes to colorfastness and vibrancy. Beyond these, the compound acts as a developing agent in black-and-white to enhance image contrast and tonal range, and it finds employment as a , , and additive in oils and polymers. Despite its utility, 4-aminophenol is highly toxic, with an acute oral LD₅₀ of 375 mg/kg in rats, and it can induce , , and renal damage through mechanisms involving and tissue respiration inhibition. It causes severe skin and eye irritation, respiratory issues, and upon exposure, and is readily absorbed through the skin. Environmentally, it is very toxic to aquatic organisms and persists as a from industrial effluents. Regulatory standards limit its presence as an impurity in pharmaceuticals to 50 ppm, reflecting its potential for maternal toxicity, mutagenicity, and teratogenicity.

Chemical Identity

Nomenclature

The for this is 4-aminophenol. Common synonyms include para-aminophenol (often abbreviated as p-aminophenol), 4-hydroxyaniline, 4-amino-1-hydroxybenzene. In early chemical literature, it was known as para-amidophenol. This compound is distinguished from its isomers, and 3-aminophenol, by the para positioning of the amino group relative to the phenolic hydroxy group.

Molecular Formula and Structure

4-Aminophenol has the C₆H₇NO and a of 109.13 g/mol. The compound features a benzene ring substituted with an amino group (-NH₂) and a hydroxyl group (-OH) in the para position, denoted structurally as H₂N-C₆H₄-OH, where the para arrangement imparts significant stabilization between the electron-donating substituents. Key structural parameters include a C-N of approximately 1.40 , an O-H of 0.96 , and a planar aromatic ring configuration, consistent with the conjugated π-system of the core. In the solid state, 4-aminophenol crystallizes in the orthorhombic system with P2₁2₁2₁, featuring dimensions of a = 7.25 , b = 9.13 , and c = 11.47 , which supports intermolecular hydrogen bonding networks involving the -NH₂ and -OH groups. Although capable of tautomerism to zwitterionic and keto-imine forms via proton transfer from the hydroxyl to the amino group, the neutral phenolic form (H₂N-C₆H₄-OH) predominates in both and the crystalline phase due to favorable aromatic stabilization.

Physical and Chemical Properties

Physical Properties

4-Aminophenol appears as white to light yellow or reddish crystals or powder, which tends to darken to gray or violet upon exposure to air and light. The compound has a density of 1.29 g/cm³ at 20°C. It melts at 188°C and decomposes at approximately 284 °C without reaching a boiling point. In terms of solubility, 4-aminophenol exhibits limited solubility in water, approximately 1.6 g/L at 20°C, but is soluble in polar organic solvents such as methanol and acetone, slightly soluble in ethanol, while remaining insoluble in nonpolar solvents like benzene and diethyl ether. As an amphoteric compound, 4-aminophenol has pKa values of 5.48 for the conjugate acid of the amino group and 10.46 for the phenolic hydroxyl group. The standard enthalpy of formation (ΔH_f°) for solid 4-aminophenol is -190.6 kJ/mol.

Chemical Properties

4-Aminophenol features an aromatic amine (-NH₂) and a phenolic hydroxyl (-OH) group attached to the benzene ring in a para position, conferring amphoteric character and enabling both nucleophilic and electrophilic substitution reactions. The amino group acts as a nucleophile, facilitating reactions such as diazotization to form 4-hydroxybenzenediazonium salts under acidic conditions with nitrous acid, a process widely used in azo dye synthesis. Similarly, the amino group undergoes selective acetylation with acetic anhydride to yield acetaminophen (paracetamol), while the hydroxyl group can also be acetylated under different conditions, though less preferentially. The phenolic hydroxyl group imparts weak acidity with a pKa of 10.46, allowing in basic media to form phenolate ions that participate in electrophilic aromatic substitutions, particularly at ortho and para positions relative to the oxygen. Conversely, the amino group provides basicity, with the pKa of its conjugate acid 5.48, enabling in acidic environments and salt formation. This amphoteric nature arises from the combined electron-donating effects of both substituents, influencing the compound's and reactivity in various pH conditions. In the solid state, 4-aminophenol molecules engage in intermolecular hydrogen bonding via N-H···O and O-H···N interactions, forming a three-dimensional network that stabilizes the crystal lattice, though specific dimer motifs contribute to this assembly. The compound exhibits sensitivity to oxidation, readily converting to p-benzoquinone in the presence of air, oxygen, or mild oxidants, leading to discoloration from white to violet or brown; this reactivity is exacerbated in alkaline conditions and contributes to its use as a . 4-Aminophenol is chemically stable under neutral conditions but decomposes in strong acids or bases, reacting violently with the latter to release toxic gases such as and upon heating. It is also slightly hygroscopic, absorbing moisture from the atmosphere due to its polar functional groups, which can affect handling and storage.

Synthesis

From Phenol

The hydroxyl group of phenol acts as a strong ortho/para-directing group in , favoring at these positions. In the industrial nitration-reduction route to 4-aminophenol, phenol undergoes to yield a primarily consisting of ortho- and para- isomers, with subsequent selective reduction targeting the para isomer. The proceeds via employing a mixed acid system of (HNO₃) and (H₂SO₄), conducted under controlled low-temperature conditions to minimize poly-nitration and enhance mono-substitution. The isomeric mixture is then separated, typically by or crystallization, to isolate for the subsequent step. Reduction of the isolated to 4-aminophenol is performed using iron powder in a weakly acidic medium, such as acetic acid or (HCl), which facilitates the transfer of electrons from iron to the nitro group while avoiding over-reduction. This classical iron-based reduction, known as the Béchamp process variant, proceeds efficiently with high yields for the para . The overall transformation is: \ceC6H5OH−>[HNO3/H2SO4]p−O2N−C6H4OH−>[Fe,AcOHorHCl]p−H2N−C6H4OH\ce{C6H5OH ->[HNO3/H2SO4] p-O2N-C6H4OH ->[Fe, AcOH or HCl] p-H2N-C6H4OH} This route, an early industrial method developed in the late , was initially applied for synthesizing intermediates.

From Nitrobenzene

The synthesis of 4-aminophenol from utilizes the Bamberger rearrangement, a process that begins with the partial reduction of to phenylhydroxylamine, followed by an acid-catalyzed rearrangement to selectively yield the para isomer. This method is particularly suited for laboratory-scale preparation due to its mechanistic para selectivity. The overall transformation can be represented as: \ceC6H5NO2−>[partialreduction]C6H5NHOH−>[acid]p−H2N−C6H4OH\ce{C6H5NO2 ->[partial reduction] C6H5NHOH ->[acid] p-H2N-C6H4OH} The partial reduction step employs selective hydrogenation techniques to generate phenylhydroxylamine without further reduction to aniline. Common laboratory methods include treatment with zinc dust in ammonium chloride solution or catalytic hydrogenation using palladium on carbon under mild conditions, such as atmospheric pressure and room temperature, to achieve high selectivity for the hydroxylamine intermediate. The subsequent rearrangement involves dissolving the phenylhydroxylamine in concentrated at low temperature (0-5°C) to promote the migration and formation of 4-aminophenol while minimizing side reactions like or over-reduction. This step is typically complete within hours, followed by neutralization and isolation of the product. Yields for the overall process range from 50-60% based on , reflecting the efficiency of traditional conditions. Mechanistically, the rearrangement proceeds via of phenylhydroxylamine, leading to and formation of a intermediate; this tautomerizes to the quinone monoxime, which undergoes further acid-catalyzed rearrangement to 4-aminophenol through migration of the hydroxyl group to the para position. The para selectivity arises from the stability of the quinoid intermediate, avoiding ortho or meta products. This approach circumvents the need for isomer separation required in alternative routes, making it advantageous for targeted synthesis. The Bamberger rearrangement was first described by German chemist Eugen Bamberger in 1894, who reported the acid-induced conversion of N-phenylhydroxylamine to 4-aminophenol in his seminal publications.

From

The synthesis of 4-aminophenol from involves the direct reduction of the nitro group to an amino group, typically employing hydrogen gas or metal-based reductants under controlled conditions to achieve high selectivity and minimize side reactions. This approach leverages the pre-isolated para-substituted starting material, avoiding complications from isomer formation seen in other routes. A primary method is catalytic , where is reduced using hydrogen gas in the presence of catalyst, often in solvents like or at temperatures of 50–60°C and moderate pressure (1–5 atm). The reaction proceeds via stepwise addition of hydrogen to the nitro group, forming intermediates like before yielding the amine. Alternative non-catalytic reductions include treatment with stannous chloride (SnCl₂) in (HCl), typically at conditions (around 100°C) in aqueous or alcoholic media, or iron powder (Fe) in HCl or acetic acid at 80–110°C. These metal reductant methods generate the corresponding metal salts as byproducts but offer simplicity for laboratory-scale preparations. The overall for the pathway is represented by the equation: O2N−C6H4OH+3H2→H2N−C6H4OH+2H2O\mathrm{O_2N-C_6H_4OH + 3H_2 \rightarrow H_2N-C_6H_4OH + 2H_2O} Yields typically exceed 90%, with the Fe/HCl process achieving up to 99% in optimized industrial setups, and purity remaining high (>95%) due to the specificity of the starting material, which limits byproduct formation to trace or azoxy compounds. This method is particularly favored for pharmaceutical-grade production, as it enables straightforward purification via and , supporting the synthesis of intermediates like without introducing metallic impurities. As a greener variation, electrochemical reduction employs nickel-iron electrodes in neutral aqueous electrolytes ( ~7) at low potentials (e.g., -0.15 V vs. RHE), using water as the proton source to deliver 85–93% conversion and selectivity to 4-aminophenol over 3–8 hours, with high current efficiency (>90%) and no sacrificial reductants. This approach reduces waste generation compared to traditional metal-based methods and aligns with sustainable goals.

Applications

Photographic Developer

4-Aminophenol, also known as p-aminophenol, serves as a key in black-and-white photographic developers, where it selectively reduces exposed crystals in emulsions to metallic silver, thereby forming the visible image. During this process, p-aminophenol undergoes a two-electron oxidation, typically proceeding through a one-electron transfer to form a semiquinone radical intermediate, followed by further oxidation to the stable quinonimide byproduct, p-benzoquinone monoimine. This mechanism ensures targeted development at sites of formation, minimizing fogging in unexposed areas, and is particularly effective in alkaline environments that activate the phenolic hydroxyl group for enhanced reducing power. In classic formulations, p-aminophenol is the primary active ingredient in Rodinal, a long-lasting liquid developer first patented in 1891 by Agfa, consisting of approximately 5-10 g/L of p-aminophenol hydrochloride dissolved in water with as an and for . It is also incorporated into combination developers, such as systems, where it synergizes with for balanced activity, or paired with (N-methyl-p-aminophenol) derivatives in modified metol- (M-HQ) formulations to achieve superadditive effects for improved contrast and shadow detail. These mixtures leverage p-aminophenols' and stability, allowing for concentrated stock solutions that dilute 1:25 to 1:100 for use. The advantages of p-aminophenol-based developers include high , which enhances edge sharpness and image definition through localized development, alongside relatively fine structure compared to more vigorous agents like , though its slow working speed necessitates longer development times. Introduced in the late , it gained prominence in the early for professional and amateur analog workflows and remains popular in niche applications for its compensating effects that preserve highlight detail in high-contrast scenes. Optimal performance occurs at 10-11, with typical development times of 9-13 minutes at 20°C for standard dilutions, depending on film type and agitation.

Pharmaceutical Intermediate

4-Aminophenol serves as a key pharmaceutical intermediate, primarily through its to form (acetaminophen), a widely used and . The synthesis involves selective N- of the amino group with in aqueous or acidic conditions, yielding paracetamol and acetic acid as a byproduct. This reaction proceeds via nucleophilic attack by the more basic amine nitrogen on the electrophilic carbonyl of acetic anhydride, minimizing O-acylation at the phenolic hydroxyl, which would produce the isomeric 4-acetoxyacetanilide. The balanced equation for the reaction is: H2N−C6H4−OH+(CH3CO)2O→CH3CONH−C6H4−OH+CH3COOH\mathrm{H_2N-C_6H_4-OH + (CH_3CO)_2O \rightarrow CH_3CONH-C_6H_4-OH + CH_3COOH} Global production of paracetamol exceeds 275,000 metric tons annually, underscoring the industrial scale of this transformation. Beyond paracetamol, 4-aminophenol acts as a precursor in the synthesis of other therapeutics, including the antimalarial amodiaquine via coupling with 4,7-dichloroquinoline derivatives followed by Mannich reaction incorporation of diethylaminomethyl groups. 4-Aminophenol is a common regulated impurity in mesalazine, an anti-inflammatory agent for inflammatory bowel disease, arising from synthesis processes. Additionally, 4-aminophenol is conjugated with arachidonic acid by fatty acid amide hydrolase (FAAH) to generate AM404, an endogenous analgesic metabolite that modulates endocannabinoid and TRPV1 signaling pathways. Due to its potential toxicity, including and renal damage, 4-aminophenol is strictly controlled as an impurity in formulations. Regulatory limits specify no more than 50 ppm of 4-aminophenol in the active pharmaceutical ingredient, aligned with ICH Q3A(R2) guidelines for residual impurities and specifications to ensure product safety.

Other Industrial Uses

4-Aminophenol is widely utilized as an intermediate in the synthesis of azo dyes, where it undergoes diazotization to form the 4-hydroxybenzenediazonium , followed by with suitable aromatic compounds to produce colored for textiles, fur, and other materials. The diazotization process can be represented as: H2N−C6H4−OH+NaNO2/HCl→+N2−C6H4−OH\mathrm{H_2N-C_6H_4-OH + NaNO_2 / HCl \rightarrow ^+N_2-C_6H_4-OH} This reaction enables the creation of stable azo linkages essential for dye coloration. In formulations, 4-aminophenol functions as an oxidative dye precursor, though its use is regulated in the to a maximum concentration of 0.90% (as ) in the ready-for-use product after mixing under oxidative conditions. Global annual production of 4-aminophenol exceeds 250,000 metric tons, with over 60% occurring in , where a significant portion supports the dyes sector. In polymer production, 4-aminophenol serves as a key building block for high-functionality resins, contributing to enhanced thermal and mechanical properties through its amino and hydroxyl groups reacting with functionalities. Derivatives of 4-aminophenol are also employed in the synthesis of specialized polyamides, such as those derived from bis(4-aminophenoxy) structures, for applications requiring high thermal stability. Additionally, it acts as an in rubber compounding, stabilizing polymers against oxidative degradation during processing and use. Beyond these, 4-aminophenol finds application as a corrosion inhibitor in various industrial processes, including metalworking fluids, where it protects steel surfaces in acidic environments.

Safety, Toxicity, and Regulations

Health and Toxicity

4-Aminophenol poses significant health risks primarily through its ability to induce oxidative stress in biological systems, leading to hematological and renal damage. Acute exposure via ingestion or inhalation is classified as harmful under the Globally Harmonized System of Classification and Labelling of Chemicals (GHS), with hazard statements H302 (harmful if swallowed) and H332 (harmful if inhaled). Skin absorption is also a concern, as the compound can penetrate intact or abraded skin, resulting in systemic effects such as cyanosis due to methemoglobinemia. The oral LD50 in rats is 671 mg/kg body weight (95% confidence interval: 550–818 mg/kg), indicating moderate acute toxicity, with observed clinical signs including muscle weakness and behavioral changes. In addition to methemoglobinemia, acute exposure can cause Heinz body formation in erythrocytes, leading to hemolytic anemia characterized by oxidative damage to red blood cells. Chronic or repeated exposure to 4-aminophenol is associated with mutagenic potential, classified under GHS as H341 (suspected of causing genetic defects), based on positive results in chromosome aberration assays and in mouse tests, though these effects appear to occur only at cytotoxic doses with a threshold mechanism. No carcinogenic effects were observed in long-term oral studies in rats at doses up to 30 mg/kg/day, indicating no potential for tumor induction under tested conditions. is a prominent chronic effect, primarily affecting the proximal tubules, mediated by bioactivation to reactive metabolites and subsequent conjugation, which generates toxic conjugates that accumulate in renal tissue and cause . These metabolites arise from oxidation and prostaglandin endoperoxide synthase activity in the . The primary treatment for methemoglobinemia induced by 4-aminophenol is intravenous administration of 1% to reduce back to , with supportive care including . However, should be used cautiously or avoided in cases involving (acetaminophen) overdose, as 4-aminophenol is a minor of , and the treatment may exacerbate in glucose-6-phosphate dehydrogenase-deficient individuals or complicate management. No specific occupational exposure limits, such as a (TLV), have been established for 4-aminophenol by major agencies like ACGIH or OSHA, though exposure should be minimized using and . As an impurity in pharmaceuticals like , 4-aminophenol may contribute to unintended exposure risks in therapeutic contexts.

Environmental Impact and Regulations

4-Aminophenol exhibits significant ecotoxicity in aquatic environments, with acute toxicity to fish evidenced by an LC50 value of 1.2 mg/L for rainbow trout (Oncorhynchus mykiss) over 96 hours. It also inhibits algal growth, showing an ErC50 of 0.25 mg/L for Pseudokirchneriella subcapitata after 72 hours in static tests. This compound is classified under the Globally Harmonized System (GHS) as an aquatic hazard (H410), indicating very toxic effects to aquatic life with long-lasting consequences. Regarding persistence, 4-aminophenol is moderately biodegradable under aerobic conditions, with studies demonstrating 85-100% degradation of 10 mg/L concentrations in river and samples. Its (log Kow) ranges from -0.09 at 7.5 to 0.04 at 7.4, suggesting low potential for in organisms, as supported by a factor (BCF) of 15-46 and an estimated log Koc of 1.96 indicating minimal to . Environmental release of 4-aminophenol primarily occurs through from pharmaceutical and manufacturing processes, where it appears as a degradation product of and other precursors. It has been detected in urban effluents and surface waters, including rivers near industrial sites, contributing to phenolic contamination in aquatic systems. Regulatory frameworks address 4-aminophenol's environmental risks globally. In the , it is restricted under REACH Annex III (entry 84) for use in , particularly hair dyes, with maximum concentrations of 0.90% in oxidative products and specific labeling requirements. It is listed as an active substance on the Toxic Substances Control Act (TSCA) inventory, subjecting it to reporting and control measures for industrial handling and emissions. The (WHO), through pharmacopoeial specifications for , limits 4-aminophenol as an impurity to less than 0.005% (50 ppm) in drug substances to minimize environmental release via pharmaceutical waste. Mitigation strategies for 4-aminophenol in include advanced treatments such as adsorption onto , which effectively removes it from phenolic effluents, and ozonation, which achieves up to 99% degradation when combined with sonolysis or catalysts. Global production of 4-aminophenol, concentrated in the region which holds over 70% of the , contributes to elevated levels of phenolic pollutants in regional waterways, exacerbating ecological pressures from industrial discharges in pharmaceutical and dye sectors.

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  64. https://www.fortunebusinessinsights.com/para-aminophenol-market-105392
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