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Nitrophenol
Nitrophenol
Nitrophenol
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Nitrophenols are compounds of the formula HOC6H5−x(NO2)x. The conjugate bases are called nitrophenolates. Nitrophenols are more acidic than phenol itself.[1]

Mono-nitrophenols

[edit]
Mono-nitrophenols
2-Nitrophenol (o-)
3-Nitrophenol (m-)
4-Nitrophenol (p-)
Identifiers
  • Compounds
  • o: o-nitrophenol (2-)
  • m: m-nitrophenol (3-)
  • p: p-nitrophenol (4-)
  • p: p tautomer
3D model (JSmol)
ChEBI
ChEMBL
ChemSpider
DrugBank
KEGG
UNII
  • o: InChI=1S/C6H5NO3/c8-6-4-2-1-3-5(6)7(9)10/h1-4,8H
    Key: IQUPABOKLQSFBK-UHFFFAOYSA-N
  • m: InChI=1S/C6H5NO3/c8-6-3-1-2-5(4-6)7(9)10/h1-4,8H
    Key: RTZZCYNQPHTPPL-UHFFFAOYSA-N
  • p: InChI=1S/C6H5NO3/c8-6-3-1-5(2-4-6)7(9)10/h1-4,8H
    Key: BTJIUGUIPKRLHP-UHFFFAOYSA-N
  • o: Oc1ccccc1[N+]([O-])=O
  • m: Oc1cccc(c1)[N+]([O-])=O
  • p: c1cc(ccc1[N+](=O)[O-])O
  • p: O=[N+]([O-])c1ccc(O)cc1
Properties
C6H5NO3
Molar mass 139.110 g·mol−1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

with the formula HOC6H4NO2. Three isomeric nitrophenols exist:

  • o-Nitrophenol (2-nitrophenol; OH and NO2 groups are neighboring), a yellow solid.
  • m-Nitrophenol (3-nitrophenol, CAS number: 554-84-7), a yellow solid (m.p. 97 °C) and precursor to the drug mesalazine (5-aminosalicylic acid). It can be prepared by nitration of aniline followed by replacement of the amino group via its diazonium derivative.[2]

The mononitrated phenols are often hydrogenated to the corresponding aminophenols that are also useful industrially.[1]

Di- and trinitrophenols

[edit]
2,4-dinitrophenol
2,4-dinitrophenol
  • 2,4,6-Trinitrophenol is better known as picric acid, which has a well-developed chemistry.
picric acid
picric acid

Safety

[edit]

Nitrophenols are poisonous. Occasionally, nitrophenols contaminate the soil near former explosives or fabric factories and military plants, and current research is aimed at remediation.[3]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Nitrophenol

View on Grokipedia
from Grokipedia
Nitrophenols are a class of synthetic organic compounds derived from phenol by the addition of one or more nitro groups to the ring, with the mononitrophenols—specifically 2-nitrophenol (ortho-nitrophenol), 3-nitrophenol (meta-nitrophenol), and (para-nitrophenol)—being the most common isomers, all sharing the molecular formula C₆H₅NO₃. Nitrophenols, particularly 2-nitrophenol and , are included on the U.S. EPA's list of priority pollutants. These pale yellow to colorless crystalline solids are characterized by their acidity, high (ranging from 2,100 mg/L for 2-nitrophenol to 16,000 mg/L for at 25°C), and low volatility, with melting points varying from 44–45°C for 2-nitrophenol to 113–114°C for . Produced industrially through the of phenol with , nitrophenols serve as key intermediates in the manufacture of dyes, pharmaceuticals, pesticides (such as and methyl parathion), fungicides, rubber chemicals, and preservatives, with annual U.S. production exceeding 35 million pounds for alone in the late 1970s, though current volumes are lower due to regulatory restrictions. They also occur environmentally as byproducts of processes, including exhaust and burning, leading to widespread detection in air (e.g., 0.106–0.120 μg/m³ for in urban settings), (up to 27 μg/L), and sediments (up to 49.5 μg/kg). by microorganisms occurs relatively quickly in acclimated environments, with half-lives of 1–8 days in and 1–40 days in , though persistence can be longer under anaerobic conditions. Nitrophenols exhibit moderate to high , acting as irritants to , eyes, and , with oral LD₅₀ values in rats ranging from 170–620 mg/kg for and similar for other isomers; chronic exposure in animals has been linked to , liver damage, cataracts (particularly from at concentrations as low as 29.18 mg/m³ via inhalation), and reproductive effects like , though human data are limited and no carcinogenic effects have been observed in studies. Due to their environmental persistence and potential, nitrophenols are priority pollutants under regulations like the U.S. EPA's ambient criteria; in its 1980 document, the EPA set protective levels at 230 μg/L for acute aquatic and emphasized monitoring at contaminated sites.

Introduction

Definition and Nomenclature

Nitrophenols are a class of organic compounds consisting of a phenolic hydroxyl group attached to a ring bearing one or more nitro substituents (-NO₂). The term "nitrophenol" serves as a collective designation for these nitro-substituted derivatives of phenol. The general formula for nitrophenols is HOC₆H₅₋ₓ(NO₂)ₓ, where x ≥ 1, which distinguishes them from the parent compound phenol (C₆H₅OH). For mononitrophenols, where x = 1, the molecular formula is C₆H₅NO₃. In , mononitrophenols are commonly identified by the position of the nitro group relative to the hydroxyl group using traditional descriptors: ortho- (or 2-), meta- (or 3-), and para- (or 4-) nitrophenol. Their systematic IUPAC names follow the format #-nitrophenol, such as 2-nitrophenol (o-nitrophenol), 3-nitrophenol (m-nitrophenol), and (p-nitrophenol). The conjugate bases of nitrophenols, known as nitrophenolates, result from of the hydroxyl group. The electron-withdrawing nature of the nitro group stabilizes the phenolate anion through inductive and effects, thereby increasing the acidity of nitrophenols relative to phenol.

Historical Background

Mononitrophenols were first prepared in the mid-19th century through the of phenol, marking an early advancement in aromatic chemistry. This process involved treating phenol with , yielding the ortho isomer as a key product, which laid the groundwork for understanding substituted . A significant development occurred with , or 2,4,6-trinitrophenol, first synthesized in 1771 by British chemist Peter Woulfe through the action of on , though its phenolic nature was not fully recognized until later. By the 1840s, gained prominence as a vibrant for and wool, with commercial production scaling after phenol became available from distillation. Its properties were harnessed in the late 19th century, notably during the in the 1850s for experimental munitions, and it saw widespread military adoption as a high in subsequent conflicts. In the early 20th century, 4-nitrophenol emerged as an industrial milestone, with production ramping up for use as an intermediate in azo dye synthesis and pharmaceuticals, including precursors to analgesics like acetaminophen. During World War II, polynitrophenols such as picric acid continued to play a role in explosives and chemical warfare agents, with facilities like the U.S. Little Rock Picric Acid Plant producing it for grenades and related compounds. Post-1940s, growing awareness of nitrophenols' toxicity prompted a shift toward safer alternatives in dyes, pesticides, and munitions, driven by reports of severe health effects including skin discoloration and organ damage among workers.

Mononitrophenols

2-Nitrophenol

2-Nitrophenol is an featuring a nitro group (-NO₂) attached at the ortho position relative to the hydroxyl group (-OH) on a benzene ring, with the molecular formula C₆H₅NO₃. This positioning allows for intramolecular , where the of the hydroxyl group forms a bond with one of the oxygen atoms in the nitro group, resulting in a chelate-like that reduces intermolecular and thereby increases its volatility compared to without such interactions. As a crystalline solid, 2-nitrophenol has a of 45 °C and a of 214 °C at standard pressure. Its in is low, approximately 2 g/L at 20 °C, reflecting limited hydrogen bonding with molecules due to the intramolecular interaction. This compound exhibits a higher and a distinct attributable to its enhanced volatility, distinguishing it from less volatile isomers. Chemically, 2-nitrophenol is more acidic than unsubstituted phenol, with a pKa of 7.23 compared to phenol's 10, owing to the electron-withdrawing nitro group that stabilizes the conjugate base (phenolate ) through delocalization. In ultraviolet-visible , it shows absorption at approximately 350 nm, attributed to an n-π* transition influenced by the intramolecular hydrogen bonding. This property makes 2-nitrophenol a useful analytical for demonstrating intramolecular hydrogen bonding in educational and laboratory settings.

3-Nitrophenol

3-Nitrophenol, also known as m-nitrophenol, features a nitro group attached to the ring at the 3-position relative to the phenolic , resulting in the molecular formula C₆H₅NO₃. This meta substitution prevents the formation of intramolecular hydrogen bonding between the nitro and s, a feature present in the ortho isomer due to their proximity, which influences its physical and spectral properties. The compound is a pale yellow crystalline solid with a of 97 °C and a of 194 °C at 70 mmHg, often decomposing at higher temperatures. It demonstrates higher solubility at 13.5 g/L (25 °C) compared to 2-nitrophenol, owing to greater intermolecular bonding capabilities in the absence of intramolecular interactions that reduce the ortho isomer's polarity and . This enhanced solubility makes 3-nitrophenol less volatile than the ortho isomer, which can be steam-distilled due to weaker intermolecular associations. Chemically, 3-nitrophenol exhibits a pKa of 8.36, reflecting moderate acidity driven primarily by the inductive withdrawal of electrons by the meta-nitro group, without the enhancement seen in the para . reveals the O-H stretching vibration at approximately 3300 cm⁻¹, appearing without the characteristic broadening or shift associated with intramolecular hydrogen bonding in 2-nitrophenol. As an industrial intermediate, 3-nitrophenol is utilized in the synthesis of dye precursors like m-aminophenol through , and it serves as a model compound in investigations of positional effects on phenolic acidity, highlighting the nitro group's influence on and . Compared to other mononitrophenols, its properties occupy an intermediate position in terms of and .

4-Nitrophenol

4-Nitrophenol features a nitro group (-NO₂) attached to the ring at the para position relative to the hydroxyl (-OH) group, resulting in the molecular formula C₆H₅NO₃. This arrangement facilitates extensive delocalization between the electron-withdrawing nitro group and the phenolic hydroxyl, particularly stabilizing the conjugate base upon and enhancing the compound's acidity compared to unsubstituted phenol. The compound appears as a colorless to pale yellow crystalline solid, with a of 114 °C and a of 279 °C at standard pressure. Its in is approximately 16 g/L at 25 °C, reflecting moderate hydrophilicity influenced by the polar functional groups. In the solid state, exhibits two polymorphs: the alpha form consists of colorless pillars that are unstable at but photostable, while the beta form forms yellow pillars stable under ambient conditions. Chemically, has a pKₐ of 7.15, the lowest among the mononitrophenol isomers (compared to 7.23 for 2-nitrophenol and 8.36 for 3-nitrophenol), due to the effective stabilization of the phenolate anion by the para-nitro group. The neutral form absorbs in the UV around 317 nm, but the deprotonated phenolate ion displays a prominent absorption band at 400 nm, arising from intramolecular charge-transfer excitation from the oxygen to the nitro group's π* orbital. This spectral shift enables its use as a , remaining colorless below pH 5.4 and turning yellow above pH 7.5 in basic conditions.

Polynitrophenols

Dinitrophenols

Dinitrophenols are polynitrophenol compounds featuring two nitro groups attached to the phenol ring, typically synthesized as further products of mononitrophenols. Among the six possible isomers (2,3-, 2,4-, 2,5-, 2,6-, 3,4-, and 3,5-dinitrophenol), (DNP) is the most studied and significant due to its unique chemical and biological properties. 2,4-Dinitrophenol, with the molecular formula C₆H₄N₂O₅, appears as a solid. Its physical properties include a that sublimes at approximately 112 °C and moderate in at 5.6 g/L (18 °C). The compound exhibits high acidity, with a pKa of 4.09, attributed to the strong electron-withdrawing effects of the ortho and para nitro groups, which stabilize the phenolate anion. Chemically, the dual nitro substitutions enhance electron withdrawal, enabling 2,4-DNP to act as a protonophore that uncouples in mitochondria by dissipating the proton gradient across the inner membrane, thereby increasing metabolic rate without ATP production. This property also contributes to its explosive potential as a solid, though it is less sensitive than trinitrophenol derivatives and is often handled wetted to reduce hazards. In the 1930s, 2,4-DNP was marketed as a weight-loss drug due to its metabolism-boosting effects but was banned by the U.S. in 1938 following reports of severe , cataracts, and fatalities. Other dinitrophenol isomers, such as 2,6-dinitrophenol, are less commonly encountered and studied, with similar acidic properties (pKa 3.97) but reduced compared to 2,4-DNP. These compounds generally share the coloration and nitro-enhanced reactivity of the class but vary in stability and applications.

Trinitrophenols

Trinitrophenols are polynitrophenol derivatives featuring three nitro groups attached to the phenolic ring, with 2,4,6-trinitrophenol, commonly known as , serving as the primary and most studied compound in this class. has the molecular formula C₆H₃N₃O₇ and manifests as bright crystals, a characteristic that contributed to its early identification and applications. Physically, picric acid exhibits a melting point of 122 °C, a density of 1.76 g/cm³, and limited solubility in water at 12.7 g/L (25 °C), rendering it sparingly soluble compared to less substituted nitrophenols. Chemically, it is a strong acid with a pKa of 0.38, reflecting the electron-withdrawing effects of the three nitro groups that stabilize the phenolate anion far more effectively than in dinitrophenols (pKa ≈ 4). This heightened acidity enables picric acid to form salts readily, particularly with metals, resulting in highly sensitive explosive picrates such as lead or copper picrate. While picric acid itself is relatively insensitive to shock or friction, it detonates with high velocity upon initiation, a property stemming from its nitroaromatic structure. Historically, was first synthesized in 1771 by Peter Woulfe through of , marking it as the earliest known synthetic due to its vibrant coloration used in textiles. By the mid-19th century, its dyeing properties were commercially exploited, but its explosive potential was recognized in the 1880s, leading to adoption as a high explosive in munitions. During , filled shells and grenades for major powers including , Britain, and , prized for its despite handling risks. Concurrently, its qualities—attributed to the phenolic hydroxyl—saw use in medical treatments for burns, wounds, and infections, with solutions applied until the 1940s when safer alternatives like sulfa drugs emerged.

Synthesis

Nitration of Phenol

The of phenol proceeds via , in which the nitronium ion (NO₂⁺) serves as the . This ion is generated from a mixture of concentrated (HNO₃) and (H₂SO₄), where H₂SO₄ protonates HNO₃ to facilitate the formation of NO₂⁺ and water. The reaction is conducted at controlled temperatures ranging from 0 to 50 °C to promote mononitration while suppressing unwanted side reactions, such as polynitration or oxidative degradation. The general equation for the process is: \ceC6H5OH+HNO3−>[H2SO4][0−50∘C]o−orp−HOC6H4NO2+H2O\ce{C6H5OH + HNO3 ->[H2SO4][0-50^\circ C] o- or p-HOC6H4NO2 + H2O} The hydroxyl group (-OH) of phenol is a strong ortho-para director due to its electron-donating resonance effect, which increases the electron density at the ortho and para positions, facilitating electrophilic attack. Under standard conditions using HNO₃/H₂SO₄ mixtures, the mononitration yields a mixture of 2-nitrophenol (ortho) and 4-nitrophenol (para) isomers in an approximate distribution of 60% ortho and 40% para. This regioselectivity can vary slightly with acid concentration and temperature; for instance, in 58–80% aqueous sulfuric acid, the ortho:para ratio ranges from 2.4:1 to 0.9:1 as acidity increases. Phenol's high reactivity poses significant challenges in direct nitration, as the activated aromatic ring is prone to oxidation by the nitrating mixture, leading to tarry byproducts and reduced yields of desired mononitrophenols. To mitigate these issues and enhance selectivity, particularly for the para isomer, protecting groups such as the sulfate ester (phenyl hydrogen sulfate) are employed. The phenol is first converted to its sulfate ester, which moderates the activating effect of the -OH group, directing predominantly to the para position; subsequent removes the protecting group to yield . This strategy improves overall efficiency in both and industrial settings by minimizing oxidation and isomer mixtures.

Alternative Synthetic Routes

One alternative route to nitrophenols involves the hydroxylation of nitrobenzene derivatives through alkali fusion. Specifically, 4-nitrophenol can be produced by heating p-nitrochlorobenzene (derived from nitration of chlorobenzene) with sodium hydroxide at elevated temperatures around 200–300°C, often under phase-transfer catalysis to improve efficiency and yield under milder conditions. Reduction routes provide another pathway, particularly for obtaining mononitrophenols from polynitrophenol precursors. Partial hydrogenation of dinitrophenols, such as 2,4-dinitrophenol, to the corresponding mononitrophenols can be achieved using palladium on carbon (Pd/C) catalysts under controlled hydrogen pressure and temperature, allowing selective reduction of one nitro group while preserving the other. Specialized methods include diazotization and for the meta isomer. 3-Nitrophenol is synthesized by diazotizing m-nitro (an aniline derivative) with in acidic medium to form the diazonium salt, followed by in to replace the diazonium group with a hydroxyl group, yielding the product in good efficiency. Enzymatic nitration represents a biotechnological approach, where enzymes like catalyze the introduction of nitro groups onto using and as nitrating agents under mild aqueous conditions, offering regioselective control and reduced waste compared to traditional methods. Post-2000 developments in have introduced alternatives using nitrate salts and oxidants to bypass strong mineral acids. For instance, regioselective nitration of to ortho-nitrophenols employs with as an acidic promoter in solvent-free conditions, achieving high yields while minimizing environmental impact through recyclable reagents and avoidance of .

Applications

Industrial and Chemical Uses

Nitrophenols serve as essential intermediates in various , predominantly for applications. These compounds are valued for their reactivity, enabling the formation of more complex molecules in large-scale . In the dye industry, plays a critical role as a precursor for azo dyes and pigments, particularly those applied in coloration to achieve vibrant hues. This application underscores 's importance in the global sector. Picric acid (2,4,6-trinitrophenol), a polynitrophenol derivative, has been historically utilized in for military munitions and mining, serving as a high explosive since the late . Its use peaked during major conflicts, including , where it was incorporated into shells and other ordnance by various nations, though its role diminished post-war in favor of safer alternatives like TNT. In agrochemicals, derivatives such as and its analogs function as active components in herbicides and pesticides, targeting and pest management in . These compounds disrupt metabolic processes in target organisms, with dinitrophenolic pesticides derived from forming a class of broad-spectrum agents historically applied in crop protection.

Pharmaceutical and Other Applications

In pharmaceutical synthesis, acts as an important intermediate for producing (acetaminophen), one of the most widely used analgesics and antipyretics. The process involves selective reduction of to , followed by with to yield the final compound. This route leverages the para-directing effect of the hydroxyl group in phenol during , ensuring high selectivity for the desired . Picric acid, known chemically as 2,4,6-trinitrophenol, finds application in as a precipitating fixative, particularly in Bouin's solution, which combines picric acid with and glacial acetic acid. This mixture excels at preserving architecture and producing intense nuclear , making it valuable for detailed morphological studies in and . Its coagulative properties enhance tissue penetration while minimizing shrinkage, though it requires thorough washing to remove residual acid before . In biochemical research, (DNP) serves as a classic mitochondrial , dissipating the proton gradient across the to investigate and energy expenditure. Low doses of DNP have been shown to improve mitochondrial function, reduce oxidative damage, and provide in models of ischemia and neurodegeneration. Although banned for consumer use as a weight-loss agent since 1938 due to risks of and cataracts, DNP remains a staple in laboratory-scale studies of cellular .

Safety and Environmental Impact

Health and Toxicity Hazards

Nitrophenols pose significant health risks through various exposure routes, primarily acting as irritants and systemic toxins. Acute exposure to compounds like causes severe irritation to the skin and eyes, leading to redness, pain, and potential burns upon contact. Inhalation of vapors or dust can induce , a condition where is oxidized to , impairing oxygen transport and resulting in symptoms such as , headache, dizziness, and fatigue. Oral ingestion exhibits high , with demonstrating an LD50 of 194 mg/kg in rats, highlighting its potential for rapid onset of convulsions, respiratory distress, and lethality at moderate doses. Dinitrophenols, such as (2,4-DNP), amplify these hazards due to their uncoupling of in mitochondria, leading to excessive heat production. Acute effects include , , sweating, and , with an oral LD50 of 25–50 mg/kg in rats. Chronic exposure to nitrophenols exacerbates organ damage and long-term effects. For , prolonged intake induces and cataracts, with historical human cases showing irreversible lens opacities after weeks of use. Animal studies reveal , including increased post-implantation loss and delayed fetal development in rats exposed orally to at doses around 25 mg/kg/day. Notably, 's use as an unregulated diet pill in resulted in numerous fatalities from hyperthermic crises and cataracts, prompting the U.S. (FDA) to ban it in 1938. Environmental pathways, such as contaminated water, may contribute to low-level human exposure, though direct health impacts dominate toxicological concerns.

Environmental Persistence and Remediation

Nitrophenols are classified as biorefractory pollutants due to their resistance to natural degradation processes in the environment. In , the of varies significantly based on conditions; under aerobic environments, it ranges from 1 to 3 days, while in subsoils or anaerobic settings, it can extend to approximately 40 days. These compounds are frequently detected in effluents from and industries, where they arise as byproducts of processes and contribute to persistent in aquatic systems. Mononitrophenols exhibit moderate potential, with log Kow values around 1.8 to 2.0, indicating limited partitioning into but sufficient to pose risks in contaminated ecosystems. They are toxic to aquatic life, with values for fish species such as sheepshead minnows reported in the range of 5-10 mg/L, leading to adverse effects on survival, growth, and development in early life stages. This underscores their role as hazardous contaminants in water bodies, where even low concentrations can disrupt aquatic communities. Remediation strategies for nitrophenols focus on biological and chemical methods to enhance breakdown. is effectively achieved using species, such as and , which metabolize p-nitrophenol through pathways involving nitro group reduction and ring cleavage, often achieving near-complete removal in contaminated media under optimal aerobic conditions. , particularly (a mixture of ferrous iron and ), generate hydroxyl radicals that rapidly oxidize , with degradation efficiencies exceeding 90% in aqueous solutions at acidic . 4-Nitrophenol is included on the U.S. Agency's priority pollutant list, highlighting its regulatory significance for monitoring and control in industrial discharges.

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