# 4-Nitrophenol

**4-Nitrophenol** (4-NP, also called p-nitrophenol; CAS 100-02-7) is phenol in which the hydrogen para to the hydroxy group has been replaced by a nitro group, giving the formula C₆H₅NO₃ and a molecular weight of 139.11 g/mol.<sup>[1](https://pubchem.ncbi.nlm.nih.gov/compound/4-Nitrophenol)</sup> It is a colorless to pale-yellow crystalline solid used to manufacture drugs, fungicides, insecticides, and dyes, and it serves as a colorimetric pH indicator.<sup>[2](https://www.epa.gov/sites/production/files/2016-09/documents/4-nitrophenol.pdf)</sup><sup> • </sup><sup>[3](https://www.sciencedirect.com/science/article/pii/S0926860X26003339)</sup> Its catalytic reduction to 4-aminophenol with sodium borohydride has become one of the most widely used model reactions for evaluating new heterogeneous catalysts.<sup>[3](https://www.sciencedirect.com/science/article/pii/S0926860X26003339)</sup>

| Key fact | Detail |
|---|---|
| Identity | Phenol with a para nitro group; C₆H₅NO₃; molecular weight 139.11 g/mol; CAS 100-02-7<sup>[1](https://pubchem.ncbi.nlm.nih.gov/compound/4-Nitrophenol)</sup> |
| Physical properties | Melting point 114 °C; boiling above 279 °C with decomposition; water solubility 16,000 mg/L at 25 °C; pKa 7.15; log Kow 1.91<sup>[4](https://chemview.epa.gov/chemview/proxy?filename=HC100027.pdf)</sup> |
| Uses | Manufacture of drugs (including acetaminophen), fungicides, methyl and ethyl parathion insecticides, and dyes; darkening leather; pH indicator<sup>[2](https://www.epa.gov/sites/production/files/2016-09/documents/4-nitrophenol.pdf)</sup><sup> • </sup><sup>[3](https://www.sciencedirect.com/science/article/pii/S0926860X26003339)</sup> |
| Production | Catalytic hydrolysis of 4-nitrochlorobenzene, or nitration of phenol with dilute HNO₃ followed by steam distillation to separate the 4- from the 2- isomer<sup>[5](https://www.ncbi.nlm.nih.gov/books/NBK601133/)</sup> |
| Model reaction | Reduction to 4-aminophenol by NaBH₄ over metal nanoparticle catalysts; first reported by Pal and co-workers in 2002 and by Esumi et al.<sup>[3](https://www.sciencedirect.com/science/article/pii/S0926860X26003339)</sup> |
| Toxicity | Rat acute oral LD50 230 mg/kg; fish 96-h LC50 5.8 mg/L; not classified for carcinogenicity by EPA, DHHS, or IARC<sup>[4](https://chemview.epa.gov/chemview/proxy?filename=HC100027.pdf)</sup><sup> • </sup><sup>[6](https://wwwn.cdc.gov/tsp/ToxFAQs/ToxFAQsDetails.aspx?faqid=879&toxid=172)</sup> |
| Environmental fate | Estimated photodegradation half-life 2.5 days; hydrolysis negligible; low persistence (P1) and low bioaccumulation potential (B1)<sup>[4](https://chemview.epa.gov/chemview/proxy?filename=HC100027.pdf)</sup> |
| Human biomonitoring | Detected in 90% of urine samples of the US general population, geometric mean 0.64 µg/L; also a metabolite of methyl parathion and nitrobenzene<sup>[5](https://www.ncbi.nlm.nih.gov/books/NBK601133/)</sup> |

## Physical and chemical properties

4-Nitrophenol is a solid with low vapor pressure, 9.8 × 10<sup>−5</sup> mm Hg at 20 °C, and high water solubility, 16,000 mg/L at 25 °C.<sup>[4](https://chemview.epa.gov/chemview/proxy?filename=HC100027.pdf)</sup> Its pKa of 7.15 means that under environmental pH conditions the compound exists partly as the 4-nitrophenolate anion, which affects its behavior: anions do not volatilize and generally do not adsorb more strongly to soils containing organic carbon and clay than their neutral counterparts.<sup>[4](https://chemview.epa.gov/chemview/proxy?filename=HC100027.pdf)</sup><sup> • </sup><sup>[7](https://pubchem.ncbi.nlm.nih.gov/compound/4-Nitrofenol)</sup> The EPA hazard characterization accordingly rates volatilization as low, hydrolysis as negligible, and soil mobility as moderate.<sup>[4](https://chemview.epa.gov/chemview/proxy?filename=HC100027.pdf)</sup>

The nitro group is electron-withdrawing, which makes 4-nitrophenol a substantially stronger acid than phenol itself and underlies its use as a pH indicator, changing between colorless and yellow forms as it deprotonates.<sup>[3](https://www.sciencedirect.com/science/article/pii/S0926860X26003339)</sup>

## Production and uses

Industrial production follows two routes: catalytic hydrolysis of 4-nitrochlorobenzene, or reaction of dilute nitric acid on phenol with subsequent steam distillation to separate the 4- from the 2- isomer.<sup>[5](https://www.ncbi.nlm.nih.gov/books/NBK601133/)</sup> The United States imported 5,035 kg of 4-nitrophenol for consumption in 2021.<sup>[5](https://www.ncbi.nlm.nih.gov/books/NBK601133/)</sup>

**Uses.** 4-Nitrophenol is used to manufacture drugs such as acetaminophen (paracetamol), fungicides, methyl and ethyl parathion insecticides, and dyes, and to darken leather.<sup>[2](https://www.epa.gov/sites/production/files/2016-09/documents/4-nitrophenol.pdf)</sup> In pharmaceutical synthesis it is a precursor to paracetamol (an analgesic and antipyretic), [phenetidine](https://www.edgepedia.org/phenetidine) (an anticonvulsant), and acetophenetidine (an analgesic).<sup>[3](https://www.sciencedirect.com/science/article/pii/S0926860X26003339)</sup>

## The model catalytic reduction

The reduction of 4-nitrophenol to 4-aminophenol (4-AP) using excess sodium borohydride (NaBH₄) in aqueous medium has received wide attention since its first report by Pal and co-workers in 2002 and by Esumi et al., because the reaction is simple to operate, easy to monitor by UV–vis spectroscopy, and runs under mild conditions.<sup>[3](https://www.sciencedirect.com/science/article/pii/S0926860X26003339)</sup> Gold nanoparticles show excellent catalytic activity toward this transformation, and a broad range of metal nanoparticles and supports have been evaluated with it.<sup>[8](https://www.sciencedirect.com/science/article/pii/S0010854515000119)</sup> The product, 4-aminophenol, is less toxic than 4-nitrophenol and is itself a precursor for analgesic drugs, photographic developers, dyes and pigments, corrosion inhibitors, antioxidants, and agrochemical intermediates.<sup>[3](https://www.sciencedirect.com/science/article/pii/S0926860X26003339)</sup>

**Mechanism.** The conventional picture involves adsorption of 4-nitrophenol and borohydride ions (BH₄<sup>−</sup>) on the catalyst surface, electron transfer, and protonation steps leading to 4-AP, described by Langmuir-Hinshelwood (both reactants adsorbed) or Eley-Rideal (one reactant adsorbed) mechanisms.<sup>[3](https://www.sciencedirect.com/science/article/pii/S0926860X26003339)</sup><sup> • </sup><sup>[9](https://pmc.ncbi.nlm.nih.gov/articles/PMC9228544/)</sup> Recent work using spectroscopic, electrochemical, and computational techniques has uncovered mechanisms beyond these two classical models.<sup>[3](https://www.sciencedirect.com/science/article/pii/S0926860X26003339)</sup> [Kinetic isotope effect](https://www.edgepedia.org/kinetic-isotope-effect) studies using deuterated water (D₂O) or deuterated reducing agent (NaBD₄) indicate that the solvent water is not a spectator: water serves as the hydrogen source in the final product 4-AP, while NaBH₄ contributes to the ionization of H<sup>+</sup> from water and facilitates its adsorption on the nitro group.<sup>[3](https://www.sciencedirect.com/science/article/pii/S0926860X26003339)</sup><sup> • </sup><sup>[10](https://www.sciencedirect.com/science/article/pii/S0009261417306036)</sup> Among the factors identified as controlling the reaction are the size, shape, and phase of the particles, the d-band center, the solvent, ligands on the surface, metal–support interactions, defects, [hydrogen spillover](https://www.edgepedia.org/hydrogen-spillover), and the electronic states of the surface.<sup>[3](https://www.sciencedirect.com/science/article/pii/S0926860X26003339)</sup>

**Kinetic caveats.** Although the reaction is commonly analyzed as pseudo-first-order, a 2025 study argued that this treatment oversimplifies the kinetics because hydrogen transport by bubbles governs the apparent rate.<sup>[11](https://arxiv.org/abs/2510.24176)</sup> The counter-cation of the borohydride salt also has a strong, previously overlooked effect on gold-catalyzed reduction.<sup>[12](https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/smtd.202502277)</sup>

## Environmental occurrence and fate

Nitrophenols are manufactured chemicals that do not occur naturally, but 4-nitrophenol enters the environment from several sources: it is a degradation product of organophosphorus insecticides such as [methyl parathion](https://www.edgepedia.org/methyl-parathion), and it forms in auto exhaust and coal combustion; in one Chinese study, coal combustion accounted for 45% of wintertime 4-nitrophenol and vehicular exhaust 40% in spring.<sup>[5](https://www.ncbi.nlm.nih.gov/books/NBK601133/)</sup><sup> • </sup><sup>[6](https://wwwn.cdc.gov/tsp/ToxFAQs/ToxFAQsDetails.aspx?faqid=879&toxid=172)</sup> In US surface water monitoring from 2000 to 2022, 4-nitrophenol was detected in 47% of 636 samples at concentrations below 0.28 to 27 µg/L, and in 30% of 644 groundwater samples at up to 250 µg/L.<sup>[5](https://www.ncbi.nlm.nih.gov/books/NBK601133/)</sup>

The compound is removed mainly by photolysis and biodegradation. Its estimated photodegradation half-life is 2.5 days, hydrolysis is negligible, and atmospheric photooxidation is slow.<sup>[4](https://chemview.epa.gov/chemview/proxy?filename=HC100027.pdf)</sup> Ready-biodegradability test results are mixed: it achieved 90% degradation in 28 days in a Sturm test (readily biodegradable) and 92% in 10 days in a Zahn-Wellens test (inherently biodegradable), but showed 0% to 4.3% degradation in other screening tests.<sup>[4](https://chemview.epa.gov/chemview/proxy?filename=HC100027.pdf)</sup> Overall it is rated as having low persistence and low bioaccumulation potential.<sup>[4](https://chemview.epa.gov/chemview/proxy?filename=HC100027.pdf)</sup> Removal methods studied for contaminated water include adsorption, microbial degradation, photocatalysis, and chemical reduction; chemical reduction is favored for its cost-effectiveness, complete reduction efficiency, and production of economically valuable 4-AP, while adsorption suffers from incomplete removal and electrochemical methods from electrode fouling.<sup>[3](https://www.sciencedirect.com/science/article/pii/S0926860X26003339)</sup>

## Toxicity and human exposure

Acute oral toxicity in rats is moderate, with an LD50 of 230 mg/kg and all deaths occurring within the first 8 hours at lethal doses; acute dermal toxicity in rabbits is low (LD50 above 5,000 mg/kg).<sup>[4](https://chemview.epa.gov/chemview/proxy?filename=HC100027.pdf)</sup> In a 13-week oral study in rats, mortality was observed at 70 mg/kg-day and the NOAEL for systemic toxicity was 25 mg/kg-day; a four-week inhalation study in rats produced cataracts and corneal keratitis sicca at 0.03 mg/L-day.<sup>[4](https://chemview.epa.gov/chemview/proxy?filename=HC100027.pdf)</sup> Aquatic toxicity is measurable: the 96-hour LC50 for fish is 5.8 mg/L and the 48-hour EC50 for water fleas ([Daphnia magna](https://www.edgepedia.org/daphnia-magna)) is 22 mg/L.<sup>[4](https://chemview.epa.gov/chemview/proxy?filename=HC100027.pdf)</sup>

**Carcinogenicity.** 4-Nitrophenol did not increase tumor incidence in mice after 18 months of dermal exposure, and DHHS, EPA, and IARC have not classified its cancer-causing risk in humans.<sup>[4](https://chemview.epa.gov/chemview/proxy?filename=HC100027.pdf)</sup><sup> • </sup><sup>[6](https://wwwn.cdc.gov/tsp/ToxFAQs/ToxFAQsDetails.aspx?faqid=879&toxid=172)</sup> EPA has not classified the compound for potential carcinogenicity and has not established a Reference Concentration or Reference Dose for it.<sup>[2](https://www.epa.gov/sites/production/files/2016-09/documents/4-nitrophenol.pdf)</sup>

**Biomonitoring.** Because 4-nitrophenol is also a metabolite of methyl parathion, nitrobenzene, and related chemicals, urinary 4-nitrophenol cannot distinguish direct exposure to the compound from exposure to these precursors.<sup>[6](https://wwwn.cdc.gov/tsp/ToxFAQs/ToxFAQsDetails.aspx?faqid=879&toxid=172)</sup> The NHANES survey detected 4-nitrophenol in 90% of urine samples of the US general population, with a geometric mean of 0.64 µg/L, and farmworkers show significantly higher creatinine-adjusted urinary concentrations than the general population.<sup>[5](https://www.ncbi.nlm.nih.gov/books/NBK601133/)</sup> Acute inhalation or ingestion in humans causes headaches, drowsiness, nausea, and cyanosis, and eye contact causes irritation.<sup>[2](https://www.epa.gov/sites/production/files/2016-09/documents/4-nitrophenol.pdf)</sup>

## References

1. [4-Nitrophenol, CID 980, PubChem, National Library of Medicine](https://pubchem.ncbi.nlm.nih.gov/compound/4-Nitrophenol)
2. [4-Nitrophenol, Hazard Summary, US EPA](https://www.epa.gov/sites/production/files/2016-09/documents/4-nitrophenol.pdf)
3. [Mahto, B., Ali, H., Gupta, M., Hussain, S. Mechanistic understanding of the 4-nitrophenol reduction as a model catalytic reaction: Advances, controversies, and perspectives. Applied Catalysis A: General, Vol. 724 (2026)](https://www.sciencedirect.com/science/article/pii/S0926860X26003339)
4. [Screening-Level Hazard Characterization for 4-Nitrophenol (CASRN 100-02-7), US EPA, September 2015](https://chemview.epa.gov/chemview/proxy?filename=HC100027.pdf)
5. [Toxicological Profile for Nitrophenols, Chapter 5: Potential for Human Exposure, ATSDR/NCBI Bookshelf](https://www.ncbi.nlm.nih.gov/books/NBK601133/)
6. [Nitrophenols, ToxFAQs, Agency for Toxic Substances and Disease Registry](https://wwwn.cdc.gov/tsp/ToxFAQs/ToxFAQsDetails.aspx?faqid=879&toxid=172)
7. [4-Nitrophenol, PubChem (environmental fate excerpt)](https://pubchem.ncbi.nlm.nih.gov/compound/4-Nitrofenol)
8. [Basic concepts and recent advances in nitrophenol reduction by gold- and other transition metal nanoparticles, Coordination Chemistry Reviews](https://www.sciencedirect.com/science/article/pii/S0010854515000119)
9. [Reduction of 4-nitrophenol using green-fabricated metal nanoparticles, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC9228544/)
10. [Insights into the reduction of 4-nitrophenol to 4-aminophenol on catalysts, Chemical Physics Letters](https://www.sciencedirect.com/science/article/pii/S0009261417306036)
11. [When bubbles matter: hydrogen transport governs apparent kinetics in 4-nitrophenol hydrogenation reaction, arXiv (2025)](https://arxiv.org/abs/2510.24176)
12. [Smolska et al., On the Overlooked Effect of Counter-Cations From Borohydrides for the Gold- and Metal-Mediated Catalytic Reduction of 4-Nitrophenol, Small Methods (2026)](https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/smtd.202502277)

---
*Topic: Encyclopedia › Physical world and mathematics › Chemistry › Organic substances › Amines and nitrogen functional groups › Nitriles, nitro, diazo, and related nitrogen groups › Nitro compounds › Nitroarenes with other substituents*

*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026*

*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*

License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license