# Phenetidine
**Phenetidine** is the name for two of the three positional isomers of ethoxyaniline, C₈H₁₁NO, aromatic amines in which an ethoxy group and an amino group occupy a benzene ring; the commercially important para isomer, 4-ethoxyaniline (p-phenetidine, CAS 156-43-4), is a colorless to dark red liquid used as an intermediate in dyes and pharmaceuticals and known as a toxic metabolite of the analgesic phenacetin.[1](https://pubchem.ncbi.nlm.nih.gov/compound/9076) • [2](https://www.drugfuture.com/chemdata/o-phenetidine.html) • [10](https://webbook.nist.gov/cgi/cbook.cgi?ID=156-43-4)
| Key fact | Detail |
|---|---|
| Formula and weight | C₈H₁₁NO, molecular weight 137.18 g/mol[1](https://pubchem.ncbi.nlm.nih.gov/compound/9076) |
| Isomers and CAS numbers | p-phenetidine (4-ethoxyaniline) CAS 156-43-4; o-phenetidine (2-ethoxybenzenamine) CAS 94-70-2[2](https://www.drugfuture.com/chemdata/o-phenetidine.html) • [3](https://www.sigmaaldrich.com/US/en/sds/aldrich/p14815) |
| Physical state | Colorless to dark red liquid; mp 2–5 °C, bp 250 °C, density 1.065 g/cm³ at 25 °C (p-isomer)[1](https://pubchem.ncbi.nlm.nih.gov/compound/9076) • [3](https://www.sigmaaldrich.com/US/en/sds/aldrich/p14815) |
| Phenacetin link | Formed from phenacetin by hydrolysis via arylacetamide deacetylase (AADAC); further oxidized to N-hydroxyphenetidine, the proximate mutagenic metabolite[4](https://www.canada.ca/en/environment-climate-change/services/evaluating-existing-substances/screening-assessment-phenacetin.html) |
| Acute toxicity | Rat oral LD₅₀ 540 mg/kg; absorption forms methemoglobin, causing cyanosis with onset delayed 2 to 4 hours or longer[3](https://www.sigmaaldrich.com/US/en/sds/aldrich/p14815) |
| GHS hazards | Acute toxicity Category 4 (oral, dermal, inhalation), eye irritation 2A, skin sensitization 1, germ cell mutagenicity Category 2[3](https://www.sigmaaldrich.com/US/en/sds/aldrich/p14815) |
| Main uses | Manufacture of acetophenetidin (phenacetin), dyes, pharmaceutical actives, antioxidants, and fine chemicals[1](https://pubchem.ncbi.nlm.nih.gov/compound/9076) • [5](https://lanxess.com/en-us/products/products/p/p-phenetidine) |
| Transport | UN 2311, Class 6.1, Packing Group III, proper shipping name "Phenetidines"[3](https://www.sigmaaldrich.com/US/en/sds/aldrich/p14815) |
## Identity and isomers
The name phenetidine covers three aniline derivatives that differ only in where the ethoxy group sits on the ring.[10](https://webbook.nist.gov/cgi/cbook.cgi?ID=156-43-4) In **p-phenetidine** (4-ethoxyaniline, 4-aminophenetole) the ethoxy group is opposite the amino group; in **o-phenetidine** (2-ethoxybenzenamine, 2-aminophenetole) the two groups are adjacent.[1](https://pubchem.ncbi.nlm.nih.gov/compound/9076) • [2](https://www.drugfuture.com/chemdata/o-phenetidine.html) The para isomer is the one produced and traded at industrial scale; the specialty chemical supplier LANXESS describes it as a colorless to light yellow clear liquid that turns red to brown on exposure to air and light.[5](https://lanxess.com/en-us/products/products/p/p-phenetidine) The ortho isomer is an oily liquid that also rapidly becomes brown on exposure to light and air, melts below −20 °C, and boils at 228–230 °C; it is insoluble in water and soluble in alcohol.[2](https://www.drugfuture.com/chemdata/o-phenetidine.html)
## Physical properties of the para isomer
Published values for p-phenetidine come from regulatory and supplier datasets. The [Sigma-Aldrich](https://www.edgepedia.org/sigma-aldrich) safety data sheet lists a melting point of 2–5 °C, a boiling point of 250 °C, a closed-cup flash point of 122 °C, and a density of 1.065 g/cm³ at 25 °C.[3](https://www.sigmaaldrich.com/US/en/sds/aldrich/p14815) The ICSC card gives a boiling range of 253–255 °C, a melting point of 2.4 °C, water solubility of 2 g per 100 ml at 20 °C, a vapor density of 4.7 relative to air, and an autoignition temperature of 425 °C.[1](https://pubchem.ncbi.nlm.nih.gov/compound/9076) The computed partition coefficient XLogP3 is 1.2, consistent with its moderate water solubility and use as a lipophilic aromatic building block.[1](https://pubchem.ncbi.nlm.nih.gov/compound/9076)
## Role in phenacetin metabolism and toxicity
The toxicological importance of p-phenetidine comes from the drug phenacetin (N-(4-ethoxyphenyl)acetamide), an analgesic introduced into clinical medicine in 1887 and later withdrawn in many countries. A secondary metabolic pathway for phenacetin involves hydrolysis to p-phenetidine by arylacetamide deacetylase (AADAC), a microsomal serine esterase expressed in liver and gastrointestinal tissues.[4](https://www.canada.ca/en/environment-climate-change/services/evaluating-existing-substances/screening-assessment-phenacetin.html) p-Phenetidine is then further metabolized to the arylhydroxylamine N-hydroxyphenetidine, which is believed to be the proximate mutagenic metabolite that also mediates the nephro- and hematotoxicity of the parent compound.[4](https://www.canada.ca/en/environment-climate-change/services/evaluating-existing-substances/screening-assessment-phenacetin.html) A 2022 review in *Toxicology Reports* likewise describes p-phenetidine as an oxidizing toxic metabolite to which phenacetin can be deacetylated.[6](https://escholarship.org/content/qt1n62x6w6/qt1n62x6w6_noSplash_e9683a73d5b8d31360b3823519d75c32.pdf?t=rxapdc)
The competition between the two metabolic routes is quantified by enzyme affinities. CYP1A2, which converts phenacetin to acetaminophen, has a Km of 31 µM, while AADAC has a Km of 1.82 mM; at a substrate concentration of 100 µM, CYP1A2 would account for 86% of net reaction velocity. High peak blood levels of phenacetin could therefore favor the low-affinity AADAC pathway that generates the toxic metabolite.[4](https://www.canada.ca/en/environment-climate-change/services/evaluating-existing-substances/screening-assessment-phenacetin.html)
## Hazard profile
Like other aromatic amines, phenetidine's principal systemic effect is methemoglobinemia: absorption into the body leads to formation of methemoglobin, which in sufficient concentration causes cyanosis, with onset that may be delayed 2 to 4 hours or longer.[3](https://www.sigmaaldrich.com/US/en/sds/aldrich/p14815) The Sigma-Aldrich sheet reports a rat oral LD₅₀ of 540 mg/kg, classifies the substance for acute toxicity by mouth, skin contact, and inhalation (Category 4 each), for serious eye irritation (2A), skin sensitization (1), and suspected germ cell mutagenicity (Category 2, H341).[3](https://www.sigmaaldrich.com/US/en/sds/aldrich/p14815) The o-phenetidine monograph carries the caution that the liquid is absorbed through skin and its vapor is hazardous.[2](https://www.drugfuture.com/chemdata/o-phenetidine.html)
Genotoxicity test results are mixed by endpoint. In vitro Ames tests in [Salmonella](https://www.edgepedia.org/salmonella) strains TA98 and TA100 were negative without activation, with some positive results in the presence of metabolic activation; p-phenetidine inhibited DNA synthesis in Chinese hamster lung cells without activation, induced DNA strand breaks in human skin fibroblasts with activation, and was negative for unscheduled DNA synthesis in hepatocytes.[7](https://www.canada.ca/en/environment-climate-change/services/evaluating-existing-substances/screening-assessment-5.html) Repeated-dose studies in rats showed effects on blood and blood-forming organs, including increased spleen weight, hemosiderosis, decreased erythrocytes, hemoglobin, and hematocrit, and Heinz bodies.[7](https://www.canada.ca/en/environment-climate-change/services/evaluating-existing-substances/screening-assessment-5.html)
## Production and uses
p-Phenetidine is a valued aromatic intermediate. Its documented uses include the manufacture of acetophenetidin (phenacetin), phenocoll, dulcin, and dyes, and more broadly dyes, pharmaceutical actives, antioxidants for feedstuffs, pigments, and other fine chemicals.[1](https://pubchem.ncbi.nlm.nih.gov/compound/9076) • [5](https://lanxess.com/en-us/products/products/p/p-phenetidine) A patented industrial process makes p-phenetidine by hydrogenating an azo precursor: the azo bond (N═N) is cracked in the presence of hydrogen over a catalyst such as [Raney nickel](https://www.edgepedia.org/raney-nickel), yielding two moles of the p-alkoxyaniline.[8](https://patents.google.com/patent/US5493055A/en) A 2026 ACS Omega study used the reverse logic, building p-phenetidine-derived azo dyes by diazotization of the arylamine and coupling with phenolate.[9](https://pubs.acs.org/acsodf/article/11/32/47904/5242284/Bootstrap-Synthesis-Revisited-A-Solid-Cobalt)
## Exposure pathways
General-population exposure to p-phenetidine is limited and indirect. In Canada's screening assessment of aromatic amines, p-phenetidine was reported in the food and beverage sector but is not a permitted direct food additive; it can appear as an organic impurity in the antioxidant [ethoxyquin](https://www.edgepedia.org/ethoxyquin), for which the Food Chemicals Codex sets a maximum limit of 3.0%, giving up to 3 ppm in paprika and ground chili pepper where ethoxyquin is permitted at 100 ppm for color retention.[7](https://www.canada.ca/en/environment-climate-change/services/evaluating-existing-substances/screening-assessment-5.html) Two azo dyes, Solvent Red 3 (CAS 6535-42-8) and Direct Yellow 12 (2870-32-8), can in principle release p-phenetidine upon cleavage of the azo bond, though the potential for cleavage is uncertain and estimated exposures from cosmetics, textiles, and leather are low.[7](https://www.canada.ca/en/environment-climate-change/services/evaluating-existing-substances/screening-assessment-5.html) Occupational handling is governed by the GHS classification above, with storage under inert gas because the substance is air and light sensitive.[3](https://www.sigmaaldrich.com/US/en/sds/aldrich/p14815)
## References
1. [4-Ethoxyaniline, CID 9076, PubChem](https://pubchem.ncbi.nlm.nih.gov/compound/9076)
2. [o-Phenetidine, Chemical Index Database, DrugFuture](https://www.drugfuture.com/chemdata/o-phenetidine.html)
3. [Safety Data Sheet, 4-Ethoxyaniline (P14815), Sigma-Aldrich/MilliporeSigma](https://www.sigmaaldrich.com/US/en/sds/aldrich/p14815)
4. [Final Screening Assessment: Phenacetin, Environment and Climate Change Canada](https://www.canada.ca/en/environment-climate-change/services/evaluating-existing-substances/screening-assessment-phenacetin.html)
5. [p-Phenetidine product page, LANXESS](https://lanxess.com/en-us/products/products/p/p-phenetidine)
6. [Toxicology Reports 9 (2022) 1725–1727, eScholarship](https://escholarship.org/content/qt1n62x6w6/qt1n62x6w6_noSplash_e9683a73d5b8d31360b3823519d75c32.pdf?t=rxapdc)
7. [Screening Assessment: Aromatic Amines subgroup, Environment and Climate Change Canada](https://www.canada.ca/en/environment-climate-change/services/evaluating-existing-substances/screening-assessment-5.html)
8. [US5493055A: Process for the manufacture of p-phenetidine, Google Patents](https://patents.google.com/patent/US5493055A/en)
9. [Bootstrap Synthesis Revisited: A Solid, Cobalt-Derived Catalyst Enables the Hydrogenation of N═N Bonds to Afford Anilines, ACS Omega](https://pubs.acs.org/acsodf/article/11/32/47904/5242284/Bootstrap-Synthesis-Revisited-A-Solid-Cobalt)
10. [Benzenamine, 4-ethoxy-](https://webbook.nist.gov/cgi/cbook.cgi?ID=156-43-4)
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*Topic: Encyclopedia › Physical world and mathematics › Chemistry › Organic substances › Amines and nitrogen functional groups › Aromatic and aryl amines › Anilines and substituted anilines › Anisidines, phenetidines, and alkoxy anilines*
*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 15, 2026 · Edited: Sep 30, 2026; Oct 11, 2026; 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