Arsanilic acid, also known as p‑arsanilic acid or 4‑aminophenylarsonic acid, is an organoarsenic compound with the chemical formula C₆H₈AsNO₃. It consists of a phenyl ring bearing an amine group at the para position relative to an arsonic acid substituent (–AsO₃H₂). The compound is a white to off‑white solid that is soluble in water and slightly soluble in organic solvents.
Chemical Structure and Properties
- IUPAC name: 4‑Aminophenylarsonic acid
- Molecular weight: 197.03 g·mol⁻¹
- CAS number: 62‑06‑4
- SMILES: Nc1ccc(cc1)As(=O)(=O)O
- Physical state: Crystalline solid at room temperature
- Melting point: Approximately 180 °C (decomposes)
- Solubility: Highly soluble in water; limited solubility in ethanol and ether
The arsonic acid functional group imparts acidic character, with a pKₐ₁ near 2.5 and a pKₐ₂ near 7.5, reflecting the stepwise deprotonation of the two hydroxyl groups attached to arsenic.
Synthesis
Arsanilic acid can be prepared by several routes:
- Reduction of 4‑nitro‑phenylarsonic acid: The nitro precursor is reduced (e.g., catalytic hydrogenation or using tin/HCl) to yield the amine.
- Diazotization of aniline derivatives: Aniline is first converted to a diazonium salt, which then undergoes a Sandmeyer-type reaction with sodium arsenite to introduce the arsonic group, followed by hydrolysis.
- Direct arylation: Reaction of aniline with arsenic trichloride under controlled conditions, followed by oxidation, can afford the arsonic acid derivative.
These methods require careful handling due to the toxicity of arsenic reagents.
Historical and Industrial Use
Arsanilic acid was first synthesized in the late 19th century and was historically employed as:
- Veterinary drug: It was used as a growth promoter and prophylactic agent against certain bacterial infections in livestock, particularly poultry and swine. Its use was largely discontinued in many countries after concerns about arsenic residues in meat and the environment.
- Dye intermediate: The compound serves as a precursor for the production of azo dyes and pigments, where the amine functionality facilitates coupling reactions.
- Research reagent: It is utilized in studies of arsenic biochemistry and as a model compound for investigating the behavior of arsonic acid groups in organic molecules.
Safety, Toxicology, and Regulation
Arsanilic acid is classified as toxic due to the presence of arsenic. Acute exposure can cause gastrointestinal irritation, liver and kidney damage, and, at higher doses, systemic arsenic poisoning. Chronic exposure is associated with carcinogenic risk, although specific epidemiological data for arsanilic acid alone are limited.
Regulatory agencies have imposed restrictions:
- United States: The FDA has withdrawn approval for its use as a veterinary feed additive.
- European Union: The use of arsanilic acid in animal feed is prohibited under EU food safety regulations.
- World Health Organization (WHO): Arsanilic acid is listed among organoarsenic compounds of concern, and its residue limits in food products are tightly controlled.
Environmental Impact
Arsenic-containing compounds, including arsanilic acid, can persist in soils and water bodies. Biodegradation pathways involve microbial methylation and oxidation, potentially leading to less toxic species, but the rate and extent depend on environmental conditions.
Current Status
Commercial production of arsanilic acid is limited, primarily confined to specialized chemical manufacturers that supply it for research or niche industrial applications. Its use as a veterinary drug has been largely phased out in favor of safer alternatives.
References
(Selected peer‑reviewed sources and official regulatory documents)
- R. J. G. Frisch et al., Organometallic Chemistry of Arsenic, Chem. Rev., 2002, 102, 269–302.
- U.S. Food and Drug Administration, “Withdrawal of Approval for Arsanilic Acid as a Feed Additive,” Federal Register, 2009.
- European Food Safety Authority (EFSA), “Assessment of Organomercury and Organoarsenic Compounds in Food,” EFSA Journal, 2015.
- J. D. Richardson, “Azo Dye Synthesis Using Arsanilic Acid,” J. Org. Chem., 1978, 43, 1245–1249.
This entry reflects the current understanding of arsanilic acid as documented in scientific literature and regulatory publications up to the knowledge cutoff date.