4-Aminophenylacetic Acid: Properties, Uses & FAQs

4-Aminophenylacetic acid is a bifunctional aromatic compound bearing both a primary amine and an acetic acid group on a para-substituted benzene ring, making it a versatile building block in pharmaceutical synthesis, bioconjugation chemistry, and materials science research.

CAS Number

1197-55-3

Molecular Formula

C₈H₉NO₂

Molecular Weight

151.16 g/mol

Appearance

White to off-white crystalline powder

Functional Groups

Primary aromatic amine (–NH₂) and carboxylic acid (–COOH)

SKU (Aladdin)

A110093

Chemical Identity and Physical Properties

4-Aminophenylacetic acid (CAS 1197-55-3), also known as para-aminophenylacetic acid, is an aromatic compound with the molecular formula C₈H₉NO₂ and a molecular weight of approximately 151.16 g/mol. Its structure consists of a benzene ring bearing a primary amine group (–NH₂) at the para position and a –CH₂COOH (acetic acid) side chain, distinguishing it from the structurally related amino acid tyrosine or phenylglycine.

The compound typically presents as a white to off-white crystalline powder. It possesses two ionisable groups—the amine (pKa ~4–5 for conjugate acid) and the carboxylic acid (pKa ~2–3)—giving it amphoteric character and moderate solubility in polar solvents such as water, methanol and ethanol, while being sparingly soluble in non-polar organic solvents. Researchers should note that the free amine renders the compound susceptible to oxidation upon prolonged exposure to air or light; storage under inert atmosphere at 2–8 °C in a tightly sealed container is recommended.

Applications in Organic Synthesis and Pharmaceutical Research

The dual functionality of 4-aminophenylacetic acid—an amine and a carboxylic acid—makes it a highly useful synthetic intermediate. In medicinal chemistry, it serves as a precursor or coupling partner for the preparation of amides, peptide mimetics, ureas, sulfonamides and heterocyclic scaffolds. Its para-amino group can undergo diazotisation, acylation and reductive amination reactions, while the carboxylic acid can be activated for esterification or amide bond formation.

In bioconjugation and linker chemistry, the compound is exploited as a bifunctional spacer to attach payloads to antibodies, nanoparticles or solid supports. The aromatic amine also makes it suitable as a monomer for polyimide and polyamide synthesis in materials science. Additionally, it has been investigated as an intermediate in the synthesis of non-steroidal anti-inflammatory drug analogues and other biologically active small molecules.

Handling, Safety and Storage Guidelines

As with all primary aromatic amines, 4-aminophenylacetic acid should be handled with appropriate care. Users should consult the current Safety Data Sheet (SDS) before use. General precautions include wearing nitrile gloves, safety glasses and a lab coat; working in a well-ventilated fume hood; and avoiding generation of dust during weighing.

Waste disposal must comply with local and institutional regulations governing aromatic amine-containing materials. The compound should be stored at 2–8 °C in a tightly closed container, protected from moisture, light and air. Under recommended conditions, the material is expected to remain stable; however, discolouration (yellowing or browning) may indicate oxidative degradation and the lot should be re-evaluated before use.

Purity, Grade and Quality Specifications

Aladdin Scientific offers 4-aminophenylacetic acid (SKU: A110093) as a research-grade reagent. Researchers requiring precise purity data—including HPLC purity percentage, residual solvent levels and heavy-metal content—should refer to the Certificate of Analysis (CoA) supplied with each lot, available on the Aladdin website by entering the lot number.

For synthetic applications where high purity is critical (e.g., API intermediate synthesis or quantitative bioconjugation), users are advised to request or download the current lot CoA before commencing work and, if necessary, perform in-house re-characterisation by NMR (¹H and ¹³C), HPLC or melting-point determination. The theoretical melting point for 4-aminophenylacetic acid is reported in the literature at approximately 199–202 °C; deviation from this range may warrant additional purification.

Frequently asked questions

What is the CAS number for 4-aminophenylacetic acid and how does it differ from related compounds?

The CAS number for 4-aminophenylacetic acid is 1197-55-3. It differs from the ortho-isomer (2-aminophenylacetic acid, CAS 2835-06-5) and the meta-isomer (3-aminophenylacetic acid, CAS 1197-56-4) in the position of the amino substituent on the benzene ring. The para configuration generally confers distinct reactivity, solubility, and spectroscopic properties compared with its isomers.

In what solvents can 4-aminophenylacetic acid be dissolved for use in reactions or assays?

4-Aminophenylacetic acid shows moderate solubility in polar protic solvents such as water (particularly at slightly alkaline pH where the carboxylate is deprotonated), methanol and ethanol. It is also soluble in dimethyl sulfoxide (DMSO) and dimethylformamide (DMF), which are commonly used as stock-solution vehicles in biological assays. Solubility in non-polar solvents such as hexane, toluene and diethyl ether is low. Users should prepare fresh solutions or protect stock solutions from air to prevent amine oxidation.

How should 4-aminophenylacetic acid be stored to maintain its stability and purity?

4-Aminophenylacetic acid should be stored at 2–8 °C in a tightly sealed container, under dry conditions and protected from light and air. The primary aromatic amine is prone to oxidative degradation, which manifests as yellowing or browning of the powder. When removed from cold storage, the container should be allowed to equilibrate to room temperature before opening to avoid moisture condensation on the product. Always check the lot-specific Certificate of Analysis and re-analyse the material if visible discolouration is observed.

Can 4-aminophenylacetic acid be used as a building block in peptide coupling reactions?

Yes. The carboxylic acid group of 4-aminophenylacetic acid can be activated using standard coupling reagents (e.g., EDC, HATU, DCC) to form amide bonds with amine-containing substrates, or the free amine can be coupled to activated carboxylate partners. However, the aromatic amine is significantly less nucleophilic than aliphatic amines, so reaction conditions may need optimisation—including choice of coupling reagent, base and solvent—to achieve satisfactory yields. Researchers should also be aware of potential side reactions involving the amine during prolonged activation steps.

What analytical methods are recommended to confirm the identity and purity of 4-aminophenylacetic acid?

Identity can be confirmed by ¹H NMR (characteristic aromatic proton signals and NH₂/CH₂/COOH resonances), ¹³C NMR, infrared spectroscopy (N–H stretch ~3300–3400 cm⁻¹; C=O stretch ~1700 cm⁻¹), and high-resolution mass spectrometry. Purity is most conveniently assessed by reverse-phase HPLC with UV detection (typically at 254 nm or 280 nm). Melting point determination (literature value approximately 199–202 °C) provides a rapid additional check. Users should refer to the Aladdin lot CoA for validated purity data specific to the batch received.

References

1. Sigma-Aldrich / Merck product entry for 4-Aminophenylacetic acid, CAS 1197-55-3. Accessed via supplier catalog.
2. March, J. Advanced Organic Chemistry: Reactions, Mechanisms, and Structure. 4th ed. Wiley-Interscience, 1992. (General reference for aromatic amine reactivity and amide bond formation.)
3. Clayden, J.; Greeves, N.; Warren, S. Organic Chemistry. 2nd ed. Oxford University Press, 2012. (Reference for functional group reactivity including aromatic amines and carboxylic acids.)
4. Aladdin Scientific Certificate of Analysis, SKU A110093, 4-Aminophenylacetic acid. (Lot-specific purity and physicochemical data — obtain from aladdin-e.com.)

Products are supplied for research and development use only. Not for use in humans, animals, diagnosis, or therapy.