3-Aminophthalhydrazide
[CAS 521-31-3]

Identification

• Classification: Dyes and pigments >> Dyes >> Reduced dye
• Name: 3-Aminophthalhydrazide
• Synonyms: 5-Amino-2,3-dihydro-1,4-phthalazinedione; Luminol
• Molecular Formula: C₈H₇N₃O₂
• Molecular Weight: 177.16
• CAS Registry Number: 521-31-3
• EC Number: 208-309-4
• SMILES: C1=CC2=C(C(=C1)N)C(=O)NNC2=O

Properties

• Density: 1.6±0.1 g/cm³ Calc.^*
• Melting point: 325 - 330 °C (Expl.)
• Boiling point: 621.9±50.0 °C 760 mmHg (Calc.)^*
• Flash point: 329.9±30.1 °C (Calc.)^*
• Solubility: DMF: < 50 mg/mL, water: < 0.1 g/100 mL 19 °C (Expl.)
• Index of refraction: 1.838 (Calc.)^*

^* Calculated using Advanced Chemistry Development (ACD/Labs) Software.

Safety Data

• Hazard Symbols: GHS07 Warning
• Risk Statements: H302-H315-H319-H335
• Safety Statements: P261-P264-P264+P265-P270-P271-P280-P301+P317-P302+P352-P304+P340-P305+P351+P338-P319-P321-P330-P332+P317-P337+P317-P362+P364-P403+P233-P405-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Skin irritation	Skin Irrit.	2	H315
Specific target organ toxicity - single exposure	STOT SE	3	H335
Eye irritation	Eye Irrit.	2	H319
Acute toxicity	Acute Tox.	4	H302
Eye irritation	Eye Irrit.	2A	H319
Specific target organ toxicity - single exposure	STOT SE	3	H336

Discovery and Applications

3-Aminophthalhydrazide is an aromatic heterocyclic compound derived from phthalhydrazide, containing both hydrazide functionality and an amino substituent on an aromatic fused ring system. It is structurally related to luminol and belongs to the class of substituted phthalazine-1,4-dione derivatives. The compound is notable for its nitrogen-rich framework and its ability to participate in redox and condensation chemistry characteristic of phthalhydrazide systems.

The core structure is based on a fused bicyclic aromatic system consisting of a benzene ring fused to a heterocyclic ring containing two adjacent nitrogen atoms and two carbonyl groups. This arrangement creates a conjugated system in which electron density is delocalized across the aromatic and heterocyclic portions of the molecule. The carbonyl groups at the 1 and 4 positions contribute significant polarization and serve as hydrogen bond acceptors.

The hydrazide portion of the molecule contains adjacent nitrogen atoms within the ring system. These nitrogen atoms can exist in different tautomeric forms depending on conditions, involving proton shifts between nitrogen and oxygen atoms. This tautomerism is a characteristic feature of phthalhydrazide derivatives and influences both chemical reactivity and electronic structure.

At the 3-position of the aromatic ring is an amino substituent (–NH₂). This amino group is electron-donating through resonance, allowing the nitrogen lone pair to interact with the aromatic π-system. This increases electron density in the ring and can influence both reactivity and photophysical behavior. The amino group also provides a site for hydrogen bonding and protonation under acidic conditions.

The combination of electron-donating amino functionality and electron-withdrawing carbonyl groups creates an electronically polarized system. This internal electronic contrast contributes to the compound’s chemical reactivity and plays a role in its behavior in oxidation reactions.

Phthalhydrazide derivatives, including 3-aminophthalhydrazide, are known for their ability to undergo oxidation reactions that generate excited-state intermediates. In related compounds such as luminol, oxidation leads to chemiluminescence. While 3-aminophthalhydrazide shares the same core structural motif, its substituent pattern modifies the electronic distribution and reaction pathways.

The fused ring system is largely planar due to aromatic conjugation and the sp²-hybridized nature of the atoms involved. Planarity allows efficient overlap of p-orbitals and supports delocalization of electrons across the molecule. This structural feature is important for both stability and electronic behavior.

Physicochemically, 3-aminophthalhydrazide is moderately polar due to the presence of carbonyl groups and nitrogen-containing functional groups. The amino group increases hydrogen-bonding capacity, while the aromatic core contributes hydrophobic character. This combination results in a balance between solubility in polar solvents and retention of aromatic stability.

Chemically, the most reactive sites include the amino group and the nitrogen-rich hydrazide system. The amino group can undergo protonation, acylation, or diazotization under appropriate conditions. The hydrazide portion can participate in oxidation reactions, condensation reactions, and coordination with metal ions in some cases.

The carbonyl groups in the fused ring system are relatively stable but play a key role in resonance stabilization and hydrogen bonding interactions. They also contribute to the electron-withdrawing character of the heterocyclic core.

Overall, 3-aminophthalhydrazide is a nitrogen-rich fused aromatic heterocycle containing a phthalhydrazide core with an amino substituent. Its structure combines electron-donating and electron-withdrawing functional groups within a conjugated system, giving rise to distinctive chemical reactivity associated with phthalhydrazide derivatives and related chemiluminescent compounds.

References

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2026. Persistence of bloodstains on domestic surfaces treated with detergents: experimental evaluation using Combur Test®. International Journal of Legal Medicine.
DOI: 10.1007/s00414-026-03742-z

2026. Detection of profenofos based on an aptamer-regulated luminol-Au@CeO2 electrochemiluminescence-colorimetric dual-mode sensor. Microchimica Acta.
DOI: 10.1007/s00604-026-07908-9