P-Phenylenediamine (106-50-3) Physical and Chemical Properties
P-Phenylenediamine
A primary aromatic diamine used as a dye intermediate and reagent in specialty chemical and polymer manufacturing for industrial formulation and R&D.
| CAS Number | 106-50-3 |
| Family | Aromatic diamines / Phenylenediamines |
| Typical Form | Powder or crystalline solid |
| Common Grades | BP, EP, JP, Technical Grade, USP |
P‑Phenylenediamine is a primary aromatic diamine of the benzene series (structural class: para‑phenylenediamines). The molecule is benzene‑1,4‑diamine with two symmetrically disposed primary amino groups at the 1 and 4 positions of the ring, giving the formula C6H8N2 and a molecular weight of 108.14. The para substitution enforces a planar aromatic core with limited torsional freedom (RotatableBondCount = 0), two hydrogen‑bond donors and two acceptors, and a moderate topological polar surface area (TPSA = 52) that governs aqueous solvation and membrane partitioning. The free base is neutral at ambient pH but is readily protonated to mono‑ and di‑cationic forms; the measured \(\mathrm{p}K_a\) of the conjugate acid is \(\mathrm{p}K_a = 6.2\), so protonation state and salt formation strongly influence solubility and environmental fate.
Electronically the molecule behaves as a moderately activating, weak aromatic base. The lone pairs on the two amino substituents are conjugated with the aromatic ring, increasing nucleophilicity at ring positions and providing facile pathways for oxidative transformation to quinone and quinone‑imines. The compound oxidizes on exposure to air (product colours range from purple to black) and can undergo radical or enzyme‑mediated conversion to p‑quinone species; these oxidative products are mechanistically implicated in its dyeing action and in occupational sensitization. Lipophilicity indicators are low (XLogP = -0.3; measured log Kow = -0.25), consistent with appreciable aqueous solubility, but the primary amine functionality confers reactivity toward electrophiles and oxidants and the capacity to form crystalline salts for formulation.
Common commercial grades reported for this substance include: BP, EP, JP, Technical Grade, USP.
Basic Physical Properties
Density
- Relative density (water = 1): 1.1
- Descriptive statement from physical data: "Greater than 1 (water= 1)"
Qualitative note: As a crystalline solid the bulk density will depend on particle form (flakes, plates, pellets). The solid is heavier than water in bulk form; solution densities will vary with concentration and salt form.
Melting Point
Reported experimental values (various sources): - 284 °F (NTP, 1992) - 145–147 °C - 139–147 °C - 295 °F
Practical note: Solid samples often show polymorphism and may contain traces of oxidized material that alter apparent fusion behavior; reported melting ranges reflect batch and conditioning differences (sample purity, hydrate or salt forms).
Boiling Point
Reported experimental values: - 513 °F at 760 mmHg (NTP, 1992) - 267 °C - 513 °F
Operational note: The compound sublimes and decomposes on strong heating; distillation is not commonly used for purification due to thermal decomposition and oxidation products.
Vapor Pressure
Reported experimental values: - less than 1 mmHg (NIOSH, 2024) - 0.005 [mmHg] - <1 mm Hg at 21 °C (technical product) - Vapor pressure, Pa at 100 °C: 144
Interpretation: Vapor pressure is very low at ambient temperature; volatility is negligible under normal handling conditions, although vapor‑phase degradation by atmospheric radicals can be relevant for environmental fate.
Flash Point
Reported experimental values: - 311 °F (NTP, 1992) - 156 °C c.c. - 312 °F
Fire‑safety note: Classified as combustible rather than highly flammable; handling should minimize ignition sources, especially for finely divided powders that form explosive dust/air mixtures.
Chemical Properties
Solubility and Phase Behavior
Reported solubility data: - 4 % at 75 °F (NIOSH, 2024) - Solubility in water, g/100 mL at 25 °C: 4 - Water solubility = 38,000 ppm - Soluble in alcohol, chloroform, ether; soluble in hot benzene; soluble in 100 parts cold water (multiple historical statements)
Behavioral summary: p‑Phenylenediamine is moderately soluble in water in its free base form and substantially more soluble when converted to protonated salts. Solubility is solvent‑dependent: appreciable solubility in polar organics (alcohols) and in non‑polar solvents when heated. Salt formation (hydrochloride, sulfate, monooxalate, etc.) is a standard approach to enhance aqueous solubility for formulation and control dermal absorption. The measured \(\mathrm{p}K_a\) of \(\mathrm{p}K_a = 6.2\) indicates that significant protonation occurs at mildly acidic pH, shifting the liquid–solid equilibrium and partitioning behavior.
Reactivity and Stability
Stability and reactivity observations: - Oxidizes on exposure to air; "ON STANDING IN AIR, OXIDIZES TO PURPLE AND BLACK/COLOR/." - Reacts readily with oxidizing agents; contact with strong oxidizers may cause fires and explosions. - May be incompatible with isocyanates, halogenated organics, peroxides, phenols (acidic), epoxides, anhydrides, and acid halides. - Decomposition releases toxic gases and vapours (oxides of nitrogen, carbon monoxide); heating to decomposition emits acrid smoke and irritating fumes. - Autoignition temperature: 400 °C
Practical implications: Store under inert atmosphere or in the dark when long‑term stability is required; avoid oxidants and strong electrophiles. Finely divided material poses both dust‑explosion and enhanced oxidation risks.
Thermodynamic Data
Standard Enthalpies and Heat Capacity
No experimentally established value for this property is available in the current data context.
(Quantitative standard enthalpies of formation and heat capacities are not reported in the supplied experimental dataset; selection of thermal data for process design should rely on project‑specific measurements or validated literature thermochemical compilations.)
Molecular Parameters
Molecular Weight and Formula
- Molecular formula: C6H8N2
- Molecular weight: 108.14
- Exact/monoisotopic mass: 108.068748264
These values apply to the neutral free base; salt forms (hydrochloride, sulfate, etc.) alter molecular weight per formula unit.
LogP and Polarity
- XLogP: -0.3
- log Kow (reported): -0.25
- Topological polar surface area (TPSA): 52
- Hydrogen bond donors: 2
- Hydrogen bond acceptors: 2
Interpretation: Low lipophilicity (negative logP) with significant polarity for an aromatic compound. The combination of low logP and two primary amines yields appreciable aqueous solubility and limited bioaccumulation potential; however, protonated species partitioning and skin absorption depend on formulation and exposure conditions.
Structural Features
- SMILES: C1=CC(=CC=C1N)N
- InChI: InChI=1S/C6H8N2/c7-5-1-2-6(8)4-3-5/h1-4H,7-8H2
- InChIKey: CBCKQZAAMUWICA-UHFFFAOYSA-N
- Structural class: para‑substituted aromatic diamine (1,4‑phenylenediamine)
- Rotatable bonds: 0; planar aromatic ring; conjugated amino substituents.
- Crystal structure references: CCDC Number 774087 (crystallographic data available for the free base and related forms).
Reactivity consequence: The para‑diamino motif confers high nucleophilicity and facile oxidation to quinone/quinone‑imine species, which underlie dyeing mechanisms and contribute to contact sensitization.
Identifiers and Synonyms
Registry Numbers and Codes
- CAS: 106-50-3
- EC (EINECS) number: 203-404-7
- UN number: 1673
- UN/NA shipping description: UN 1673; Phenylenediamines
- UN hazard class: 6.1 (as reported)
- InChIKey: CBCKQZAAMUWICA-UHFFFAOYSA-N
- InChI: InChI=1S/C6H8N2/c7-5-1-2-6(8)4-3-5/h1-4H,7-8H2
- SMILES: C1=CC(=CC=C1N)N
- UNII: U770QIT64J
- ChEBI: CHEBI:51403
- ChEMBL: CHEMBL403741
- DrugBank: DB14141
- RTECS: SS8050000
Synonyms and Structural Names
Common synonyms reported in technical usage include: - benzene‑1,4‑diamine - 1,4‑phenylenediamine - para‑phenylenediamine - p‑phenylenediamine - paraphenylenediamine - 4‑phenylenediamine - p‑diaminobenzene - 4‑aminoaniline - Orsin - "black henna" (term used in hair dye contexts)
(Commercial and literature sources may use a wider set of trade and depositor synonyms; selection of a specific name should match regulatory and procurement documents.)
Industrial and Commercial Applications
Representative Uses and Industry Sectors
- Main industrial roles: dye intermediate and oxidation dye (permanent hair dyes, fur dyes), photographic developing agent, and intermediate in the manufacture of antioxidants and vulcanization accelerators for rubber.
- Chemical intermediate uses: synthesis of aramid fibres (e.g., precursors for high‑performance polyamides/diisocyanates), azo and other dye production routes.
- Formulation uses: component in hair dye formulations (requires attention to sensitization), developers and oxidation bases for dye systems.
- Industry sectors: textile dyeing and finishing, leather processing, photographic chemicals, rubber compounding, specialty chemical intermediates, cosmetics/personal care (restricted use and regulatory labelling applies).
Market and process note: High production volumes reflect wide use as a chemical intermediate; occupational exposure can occur in dye and fiber manufacturing, dyeing operations, and formulation facilities.
Role in Synthesis or Formulations
- Acts as a nucleophilic coupling partner and can be oxidatively converted to reactive quinone species used to form coloured oxidation products (the basis of permanent oxidative hair dyes and certain photographic developer chemistries).
- Used as a precursor for N,N'‑disubstituted p‑phenylenediamines, antioxidants in gasoline and rubber, and as a vulcanization accelerator in elastomer chemistry.
- Salts (hydrochloride, sulfate, monooxalate) are used to control solubility and stability in formulations; specification and impurity controls (e.g., o‑phenylenediamine content, metal contamination) are important for product performance and safety.
Safety and Handling Overview
Acute and Occupational Toxicity
Key toxicity and exposure data: - Acute oral/inhalation/dermal hazards reported: GHS hazard statements include H301 (toxic if swallowed), H311 (toxic in contact with skin), H331 (toxic if inhaled), H317 (may cause an allergic skin reaction), H319 (causes serious eye irritation), and aquatic toxicity statements (H400/H410). - LC50 (rat, inhalation): 920 mg/m3/4H - Reported LD/LDLo values: LDLo rat oral 100 mg/kg; LDLo rat ip 50 mg/kg; LDLo rat sc 170 mg/kg; LD50 dog iv 17 mg/kg - Occupational exposure limits: - Permissible Exposure Limit (PEL‑TWA 8‑hr): \(\,0.1\,\mathrm{mg}\,\mathrm{m}^{-3}\) (skin designation) - Recommended Exposure Limit (REL‑TWA): \(\,0.1\,\mathrm{mg}\,\mathrm{m}^{-3}\) (skin) - Threshold Limit Value (TLV‑TWA): \(\,0.1\,\mathrm{mg}\,\mathrm{m}^{-3}\) - Immediately Dangerous to Life or Health (IDLH): 25 mg/m3
Hazard summary and occupational considerations: P‑Phenylenediamine is a potent skin and respiratory sensitizer that can cause allergic contact dermatitis, asthma, and systemic effects following dermal absorption or high acute exposures. Sensitized individuals may react to very low airborne or dermal concentrations. Medical surveillance for exposed workers (skin exams, liver function tests where indicated) is recommended. Appropriate PPE and engineering controls are essential for exposure prevention.
Storage and Handling Considerations
- Storage: Keep containers well‑closed, protected from light, and separated from strong oxidizing agents, acids, and incompatible materials. Recommended storage in a cool, ventilated area.
- Handling controls: Use local exhaust ventilation and closed systems where feasible; eliminate ignition sources for combustible dusts; control dust formation to avoid explosion hazards.
- Personal protective equipment: Impervious gloves, eye/face protection, and protective clothing to prevent skin contact; respiratory protection when airborne concentrations may exceed occupational limits (selection per established respirator guidance and exposure scenario).
- Spill and release: Avoid dispersal of dust; dampen spills where appropriate to minimize dust, collect into suitable containers, and prevent release to waterways. Do not absorb with combustible materials (e.g., sawdust).
- First aid basics: For inhalation, move to fresh air and seek medical attention. For skin contact, wash thoroughly with soap and water for at least 15 minutes and remove contaminated clothing. For eye contact, irrigate with water for 20–30 minutes and obtain medical attention. For ingestion, do not induce vomiting; seek immediate medical attention and follow local clinical guidance.
Regulatory and emergency note: The substance is classified for transport and is associated with hazardous goods protocols (UN 1673 reported); packaging and shipping must comply with applicable transport regulations. For detailed hazard, transport and regulatory information, users should refer to the product‑specific Safety Data Sheet (SDS) and local legislation.
