Molecular structure of p-Phenylenediamine (CAS NO.106-50-3) is:

Product Name: p-Phenylenediamine
CAS Registry Number: 106-50-3
IUPAC Name: Benzene-1,4-diamine
Molecular Weight: 108.14112 [g/mol]
Molecular Formula: C₆H₈N₂
XLogP3: -0.3
H-Bond Donor: 2
H-Bond Acceptor: 2 
EINECS: 203-404-7
Melting Point: 139 °C
Storage temp.: 2-8°C
Water Solubility: 47 g/L (25 ºC)
Stability: Stable, but oxidizes when exposed to air. Incompatible with oxidizing agents. Store under inert atmosphere.
Surface Tension: 57.5 dyne/cm
Density: 1.15 g/cm³
Flash Point: 135.9 °C
Enthalpy of Vaporization: 50.54 kJ/mol
Boiling Point: 267.4 °C at 760 mmHg
Vapour Pressure: 0.00817 mmHg at 25°C
Other Registry Number: 56481-76-6
Related Registry Number: 16245-77-5 (monosulfate); 540-24-9 (mono-hydrochloride); 624-18-0 (di-hydrochloride); 62654-17-5 (monooxalate)
Product Categories: Variousamine; Intermediates of Dyes and Pigments; 1,4-Benzenediamine; Fluorenes, etc. (reagent for high-performance polymer research); Functional Materials; Reagent for High-Performance Polymer Research; Chemistry

 p-Phenylenediamine (CAS NO.106-50-3) is added to real henna to create so called "Black Henna".In many cases, it can cause allergic reaction, and cause scarring in some people. PPD should never be applied directly to the skin in its pure form or mixed with anything else. This  product is used in almost every hair dye on the market, whatever brand. Also it  is a precursor to aramid plastics and fibers such as Kevlar. It is mainly used as a component of engineering polymers and composites. Its use is controversial because of its high toxicity.It may cause allergic reactions, which makes it a controversial chemical to be included in hair dyes.

When tested by the United States Environmental Protection Agency, the EPA reported that in rats and mice chronically exposed to PPD in their diet, it simply depressed body weights, but there were no other clinical signs of toxicity, as were observed in several studies. From 1992 to 2005 years, thirty-one published articles had been investigating a causal association between PPD and cancer . Associations between personal hair dye use and non-Hodgkin's lymphoma, multiple myeloma, acute leukemia, and bladder cancer  were observed in at least one well-designed study with detailed exposure assessment, but were not consistently observed across studies. 
Hazard Codes: T,N,T+,Xn
Risk Statements: 23/24/25-36-43-50/53-63-36/37/38-45-40 
R23/24/25:Toxic by inhalation, in contact with skin and if swallowed. 
R36:Irritating to eyes. 
R43:May cause sensitization by skin contact. 
R50/53:Very toxic to aquatic organisms, may cause long-term adverse effects in the aquatic environment. 
R63:Possible risk of harm to the unborn child. 
R36/37/38:Irritating to eyes, respiratory system and skin. 
R45:May cause cancer. 
R40:Limited evidence of a carcinogenic effect.
Safety Statements: 28-36/37-45-60-61-28A-24/25-23-53 
S28:After contact with skin, wash immediately with plenty of soap-suds. 
S36/37:Wear suitable protective clothing and gloves. 
S45:In case of accident or if you feel unwell, seek medical advice immediately (show the label whenever possible.) 
S60:This material and its container must be disposed of as hazardous waste. 
S61:Avoid release to the environment. Refer to special instructions / safety data sheets. 
S24/25:Avoid contact with skin and eyes. 
S23:Do not breathe vapour. 
S53:Avoid exposure - obtain special instructions before use.
RIDADR: UN 1673 6.1/PG 3
WGK Germany: 3
RTECS: SS8050000
F: 8-10-23
HazardClass: 6.1
PackingGroup: III
HS Code: 29215119

 p-Phenylenediamine , its cas register number is 106-50-3. It also can be called 1,4-Benzenediamine ; 1,4-Phenylenediamine ; 4-Phenylenediamine ; CI 76060 ; p-Phenylenediamine ; 1,4-Diaminobenzene ; 4-Aminoaniline ; 6PPD ; AI3-00710 ; BASF ursol D ; Benzofur D ; C.I. 76060 ; C.I. Developer 13 ; C.I. Oxidation Base 10 ; CCRIS 509 ; Fenylenodwuamina ; Fouramine D ; Paraphenylenediamine ; Pelagol D ; p-Aminoaniline ; p-Benzenediamine ; p-Diaminobenzene ; p-Fenylendiamin ; p-Phenyldiamine ; p-Phenylenediamine ; 1,4-Benzenediamine ; Phenylenediamine, para- ; p-Phenylenediamine .It is a white to light purple solid.