2-Propenoicacid, 3-(3,4-dihydroxyphenyl)-

Molecular Formula: C₉H₈O₄
Molecular Weight: 180.16 g/mol 
EINECS: 206-361-2 
Index of Refraction: 1.706
Molar Refractivity: 47.47 cm³
Molar Volume: 121.8 cm³ 
Surface Tension: 77.8 dyne/cm
Density: 1.478 g/cm³
Flash Point: 220 °C
Enthalpy of Vaporization: 70.65 kJ/mol
Boiling Point: 416.8 °C at 760 mmHg
Vapour Pressure: 1.08E-07 mmHg at 25 °C
Melting point: 34-39 °C
Storage tempreture: Store at RT.
Water solubility: Soluble in hot water
Solubility: Ethanol : 50 mg/mL
Appearance: Light yellow to greenish-yellow powder
Structure of Caffeic acid (CAS NO.331-39-5):
                      
IUPAC Name: (E)-3-(3,4-Dihydroxyphenyl)prop-2-enoic acid
Canonical SMILES: C1=CC(=C(C=C1C=CC(=O)O)O)O
Isomeric SMILES: C1=CC(=C(C=C1/C=C/C(=O)O)O)O
InChI: InChI=1S/C9H8O4/c10-7-3-1-6(5-8(7)11)2-4-9(12)13/h1-5,10-11H,(H,12,13)/b4-2+
InChIKey: QAIPRVGONGVQAS-DUXPYHPUSA-N
Product Category of Caffeic acid (CAS NO.331-39-5): Pharmaceutical Intermediates;Aromatic Cinnamic Acids, Esters and Derivatives;Cinnamic acid;Organic acids;Antioxidant;Biochemistry

 Caffeic acid (CAS NO.331-39-5) may be the active ingredient in caffenol, a do-it-yourself black and white photographic developer made from instant coffee. It is also used as a matrix in MALDI mass spectroscopy analyses, and is a key intermediate in the biosynthesis of lignin, one of the principal sources of biomass.

Caffeic acid (CAS NO.331-39-5) hasn't been listed as a carcinogen by NTP, IARC, ACGIH, or CA Prop 65. And the toxicological properties have not been fully investigated. You can see actual entry in RTECS for complete information.

Moderately toxic by intraperitoneal route. An experimental teratogen. Mutation data reported. When heated to decomposition it emits acrid smoke and fumes.
Hazard Codes: Xn,Xi
Risk Statements: 36/37/38-40-63-68
R36/37/38:Irritating to eyes, respiratory system and skin. 
R40:Limited evidence of a carcinogenic effect. 
R63:Possible risk of harm to the unborn child. 
R68:Possible risk of irreversible effects.
Safety Statements: 26-36/37/39-36 
S26: In case of contact with eyes, rinse immediately with plenty of water and seek medical advice.
S36:Wear suitable protective clothing. 
S36/37/39:Wear suitable protective clothing, gloves and eye/face protection.

 Caffeic acid , its cas register number is 331-39-5. It also can be called 3,4-Dihydroxycinnamic acid ; 3,4-Dihydroxybenzeneacrylic acid ; and 3-(3,4-Dihydroxy phenyl)-2-propenoic acid . It is a naturally occurring organic compound, and is found in all plants because it is a key intermediate in the biosynthesis of lignin. It is hazardous, so the first aid measures and others should be known. Such as: When on the skin: first, should flush skin with plenty of water immediately for at least 15 minutes while removing contaminated clothing. Secondly, get medical aid. Or in the eyes: Flush eyes with plenty of water for at least 15 minutes, occasionally lifting the upper and lower eyelids. Then get medical aid soon. While, it's inhaled: Remove from exposure and move to fresh air immediately. Give artificial respiration while not breathing. When breathing is difficult, give oxygen. And as soon as to get medical aid. Then you have the ingesting of the product: Wash mouth out with water, and get medical aid immediately.
In addition, Caffeic acid (CAS NO.331-39-5) could be stable under normal temperatures and pressures. It is not compatible with strong oxidizing agents, strong acids, strong bases, and you must not take it with incompatible materials. And also prevent it to broken down into hazardous decomposition products: Carbon monoxide, carbon dioxide.