Butadiene

Appearance and odour

 Colourless gas; a colourless mobile liquid under pressure or at low temperature; faint aromatic or gasoline-like odour

 Odour threshold in air about 0.23 ppm with a level of distinct odour awareness of about 3.7 ppm; odour is unreliable and must not be used to detect a leak

Melting / freezing point

approx. −108.9 °C

101.3 kPa  BASF and OSHA agree (−164 °F); at 101.3 kPa

Boiling point

approx. −4.4 °C

BASF gives −4.41 °C at 101.3 kPa. The atmospheric boiling point of −4.4 °C means the substance must be kept in pressure vessels at ambient temperature

 Density

 Liquid approx. 0.615 g/cm³ (25 °C) / 0.62 g/cm³ (20 °C); gas approx. 2.24 – 2.3 kg/m³

 Relative density (water = 1) about 0.62 at 68 °F; gas density at 101.3 kPa and 15 – 21 °C

Vapour density

 approx. 1.87 – 1.93

Air = 1; calculated 54.09/28.96 = 1.87, BASF gives 1.93 and OSHA 1.88. Heavier than air, so a leak may travel along the ground and through low-lying areas to distant ignition sources and flash back

Vapour pressure

 approx. 217 kPa (16.85 °C); approx. 255 kPa (21.85 °C)

 BASF gives 2 170 hPa at 16.85 °C and 2 550 hPa at 21.85 °C, equivalent to about 245 kPa at 20 °C; OSHA gives 2.4 atm. Vapour pressure rises steeply with temperature, so storage temperature must be tightly controlled

 Flash point

approx. −76 °C (closed cup)

BASF gives −76.1 °C and OSHA −105 °F (−76 °C); GESTIS gives −85 °C. The extremely low flash point means any flammable concentration at ambient temperature is far above it, which is the primary fire characteristic of the substance

Autoignition temperature

approx. 415 – 420 °C

 BASF gives 420 °C, GESTIS 415 °C and Chinese sources mostly 414 – 420 °C

 Explosion limits

2.0 % – 12.0 %（v/v） | 2.0 % – 12.0 % (v/v)

 In air; BASF, NIOSH and OSHA agree . GESTIS gives a wider range of 1.4 % – 16.3 % (v/v), which should be quoted with its source. With a lower limit as low as 2 %, a leak readily produces a flammable atmosphere

Critical parameters

 approx. 152.0 °C / approx. 4.33 MPa / approx. 0.245 g/cm³

BASF gives a critical temperature of 152.0 °C. Above it the substance cannot be liquefied , so the temperature limit for refrigerated storage and transport must be set strictly

Solubility in water

 approx. 0.735 g/L (20 °C)

 BASF and ECHA agree (735 mg/L); slightly soluble in water and readily soluble in ethanol, diethyl ether, acetone, benzene, carbon tetrachloride and chloroform

 Partition coefficient

 log Pow approx. 1.99

 n-Octanol / water (25 °C, pH 7); weakly lipophilic, with no bioaccumulation potential

Soil adsorption coefficient

 KOC approx. 39.6; log KOC approx. 1.6

 Calculated by BASF; moderately mobile in soil, but the substance is removed mainly by volatilisation and biodegradation

Viscosity

 approx. 0.149 mPa·s (20 °C)

 Dynamic viscosity of the liquid phase; the gas phase has a very low viscosity. Kinematic viscosity is not applicable to a gas

Other constants

 Refractive index 1.429 2; ionisation energy 9.07 – 9.08 eV; liquid heat capacity 123.6 J/(g·K) at 25 °C; combustion products CO and CO₂

 Combustion in air yields mainly carbon monoxide and carbon dioxide, with a marked increase in carbon monoxide under incomplete combustion

Stability and incompatibilities

Stable when inhibited; incompatible with strong oxidising agents

 BASF lists strong oxidising agents only; Chinese sources additionally list acids, halogens and copper/copper alloys and note that transition-metal compounds such as cobalt salts may trigger violent polymerisation. Violent reaction occurs with oxidants, and explosive decomposition is possible above about 200 °C at high pressure