Ethyne

Acetylene

Names
Preferred IUPAC name Acetylene
Systematic IUPAC name Ethyne
Identifiers
CAS Number	74-86-2 Y
3D model (Jmol)	Interactive image
ChEBI	CHEBI:27518 Y
ChEMBL	ChEMBL116336 Y
ChemSpider	6086 Y
ECHA InfoCard	100.000.743
KEGG	C01548 Y
UNII	OC7TV75O83 Y
UN number	1001 (dissolved) 3138 (in mixture with ethylene and propylene)
InChI InChI=1S/C2H2/c1-2/h1-2H Y Key: HSFWRNGVRCDJHI-UHFFFAOYSA-N Y InChI=1/C2H2/c1-2/h1-2H Key: HSFWRNGVRCDJHI-UHFFFAOYAY
SMILES C#C
Properties
Chemical formula	C2H2
Molar mass	26.04 g·mol−1
Appearance	Colorless gas
Odor	Odorless
Density	1.097 g/L = 1.097 kg/m3
Melting point	−80.8 °C (−113.4 °F; 192.3 K) Triple point at 1.27 atm
Sublimation conditions	−84 °C; −119 °F; 189 K (1 atm)
Solubility in water	slightly soluble
Vapor pressure	44.2 atm (20 °C)
Acidity (pKa)	25
Magnetic susceptibility (χ)	-12.5·10−6 cm3/mol
Structure
Molecular shape	Linear
Thermochemistry
Std molar entropy (So298)	201 J·mol−1·K−1
Std enthalpy of formation (ΔfHo298)	+226.88 kJ/mol
Hazards
NFPA 704	4 1 3
Autoignition temperature	300 °C (572 °F; 573 K)
US health exposure limits (NIOSH):
PEL (Permissible)	none
REL (Recommended)	C 2500 ppm (2662 mg/m3)
IDLH (Immediate danger)	N.D.
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Y  (what is YN ?)
Infobox references

Acetylene (systematic name: ethyne) is the chemical compound with the formula C₂H₂. It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in its pure form and thus is usually handled as a solution. Pure acetylene is odorless, but commercial grades usually have a marked odor due to impurities.

As an alkyne, acetylene is unsaturated because its two carbon atoms are bonded together in a triple bond. The carbon–carbon triple bond places all four atoms in the same straight line, with CCH bond angles of 180°.

Acetylene was discovered in 1836 by Edmund Davy, who identified it as a "new carburet of hydrogen". It was rediscovered in 1860 by French chemist Marcellin Berthelot, who coined the name "acétylène". Berthelot's empirical formula for acetylene (C₄H₂), as well as the alternative name "quadricarbure d'hydrogène" (hydrogen quadricarbide) were incorrect because chemists at that time used the wrong atomic mass for carbon (6 instead of 12). Berthelot was able to prepare this gas by passing vapours of organic compounds (methanol, ethanol, etc.) through a red-hot tube and collecting the effluent. He also found acetylene was formed by sparking electricity through mixed cyanogen and hydrogen gases. Berthelot later obtained acetylene directly by passing hydrogen between the poles of a carbon arc. Commercially available acetylene gas could smell foul due to the common impurities hydrogen sulfide and phosphine. However, as purity increases it will become odourless.

Since the 1950s, acetylene has mainly been manufactured by the partial combustion of methane or appears as a side product in the ethylene stream from cracking of hydrocarbons. Approximately 400,000 tonnes were produced by this method in 1983. Its presence in ethylene is usually undesirable because of its explosive character and its ability to poison Ziegler-Natta catalysts. It is selectively hydrogenated into ethylene, usually using Pd–Ag catalysts.