Acetylene

Acetylene (systematic name: ethyne) is a chemical compound with the formula C₂H₂ and structure HC≡CH. 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 such as divinyl sulfide and phosphine.

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°. Although indefinitely persistent (i.e., kinetically "stable") at 1 atm at room temperature, acetylene is nevertheless a highly endothermic (i.e., thermodynamically unstable) molecule, with a standard enthalpy of formation of +54.2 kcal/mol. This is a result of the high energy content of the triple bond. The energy stored therein is released when acetylene is burned (or otherwise undergoes a chemical reaction).