Ethyne (i.e. Acetylene)

Ethyne, C2H2, (i.e. Acetylene) is the first member of the ethyne homologous series of hydrocarbons and is the only member of the series of major industrial importance.

Preparation of Ethyne

Ethyne is made industrially from hydrocarbons obtained by cracking petroleum. The thermal cracking of butane with steam as a diluent. The first reaction is

   3 C4H10  +  2 H2O   ==> 2 HCCH  +  2 C2H4 + 2 CH4 +  2 CO +  7 H2 
                            Ethyne    Ethene   Methane  Carbon  Hydrogen

Subsequent reactions in this gas stream are

         C2H4     ==>     HCCH +    H2      
        Ethene            Ethyne   Hydrogen        


         2 CH4    ==>     HCCH +       3H2     
        Methane           Ethyne     Hydrogen        

Ethyne is also made industrially by the combustion of natural gas, which is mainly methane using insufficient oxygen. The heat evolved in the incomplete combustion is used to heat a further supply of methane to about 1773 degK.

        	2 CH4   +   O2   ==>   2 CO   +   2 H2  

1773 degK 2CH4 ==> HCCH + 3H2

The reaction time is very short (between 0.01 and 0.05 seconds). The optimum yield of ethyne is obtained by rapid cycling of the products.

Ethyne is also made industrially by the arc process in which a temperature of 1773 degK is achieved by an electric discharge. The feedstock is usually a mixture of C3 hydrocarbons and C4 hydrocarbons and the products are mainly ethyne, methane and hydrogen. Contact time is again very short.

The initial step in the purification of ethyne from these thermal processes to absorb it selectively into a suitable solvent, and then to separate the ethyne by fractional distillation. The solvent for the extraction is recycled.

Traditional Preparation of Ethyne

Calcium carbide is prepared by the reaction between calcium oxide and coke at very high temperatures in an electric furnace. The action of water on the calcium carbide then results in the formation of ethyne.

Because calcium oxide and coke are obtained from limestone and coal, respectively, acetylene gas is obtained in a few steps from three abundant cheap raw materials (i.e. water, coal, and limestone).

       Coal +  Coke   +   Ca(OH) 2    ==>     CaC2    
              Carbon     Limestone         Calcium Carbide 

CaC2 + H2O ==> HCCH Calcium Carbide 2000 degC Ethyne

Enormous quantities of acetylene gas are consumed each year as a as fuel for the oxy-acetylene welding torch. The acetylene gas \ used for this purpose is distributed as a solution of acetylene in acetone under pressure contained in tanks.

Acetylene gas is also the starting material for the large scale synthesis of important organic compounds, including ethanoic acid (i.e. acetic acid) and a number of unsaturated compounds that are used to make plastics and synthetic rubber.

Many of the synthetic uses of acetylene have grown out of work done by W.Reppe in Germany before and during World War II. Because of the need to replace petroleum by the more abundant coal as the primary organic source, his work revolutionized the industrial chemistry of acetylene.

Uses of Ethyne

Ethyne is used
