Preparation of Ethyne

Alkynes are hydrocarbons containing one or more than one carbon-carbon triple bonds. According to IUPAC conventions, the names of alkynes are derived from those of the corresponding alkanes by altering the suffix - ane to - yne which indicates a triple bond. Until the 50s, because side reactions were rare, ethyne was used in pharmaceuticals as an anaesthetic medium under the trade name Narcyclen. The gaseous agent in a mixture with oxygen for breathing purposes had to contain 60% of largely purified ethyne. Formerly, ethyne was substantially used for illumination purposes and combusted in mobile acetylene gas lights or carbide lamps, thus making it a very important chemical compound.

Molecular Structure of Ethyne

What is Ethyne?

Ethyne (acetylene) was first discovered in 1836 by the Irish chemist Edmund Davy as a by-product during the medication of metallic potassium by heating an intermixture of calcined potassium tartrate with charcoal.

The chemical formula of ethyne is ${{C}_{2}}{{H}_{2}}$. This compound is an unsaturated hydrocarbon and it's enormously unstable when present in its pure state. Ethyne is described as the simplest alkyne since it consists of only two carbon atoms, which are triply bonded to each other.

Structure of Acetylene

The structure of ethyne (the simplest alkyne) is :

Hybridization of Acetylene

The molecule of ethyne is linear which means all four atoms lie in a straight line. The carbon-carbon triple bond length is only 1.20 Å long. Both carbons are sp- hybridised in the hybrid orbital illustration of acetylene. In an sp-hybridised carbon, the two sp- hybridised orbitals are attained by combining the 2s orbital with the 2px orbital that's acquainted at an angle of 180° with respect to each other (e.g., along the x-axis). The 2p_y and 2p_z orbitals remain unhybridized and are acquainted perpendicularly along the y and z axes, respectively.

Bonding in Ethyne Molecules

The overlap of one sp orbital from each carbon atom forms the C-C sigma bond, while the two C-H sigma bonds are formed by the overlap of the alternate sp orbital on each carbon with a 1s orbital on hydrogen. Each carbon atom still has two half-filled 2p_y and 2p_z orbitals. These two perpendicular pairs of p orbitals form two pi bonds between the carbons, resulting in one sigma bond plus two pi bonds (triple bond).

Preparation of Ethyne

${{C}_{2}}{{H}_{2}}$ (Ethyne gas) is produced by the hydrolysis of calcium carbide in the laboratory. Ethyne can also be prepared by subjecting methane to partial combustion.

$Ca{{C}_{2}}+2{{H}_{2}}O\to {{C}_{2}}{{H}_{2}}+Ca{{\left( OH \right)}_{2}}$

Therefore, acetylene is produced by the hydrolysis of $Ca{{C}_{2}}$ and forms ethyne and calcium hydroxide as final products. Industrial ethyne is prepared by the thermal decomposition or pyrolysis of methane or natural gas at 1500 ℃.

$2C{{H}_{4}}\xrightarrow[\text{electricarc}]{{{1500}^{\circ}}C}{{C}_{2}}{{H}_{2}}+3{{H}_{2}}$

Also, hydrolysis of $Ca{{C}_{2}}$ forms a gas (ethyne gas), which on trimerization gives benzene.

Formation of Benzene from Ethyne

Uses of Ethyne

Until several decades ago, when it was replaced by cheaper ethenes, ethyne was one of the most important raw materials in the chemical industry. Still, ethyne continues to be of significance for numerous uses.

• Ethyne is useful for the artificial maturing and preservation of fruits.

• It's helpful in acetylene lights to induce light.

• Ethyne polymerizes to polyacetylene (polyethylene) in the presence of copper catalysts. When prepared by an alternative methodology, polyacetylene (CH)_x exhibits moderate electrical conductivity.

• It's also helpful to manufacture organic compounds like acetic acid, acetaldehyde, ethyl alcohol, and polymers like PVC, etc.

• It's useful to produce an oxyacetylene flame.

Oxyhydrogen, a combination of ethyne and pure oxygen, can generate a temperature of 2800℃ and is used for cutting and welding steel.

Properties of Ethyne

Physical Properties of Ethyne:

• It's a colourless gas and is lighter than air.

• It has an odour analogous to ether, which resembles a faint garlic odour.

• Slightly soluble in water whereas fully soluble in organic solvents.

• Melting point: – 82°C

• Boiling point: – 75°C

• Liquefies at: – 84°C

• The molar mass of ethyne is equal to 26.038 grams per mole.

Chemical Properties of Ethyne:

1. Combustion

Ethyne burns in the air with sooty flame and gives carbon dioxide, water, and heat.

${{C}_{2}}{{H}_{5}}+5{{O}_{2}}\to 4C{{O}_{2}}+2{{H}_{2}}O$

2. Addition of hydrogen

Ethyne in addition to hydrogen forms initially ethene and eventually forms ethane.

3. Reaction with Halogens

Ethyne reacts with halogen acids to form 1,2-Tetra halo ethanes.

${{C}_{2}}{{H}_{2}}+2{{X}_{2}}\to CH{{X}_{2}}CH{{X}_{2}}$

4. Oxidation of ethyne

Ethyne undergoes oxidation or combustion reactions. Ethyne is oxidised to form Oxalic acid in presence of dilute $KMn{{O}_{4}}$.

${{C}_{2}}{{H}_{2}}+4\left[ O \right]\xrightarrow{KMn{{O}_{4}}}HOOC-COOH$

Interesting Fact

• There are three ways to produce Acetylene; a reaction of water and calcium carbide, passage of hydrocarbons through an electric arc, or by partial combustion of methane with air/ oxygen.

• Carbide lights and Acetylene weren’t just used for movable lighting - headlamps and such - they were also used for other wide-ranging illumination. In the early 20^th century, it was used for street lights, and earlier on, many motor vehicles used carbide lights before electric headlights.

Important Question

1. Is ethyne ignitable?

Ans: Ethyne is highly ignitable and only small measures should be made. Indeed, small quantities of ethyne/ air mixtures can produce dangerous explosions if the mix is close to stoichiometric.

2. How are the natural sources of ethylene used?

Ans: It's a colourless, ignitable gas having a sweet taste and odour. Natural sources of ethylene include both natural gas and petroleum; it's also a naturally occurring hormone in plants, in which it inhibits growth and promotes leaf fall, and in fruits, in which it promotes growth.

Conclusion

Ethyne is the simplest triple-bonded carbon-carbon organic compound, also known as acetylene. Acetylene can be disintegrated into its elements with the liberation of heat. The combustion of acetylene produces a large amount of heat, and, in a duly designed torch, the oxyacetylene flame attains the loftiest flame temperature. Acetylene is produced by the hydrolysis of calcium carbide, by the passage of a hydrocarbon through an electric arc, etc.