C₂H₂ — Acetylene

Acetylene (C₂H₂) is the simplest member of the alkyne family of hydrocarbons, characterized by the presence of a carbon-carbon triple bond. It is a colorless and highly flammable gas with a faint garlic-like odor. Acetylene is industrially significant due to its high flame temperature, making it ideal for welding and cutting metals in oxy-acetylene torches.

Discovered in 1836 by Edmund Davy, acetylene later became one of the most important industrial gases. Apart from its role as a fuel, it is a crucial building block in organic synthesis, used for producing compounds like vinyl chloride, acetaldehyde, and synthetic rubber. Its chemical reactivity stems from its triple bond, which allows it to participate in addition reactions easily.

The chemical formula of acetylene is \(C_2H_2\). Its structural formula can be written as \(H–C≡C–H\), showing a triple bond between the two carbon atoms. Each carbon atom forms one sigma (σ) bond with hydrogen and a triple bond (one σ and two π bonds) with the other carbon atom.

The carbon atoms in acetylene are sp hybridized. Each carbon atom uses one sp orbital to form a sigma bond with hydrogen and another sp orbital to form a sigma bond with the other carbon atom. The remaining two unhybridized p orbitals on each carbon overlap sideways to form two π bonds. This results in a linear molecule with a bond angle of 180°.

The C≡C bond in acetylene is very strong, with a bond energy of approximately 839 kJ/mol. This triple bond makes acetylene more reactive than alkenes or alkanes and capable of undergoing various addition reactions.

Acetylene can be prepared by several laboratory and industrial methods:

1. Laboratory Preparation from Calcium Carbide

When calcium carbide reacts with water, acetylene gas is liberated along with calcium hydroxide. This is the most common laboratory method:

The reaction is exothermic, and the gas is collected over water. This method is also used for small-scale industrial production.

2. Industrial Preparation by Partial Combustion of Methane

On a large scale, acetylene is manufactured by the partial combustion of methane in a controlled oxygen supply:

This process involves very high temperatures (around 1500°C) and yields acetylene along with hydrogen gas.

3. Thermal Cracking of Hydrocarbons

Acetylene can also be produced by pyrolysis of hydrocarbons such as ethylene or propane at high temperatures (1000–1500°C):

4. Electric Arc Method

Earlier, acetylene was prepared by passing an electric arc between carbon electrodes in a hydrogen atmosphere:

This method is energy-intensive and now rarely used.

Physical Properties:

• Acetylene is a colorless gas with a faint garlic-like odor.
• It is slightly soluble in water but readily soluble in acetone and alcohol.
• It has a boiling point of -84°C and a melting point of -80.8°C.
• Acetylene is lighter than air and highly flammable.
• It can explode under pressure if not stored properly.

Chemical Properties:

• 1. Combustion: Acetylene burns in oxygen to form carbon dioxide and water with a very high flame temperature (~3300°C), making it ideal for oxy-acetylene welding.

\(2C_2H_2 + 5O_2 \rightarrow 4CO_2 + 2H_2O\)

• 2. Hydrogenation: Acetylene can be hydrogenated in the presence of a nickel or palladium catalyst to form ethylene or ethane:

\(C_2H_2 + H_2 \xrightarrow{Ni} C_2H_4 \xrightarrow{+H_2} C_2H_6\)

• 3. Addition of Halogens: Acetylene reacts with halogens like bromine or chlorine to form tetrahaloethanes:

\(C_2H_2 + 2Br_2 \rightarrow C_2H_2Br_4\)

• 4. Addition of Hydrogen Halides: Acetylene reacts with hydrogen halides to form haloalkenes and haloalkanes:

\(C_2H_2 + HBr \rightarrow C_2H_3Br \xrightarrow{+HBr} C_2H_4Br_2\)

• 5. Polymerization: Acetylene polymerizes under specific conditions to form benzene or other cyclic compounds:

\(3C_2H_2 \xrightarrow{Cu_2Cl_2} C_6H_6\)

• 6. Oxidation: Controlled oxidation of acetylene with cold dilute potassium permanganate produces oxalic acid:

\(C_2H_2 + 2[O] \xrightarrow{} (COOH)_2\)

• 1. Welding and Metal Cutting: Acetylene is widely used in oxy-acetylene torches for welding and cutting metals due to its high flame temperature when burned with oxygen.
• 2. Chemical Synthesis: Used as a raw material in the production of acetaldehyde, acetic acid, vinyl chloride, and synthetic rubbers.
• 3. Illumination: In the past, acetylene lamps were used in miners’ helmets and bicycles because of its bright luminous flame.
• 4. Organic Synthesis: Acetylene serves as a precursor in synthesizing many organic compounds like alcohols and aldehydes through addition reactions.
• 5. Polymer Production: Used in the manufacture of polyvinyl chloride (PVC) and acrylonitrile for plastics and textiles.

Acetylene is a highly flammable and explosive gas under pressure. It should always be stored in acetone-saturated cylinders to prevent spontaneous decomposition. Direct exposure to acetylene can cause dizziness, headache, or nausea due to oxygen displacement.

Although not considered toxic, acetylene must be handled carefully to prevent fire or explosion hazards. Proper ventilation, flame arrestors, and leak checks are essential for safe industrial use.

Environmentally, acetylene does not contribute significantly to greenhouse gas effects but should still be managed to avoid accidental release.