Acetylene

Acetylene, or ethyne, is the simplest alkyne and an unsaturated hydrocarbon. Its chemical formula, C₂H₂, signifies that it consists of two carbon atoms connected by a triple bond, with each carbon atom also bonded to a hydrogen atom. 

1.0Introduction

Acetylene, or ethyne, is the simplest alkyne and an unsaturated hydrocarbon. Its chemical formula, C₂H₂, signifies that it consists of two carbon atoms connected by a triple bond, with each carbon atom also bonded to a hydrogen atom. This versatile compound is commonly used as a fuel and precursor in synthesising various chemical compounds. While pure acetylene is colourless and odourless, it is often mixed with a distinct odorant for safety purposes. Due to its instability, acetylene is typically stored and dissolved in a solvent like acetone under pressure, ensuring safer handling and transportation.

2.0Physical Properties

• Appearance: Colorless gas
• Odor: Odorless
• Density: 1.1772 kg/m³ at 0°C and 101.3 kPa
• Melting point: −80.8°C
• Solubility: Slightly soluble in water and alcohol; soluble in acetone and benzene.

3.0Formula and Structure

The chemical formula of acetylene is C₂H₂, indicating it consists of two carbon atoms and two hydrogen atoms. The expanded representation can be written as HC≡CH, highlighting the triple bond between the carbon atoms. Acetylene has a molar mass of 26.038 g/mol.

Acetylene's structural formula is C₂H₂. It has a sp hybridization and a linear molecular geometry, with the carbon and hydrogen atoms arranged in a straight line at an angle of 180°.

A triple bond, consisting of connecting the two carbon atoms in acetylene:

1. One sigma bond is formed by overlapping one sp hybrid orbital from each carbon atom.
2. The side-by-side overlap of the unhybridized p orbitals on each carbon atom forms two pi bonds.

4.0Acidic Behavior of Acetylene

Acetylene exhibits weak acidic properties due to the nature of its carbon-hydrogen (C-H) bonds. The significant s-orbital character (50%) in the sp hybridized carbon atoms draws electron density closer to the carbon, weakening the C-H bond. This makes the hydrogen atom more easily removable by a strong base, forming a negatively charged acetylide ion (C₂²⁻).

5.0Chemical Properties of Acetylene

Acetylene participates in various chemical reactions due to its high reactivity, mainly due to its triple bond. Here are some key reactions:

1. Vinylation

• In the presence of a catalyst, acetylene reacts with hydrogen halides (e.g., HCl) or other compounds to form vinyl derivatives.
• Example: $C_2{H}_2+HCl\stackrel{Catalyst}{\to }C{H}_2=CHCl$

2. Hydration

• Acetylene undergoes hydration in the presence of a catalyst (e.g., Hg²⁺) to form an enol, which tautomerises to yield acetaldehyde.
• Example: $C_2{H}_2+H_{2}O\stackrel{HgS{O}_4}{\to }C{H}_{3}CHO$

3. Hydrohalogenation

• Acetylene reacts with halogens like HBr or HI to form haloalkenes or dihaloalkanes.
• Example: $C_2{H}_2+2HBr\to C{H}_{2}Br-C{H}_{2}Br$

4. Addition to Formaldehyde

• Acetylene reacts with formaldehyde to form 1,4-butanediol, which is helpful in the synthesis of polymers and other chemicals.
• Example: $C_2{H}_2+2C{H}_{2}O\to HO-C{H}_2-C\equiv C-C{H}_2-OH$

5. Carbonylation

• Acetylene reacts with carbon monoxide to produce acrylic acid or related compounds under specific conditions.

6. Organometallic Chemistry

• Acetylene reacts with metals or their derivatives to form organometallic compounds, such as metal acetylides, used in organic synthesis and catalysis.
• Example:$C_2{H}_2+NaN{H}_2\to NaC\equiv CH$
• Copper(I) acetylide is produced by reacting copper chloride with acetylene. 
• Similarly, silver acetylide is commonly prepared using silver nitrate. When acetylene reacts with a strong base such as potassium hydroxide (KOH), it forms potassium acetylide as shown in the reaction below:
• $HC\equiv CH+KOH\rightleftharpoons HC\equiv CK+H_{2}O$

7. Acid-Base Reactions

• Due to its weak, acidic nature, acetylene reacts with strong bases like sodium amide (NaNH₂) or potassium hydroxide (KOH) to form acetylides.
• Example:$C_2{H}_2+NaN{H}_2\to NaC\equiv CH+N{H}_3$

6.0Preparation of Ethyne (Acetylene)

1. From Calcium Carbide (Industrial Method):

On an industrial scale, ethyne is prepared by reacting calcium carbide (CaC₂) with water. This method involves the following steps:

Step 1: Preparation of Calcium Carbide

• Calcium carbide is produced by heating quick lime (CaO) with coke (C) in an electric arc furnace: $CaO+3C\stackrel{2000^{o}C}{\to }Ca{C}_2+CO$

Step 2: Preparation of Quick Lime

• Quick lime is obtained by heating limestone (CaCO₃):
  $CaC{O}_3\stackrel{\Delta }{\to }CaO+C{O}_2$

Step 3: Hydrolysis of Calcium Carbide

• Calcium carbide reacts with water to produce acetylene (C₂H₂​) and calcium hydroxide (Ca(OH)₂​):
• $Ca{C}_2+2H_{2}O\to Ca(OH{)}_2+C_2{H}_2$

2. From Vicinal Dihalides:

Ethyne can also be prepared by dehydrohalogenation of vicinal dihalides. This method involves the following steps:

Step 1: Formation of Alkenyl Halide

• Vicinal dihalides are treated with alcoholic potassium hydroxide (KOH), eliminating one molecule of hydrogen halide (HX) to form an alkenyl halide:

R-CHX-CHX-R+ KOH → R-CH=CHX+KX + H₂O

Step 2: Formation of Alkyne

• The alkenyl halide is further treated with sodamide (NaNH₂​), which eliminates the remaining hydrogen halide to yield the alkyne:
  R-CH=CHX + NaNH₂ → R-C≡CH + NaX + NH₃​

7.0Uses of Acetylene

• Welding and Cutting: Used in industrial processes due to its high flame temperature (3300°C).
• Lighting: Utilized for incandescent lighting in some regions.
• Fuel and Explosives: Previously, they were the primary fuel used in explosives.
• Plastics Production: Key raw material for plastics and acrylic acid derivatives.
• Radiocarbon Dating Plays a role in the dating process.
• Semiconductor: Used in making polyacetylene, the first natural semiconductor.
• Brazing and Soldering: Essential for joining metals.
• Glass and Rubber: Used in glass manufacturing and synthetic rubber production.
• Food Preservation: Added to preserve food.