What is acetylene, and what is its formula?

Acetylene is a hydrocarbon with the chemical formula C₂H₂. It is the simplest alkyne, consisting of two carbon atoms triple-bonded to each other and two hydrogen atoms.

How is acetylene prepared?

Acetylene can be prepared: 1. From calcium carbide: By reacting calcium carbide with water. 2. From vicinal dihalides: Through dehydrohalogenation using alcoholic potassium hydroxide, followed by treatment with sodamide.

Why is acetylene used in welding?

Acetylene flames can reach temperatures up to 3300°C, making them suitable for cutting and welding metals.

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Acetylene

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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:

One sigma bond is formed by overlapping one sp hybrid orbital from each carbon atom.
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:

Vinylation

In the presence of a catalyst, acetylene reacts with hydrogen halides (e.g., HCl) or other compounds to form vinyl derivatives.
Example: $C_{2} H_{2} + H Cl C a t a l ys t C H_{2} = C H Cl$

Hydration

Acetylene undergoes hydration in the presence of a catalyst (e.g., Hg²⁺) to form an enol, which tautomerises to yield acetaldehyde.
Example: $C_{2} H_{2} + H_{2} O H g S O_{4} C H_{3} C H O$

Hydrohalogenation

Acetylene reacts with halogens like HBr or HI to form haloalkenes or dihaloalkanes.
Example: $C_{2} H_{2} + 2 H B r C H_{2} B r - C H_{2} B r$

Addition to Formaldehyde

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

Carbonylation

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

Organometallic Chemistry

Acetylene reacts with metals or their derivatives to form organometallic compounds, such as metal acetylides, used in organic synthesis and catalysis.
Example: $C_{2} H_{2} + N a N H_{2} \to N a C \equiv C H$
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:
$H C \equiv C H + K O H ⇌ H C \equiv C K + H_{2} O$

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: $C_{2} H_{2} + N a N H_{2} \to N a C \equiv C H + N H_{3}$

6.0Preparation of Ethyne (Acetylene)

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: $C a O + 3 C 200 0^{o} C C a C_{2} + CO$

Step 2: Preparation of Quick Lime

Quick lime is obtained by heating limestone (CaCO₃):
$C a C O_{3} Δ C a O + C O_{2}$

Step 3: Hydrolysis of Calcium Carbide

Calcium carbide reacts with water to produce acetylene (C₂H₂) and calcium hydroxide (Ca(OH)₂):
$C a C_{2} + 2 H_{2} O \to C a (O H)_{2} + C_{2} H_{2}$

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.

Also Read:-

Nomenclature of Co-ordination Compounds	Double Salt	Aluminium Hydroxide
Aliphatic Hydrocarbons	Non-Aromatic Compounds	Nuclear Chemistry PYQs with Solution

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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:

One sigma bond is formed by overlapping one sp hybrid orbital from each carbon atom.
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:

Vinylation

In the presence of a catalyst, acetylene reacts with hydrogen halides (e.g., HCl) or other compounds to form vinyl derivatives.
Example: $C_{2} H_{2} + H Cl C a t a l ys t C H_{2} = C H Cl$

Hydration

Acetylene undergoes hydration in the presence of a catalyst (e.g., Hg²⁺) to form an enol, which tautomerises to yield acetaldehyde.
Example: $C_{2} H_{2} + H_{2} O H g S O_{4} C H_{3} C H O$

Hydrohalogenation

Acetylene reacts with halogens like HBr or HI to form haloalkenes or dihaloalkanes.
Example: $C_{2} H_{2} + 2 H B r C H_{2} B r - C H_{2} B r$

Addition to Formaldehyde

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

Carbonylation

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

Organometallic Chemistry

Acetylene reacts with metals or their derivatives to form organometallic compounds, such as metal acetylides, used in organic synthesis and catalysis.
Example: $C_{2} H_{2} + N a N H_{2} \to N a C \equiv C H$
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:
$H C \equiv C H + K O H ⇌ H C \equiv C K + H_{2} O$

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: $C_{2} H_{2} + N a N H_{2} \to N a C \equiv C H + N H_{3}$

6.0Preparation of Ethyne (Acetylene)

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: $C a O + 3 C 200 0^{o} C C a C_{2} + CO$

Step 2: Preparation of Quick Lime

Quick lime is obtained by heating limestone (CaCO₃):
$C a C O_{3} Δ C a O + C O_{2}$

Step 3: Hydrolysis of Calcium Carbide

Calcium carbide reacts with water to produce acetylene (C₂H₂) and calcium hydroxide (Ca(OH)₂):
$C a C_{2} + 2 H_{2} O \to C a (O H)_{2} + C_{2} H_{2}$

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.

Also Read:-

Nomenclature of Co-ordination Compounds	Double Salt	Aluminium Hydroxide
Aliphatic Hydrocarbons	Non-Aromatic Compounds	Nuclear Chemistry PYQs with Solution

Frequently Asked Questions

What is acetylene, and what is its formula?

How is acetylene prepared?

Why is acetylene used in welding?

Frequently Asked Questions

What is acetylene, and what is its formula?

How is acetylene prepared?

Why is acetylene used in welding?

Join ALLEN!

Acetylene

1.0Introduction

2.0Physical Properties

3.0Formula and Structure

4.0Acidic Behavior of Acetylene

5.0Chemical Properties of Acetylene

6.0Preparation of Ethyne (Acetylene)

7.0Uses of Acetylene

Table of Contents

Join ALLEN!

Acetylene

1.0Introduction

2.0Physical Properties

3.0Formula and Structure

4.0Acidic Behavior of Acetylene

5.0Chemical Properties of Acetylene

6.0Preparation of Ethyne (Acetylene)

7.0Uses of Acetylene

Table of Contents