9.3 Alkyne

Introduction

Alkynes are unsaturated hydrocarbons that contain at least one carbon-carbon triple bond. They are an important family of organic compounds, and the simplest member, ethyne (also called acetylene), has great commercial importance.

Ethyne is commonly used in cutting and welding metals because it burns with a very hot flame called the oxy-acetylene flame. It is also used in the artificial ripening of fruits.

This section explains the structure of alkynes and the commercial importance of ethyne.

Definition

Alkynes: Hydrocarbons that have at least one triple bond between carbon atoms are called alkynes.

General Formula: C_nH_2n-2

In simple words: Alkynes are unsaturated hydrocarbons with at least one carbon-carbon triple bond.

Main Idea

• Alkynes have at least one carbon-carbon triple bond.
• They are unsaturated hydrocarbons.
• Alkynes with one triple bond have the general formula C_nH_2n-2.
• The simplest member of the alkyne family is ethyne (also called acetylene).
• In ethyne, the two carbon atoms share three pairs of electrons.
• The carbon atoms are joined by a triple bond.

Core Concept

Key point: Alkynes contain a carbon-carbon triple bond and have the general formula C_nH_2n-2.

Detailed Explanation

What are Alkynes?

Hydrocarbons that have at least one triple bond between carbon atoms are called alkynes.

This means alkynes are unsaturated hydrocarbons.

Alkynes with one triple bond have the general formula: C_nH_2n-2

Key point: The triple bond is the defining feature of alkynes.

Ethyne (Acetylene)

Ethyne, also called acetylene, is the simplest member of the alkyne family.

Structure:

• In ethyne, the two carbon atoms share three pairs of electrons.
• This means the carbon atoms are joined by a triple bond.

Structure of ethyne:

H - C ≡ C - H

Condensed formula: HC ≡ CH

Key point: Ethyne has a triple bond between the two carbon atoms and one hydrogen atom attached to each carbon.

Commercial Importance of Ethyne

Ethyne has great commercial importance.

1. Welding and Cutting Metals:

• Ethyne is also known as acetylene and is commonly used in cutting and welding metals.
• When it is ignited in oxygen or air, it burns with a bluish colour producing a very hot flame called the oxy-acetylene flame.
• Its temperature is about 3200^° C.
• The oxy-acetylene flame has the highest flame temperature of all other fuel gases.
• Welding and cutting of metals require high temperatures. Therefore, ethyne is used for this purpose.

2. Artificial Ripening of Fruits:

• Another advantage of ethyne is its use in the artificial ripening of fruits.
• Have you ever seen a packet containing solid material in your fruit boxes, especially in mango crates?
• This packet contains calcium carbide, which on coming in contact with moisture produces ethyne gas.
• This ethyne promotes ripening and also induces colour changes in fruits.
• However, its use can lead to exposure to harmful chemicals that can cause respiratory issues and other health problems.

Equation:

CaC₂ + H₂O → C₂H₂ + Ca(OH)₂

Key point: Ethyne is used in welding, cutting metals, and artificial ripening of fruits. However, its use in ripening can be harmful.

Summary Table

Intermediate Level

Alkynes:

• Hydrocarbons with at least one triple bond.
• Unsaturated hydrocarbons.
• General formula: C_nH_2n-2.
• Simplest alkyne: Ethyne (HC ≡ CH).

Structure of ethyne:

• Two carbon atoms share three pairs of electrons.
• Triple bond between the carbon atoms.
• One hydrogen atom attached to each carbon.

Commercial importance of ethyne:

• Burns with a bluish colour producing the oxy-acetylene flame.
• Temperature: about 3200^° C.
• Highest flame temperature of all fuel gases.
• Calcium carbide reacts with moisture to produce ethyne gas.
• Ethyne promotes ripening and induces colour changes.
• Use can be harmful (respiratory issues).
• Equation: CaC₂ + H₂O → C₂H₂ + Ca(OH)₂.

Key point: Ethyne is the simplest alkyne and has important commercial uses in welding and fruit ripening.

Solved Example

Example: Write the Structure of Ethyne

Question: Write the structure of ethyne.

Solution:

• Ethyne has two carbon atoms joined by a triple bond.
• Each carbon atom is bonded to one hydrogen atom.

Structure:

H - C ≡ C - H

Final Answer: The structure of ethyne is H - C ≡ C - H.

Example: Write the Equation for the Production of Ethyne from Calcium Carbide

Question: Write the equation for the production of ethyne from calcium carbide.

Solution:

CaC₂ + H₂O → C₂H₂ + Ca(OH)₂

Final Answer: CaC₂ + H₂O → C₂H₂ + Ca(OH)₂

Important Points

• - Alkynes = hydrocarbons with at least one carbon-carbon triple bond.
• - General formula: C_nH_2n-2.
• - Type of hydrocarbon: Unsaturated.
• - Simplest alkyne: Ethyne (HC ≡ CH), also called acetylene.
• - Structure of ethyne: H - C ≡ C - H.
• - Commercial importance of ethyne:
• 1. Welding and cutting metals — oxy-acetylene flame, temperature about 3200^° C.
• 2. Artificial ripening of fruits — from calcium carbide; can be harmful.
• - Equation for artificial ripening: CaC₂ + H₂O → C₂H₂ + Ca(OH)₂.

Remember

• - Alkynes = hydrocarbons with a triple bond.
• - General formula: C_nH_2n-2.
• - Simplest alkyne: Ethyne (HC ≡ CH).
• - Structure: H - C ≡ C - H.
• - Uses:
• 1. Welding and cutting metals (oxy-acetylene flame).
• 2. Artificial ripening of fruits.
• - Oxy-acetylene flame temperature: about 3200^° C.
• - Equation: CaC₂ + H₂O → C₂H₂ + Ca(OH)₂.
• - Use in ripening can be harmful.

Common Mistakes

Quick Revision

Alkyne:

Hydrocarbons that have at least one triple bond between carbon atoms are called alkynes. This means alkynes are unsaturated hydrocarbons. Alkynes with one triple bond have the general formula C_nH_2n-2.

Ethyne, also called acetylene is the simplest member of the alkyne family. In ethyne, the two carbon atoms share three pairs of electrons. This means the carbon atoms are joined by a triple bond.

• H - C ≡ C - H

Commercial Importance of Ethyne:

Ethyne has great commercial importance. It is also known as acetylene and is commonly used in cutting and welding metals. When it is ignited in oxygen or air, it burns with a bluish colour producing a very hot flame called oxy-acetylene flame. Its temperature is about 3200^° C. Oxy-acetylene flame has the highest flame temperature of all other fuel gases. Welding and cutting of metals require high temperatures. Therefore, ethyne is used for this purpose.

Another advantage of ethyne is its use in the artificial ripening of fruits. Have you ever seen a packet containing solid material in your fruit boxes, especially in mango crates? This packet contains calcium carbide, which on coming in contact with moisture produces ethyne gas. This ethyne promotes ripening and also induces colour changes in fruits. However, its use can lead to exposure to harmful chemicals, that can cause respiratory issues and other health problems.

• CaC₂ + H₂O → C₂H₂ + Ca(OH)₂

60-Second Revision

• Alkynes = hydrocarbons with a triple bond.
• General formula: C_nH_2n-2.
• Simplest alkyne: Ethyne (HC ≡ CH).
• Structure: H - C ≡ C - H.
• Uses:
• Oxy-acetylene flame temperature: about 3200^° C.
• Equation: CaC₂ + H₂O → C₂H₂ + Ca(OH)₂.
• Use in ripening can be harmful.
• ⚠️ Warning: alkynes are unsaturated hydrocarbons — they contain at least one carbon-carbon triple bond.