Ethyne

What is Ethyne?

Ethyne is the common IUPAC name for the gas most people call acetylene. It belongs to a family of hydrocarbons called alkynes, which are characterized by having at least one carbon-carbon triple bond. The name “ethyne” comes from “eth” meaning two carbon atoms and “yne” indicating a triple bond.

The molecular formula is C₂H₂, meaning each molecule contains two carbon atoms and two hydrogen atoms. The structure can be written as HC≡CH, where the three lines (≡) represent the triple bond. This triple bond makes ethyne very reactive compared to alkanes and alkenes.

Structure and Bonding in Ethyne

In ethyne, the two carbon atoms are connected by a triple bond consisting of one sigma (σ) bond and two pi (π) bonds. Each carbon atom is also bonded to one hydrogen atom through a single sigma bond. This arrangement gives ethyne a linear shape with all four atoms in a straight line and a bond angle of 180°.

The carbon atoms in ethyne use sp hybridization, which is different from sp³ in alkanes or sp² in alkenes. The triple bond is shorter and stronger than double or single bonds. The C≡C bond length is about 120 pm, while the C-H bond length is about 106 pm.

This linear structure and the presence of the triple bond give ethyne its unique chemical properties. The electrons in the pi bonds are more exposed and available for reactions, making ethyne much more reactive than ethane or ethene.

Preparation of Ethyne

Laboratory Preparation from Calcium Carbide

The most common way to prepare ethyne in the laboratory is by the reaction of calcium carbide (CaC₂) with water. This is a simple and effective method that produces ethyne gas quickly.

Chemical Equation:
CaC₂₍ₛ₎ + 2H₂O₍ₗ₎ → C₂H₂₍g₎ + Ca(OH)₂₍ₐq₎

Procedure:

1. Place lumps of calcium carbide in a conical flask
2. Add water drop by drop through a thistle funnel or dropper
3. The reaction is vigorous and produces ethyne gas immediately
4. Collect the gas over water or in a gas jar
5. The gas may contain impurities like phosphine (PH₃) and hydrogen sulfide (H₂S) which give it a bad smell
6. To purify, pass the gas through copper(II) sulfate solution to remove H₂S and through water to remove other impurities

Safety Note: This reaction produces a lot of heat and the gas is highly flammable. Never perform this experiment near a flame unless you intend to burn the gas. Always wear safety goggles.

Industrial Production of Ethyne

In industry, ethyne is produced on a large scale by several methods:

1. Thermal Cracking of Methane: Natural gas (mainly methane) is heated to very high temperatures (around 1500°C) to break down into ethyne and hydrogen:

2CH₄ → C₂H₂ + 3H₂

2. From Calcium Carbide (Industrial Scale): Calcium carbide is first made by heating calcium oxide (quicklime) with coke in an electric arc furnace at about 2000°C:

CaO + 3C → CaC₂ + CO

The calcium carbide is then reacted with water to produce ethyne as shown earlier.

3. Partial Combustion of Hydrocarbons: Heating heavier hydrocarbons with limited oxygen can produce ethyne, though this method is less efficient.

Other Laboratory Methods

Dehydrohalogenation of 1,2-dibromoethane: When 1,2-dibromoethane is treated with alcoholic potassium hydroxide, it loses hydrogen bromide molecules to form ethyne:

BrCH₂CH₂Br + 2KOH (alcoholic) → HC≡CH + 2KBr + 2H₂O

This method is less common in school labs because it requires special chemicals and conditions.

Physical Properties of Ethyne

Chemical Properties and Reactions of Ethyne

Ethyne is highly reactive due to its triple bond. The pi electrons are available for reactions, making ethyne undergo addition reactions easily. It also shows weak acidic properties, which is unusual for hydrocarbons.

1. Combustion Reactions

Complete Combustion: When ethyne burns in excess oxygen, it produces carbon dioxide and water, releasing a lot of heat:

2C₂H₂ + 5O₂ → 4CO₂ + 2H₂O (ΔH = -1300 kJ/mol)

The flame temperature can reach 3500°C when ethyne burns in pure oxygen. This is why it is used in oxy-acetylene welding and cutting torches.

Incomplete Combustion: In limited air supply, ethyne burns with a very sooty, luminous yellow flame producing carbon monoxide or just carbon:

2C₂H₂ + 3O₂ → 4CO + 2H₂O
C₂H₂ + O₂ → 2C + H₂O

The soot (carbon) is what makes the flame yellow and smoky.

2. Addition Reactions

Hydrogenation (Addition of Hydrogen): Ethyne can add hydrogen in two stages using a catalyst like nickel or platinum:

Stage 1: HC≡CH + H₂ → H₂C=CH₂ (ethene)
Stage 2: H₂C=CH₂ + H₂ → H₃C-CH₃ (ethane)

Halogenation (Addition of Halogens): Ethyne reacts with bromine or chlorine in two stages:

Stage 1: HC≡CH + Br₂ → CHBr=CHBr (1,2-dibromoethene)
Stage 2: CHBr=CHBr + Br₂ → CHBr₂-CHBr₂ (1,1,2,2-tetrabromoethane)

In the first stage, the red-brown color of bromine water disappears quickly. This reaction is used as a test to identify unsaturated compounds like ethyne.

Addition of Hydrogen Halides: Hydrogen chloride or hydrogen bromide can add to ethyne:

HC≡CH + HCl → CH₂=CHCl (chloroethene, also called vinyl chloride)

Vinyl chloride is the starting material for making PVC plastic, which is used for water pipes, electrical insulation, and many household items.

Addition of Water (Hydration): In the presence of dilute sulfuric acid and mercury(II) sulfate catalyst at about 60°C, ethyne reacts with water to form ethanal (acetaldehyde):

HC≡CH + H₂O → CH₃CHO (ethanal)

Ethanal is then oxidized to ethanoic acid (acetic acid), which is used to make vinegar.

3. Polymerization Reactions

Cyclic Polymerization to Benzene: When ethyne is passed through a red-hot iron tube at 400-500°C, three molecules join together to form benzene:

3C₂H₂ → C₆H₆ (benzene)

This is an important industrial process because benzene is a valuable starting material for making dyes, drugs, plastics, and explosives.

Linear Polymerization: Under certain conditions with catalysts, ethyne molecules can join end to end to form long-chain polymers used in synthetic rubber and plastics.

4. Acidic Character of Ethyne

Unlike most hydrocarbons, ethyne shows weak acidic properties. This is because the hydrogen atoms are attached to sp-hybridized carbons, making them slightly easier to remove as H⁺ ions.

Ethyne reacts with very strong bases like sodium amide (NaNH₂) or metallic sodium:

HC≡CH + 2Na → NaC≡CNa + H₂ (sodium acetylide)

HC≡CH + NaNH₂ → HC≡CNa + NH₃

These reactions show that ethyne is more acidic than ethane or ethene, though it is still much weaker than water or alcohols.

Test for Ethyne

To confirm that a gas is ethyne, you can perform these tests:

1. Burning Test: Ethyne burns with a bright, luminous, sooty yellow flame. This is different from ethane which burns with a clean blue flame.
2. Bromine Water Test: Pass the gas through red-brown bromine water. If it is ethyne, the color will disappear quickly as the bromine adds to the triple bond.
3. Potassium Manganate(VII) Test: Ethyne turns purple acidified potassium manganate(VII) solution to brown, showing it is unsaturated.
4. Ammoniacal Silver Nitrate Test: Pass ethyne through ammoniacal silver nitrate solution. A white precipitate of silver acetylide (Ag₂C₂) forms. This is a specific test for terminal alkynes like ethyne.

Uses of Ethyne

1. Oxy-Acetylene Welding and Cutting

This is the most important use of ethyne. When ethyne burns in pure oxygen, the flame reaches about 3500°C. This extremely hot flame can melt steel and other metals, allowing welders to join metal pieces together or cut through thick metal sheets. In Nigeria, you will see welders at mechanic workshops in places like Ladipo Market in Lagos using oxy-acetylene torches to repair car bodies and fabricate metal gates.

2. Production of Important Chemicals

• Vinyl chloride: Used to make PVC pipes for plumbing
• Acetic acid: Used in making vinegar and cellulose acetate
• Acrylonitrile: Used to make synthetic fibers and plastics
• Acetaldehyde: Starting material for many chemicals
• 1,4-Butanediol: Used in making synthetic fibers and plastics

3. Artificial Ripening of Fruits

In some markets in Nigeria, traders use calcium carbide to ripen fruits like plantains, mangoes, and bananas quickly. When calcium carbide absorbs moisture from the air or fruit, it produces ethyne gas which acts like the natural ripening hormone ethylene. However, this practice is illegal in Nigeria because calcium carbide contains harmful impurities that can cause health problems.

4. Portable Lighting

In the past, miners used carbide lamps which produced light by burning ethyne. Water drips onto calcium carbide in the lamp, producing ethyne which burns with a bright flame. Though electric lamps have mostly replaced these, some spelunkers (cave explorers) and miners in remote areas still use carbide lamps.

5. Production of Carbon Black

When ethyne burns in limited oxygen, it produces fine carbon black (soot). This carbon black is used in making printer ink, black paint, and as a filler in rubber tires to make them stronger.

Common Exam Mistakes

WAEC Chemistry examiners have noted these common errors when students answer questions about ethyne:

• Confusing acetylene with ethylene: Ethyne (acetylene) has a triple bond, while ethene (ethylene) has a double bond. Their formulas are C₂H₂ and C₂H₄ respectively.
• Writing wrong structural formulas: Some students write CH₃-CH instead of HC≡CH. Remember, ethyne has no CH₃ groups.
• Incomplete reaction equations: When writing the preparation from calcium carbide, students forget to include calcium hydroxide as a product or fail to balance the equation.
• Not explaining the sooty flame: Students mention the yellow flame but don’t explain that it is due to incomplete combustion producing carbon particles.
• Confusing addition reactions: Forgetting that ethyne undergoes addition in TWO stages when reacting with H₂ or Br₂.
• Forgetting test details: Mentioning bromine water test but not describing that the color changes from red-brown to colorless.
• Vague answers about uses: Writing “used in welding” without explaining that it requires oxygen to produce a very hot flame.

Practice Questions

Multiple Choice Questions

1. What is the molecular formula of ethyne?
a) C₂H₄
b) C₂H₆
c) C₂H₂ ✓
d) C₃H₄

2. Ethyne is prepared in the laboratory by the action of water on:
a) Calcium oxide
b) Calcium carbide ✓
c) Calcium carbonate
d) Calcium hydroxide

3. When ethyne burns in air, it produces a flame that is:
a) Blue and clean
b) Yellow and sooty ✓
c) Colorless
d) Green

4. The bond angle in the ethyne molecule is:
a) 109.5°
b) 120°
c) 180° ✓
d) 90°

5. Which of these is NOT a use of ethyne?
a) Oxy-acetylene welding
b) Making PVC plastic
c) Making soap ✓
d) Ripening fruits

Theory Questions

1. (a) Draw the structural formula of ethyne and indicate the type of bond present. (2 marks)
(b) Describe with the aid of an equation how ethyne is prepared in the laboratory. (3 marks)
(c) State two physical properties of ethyne. (2 marks)
(d) Write an equation for the complete combustion of ethyne. (2 marks)

Examiner’s Tip: For part (a), clearly show the triple bond using three lines (≡) or mention it is a triple bond. For part (b), include the apparatus setup or at least mention adding water drop by drop to calcium carbide.

2. (a) Ethyne undergoes addition reactions with bromine. Write equations for the TWO stages of this reaction. (4 marks)
(b) Describe a chemical test to distinguish ethyne from ethane. (3 marks)
(c) Explain why ethyne is more reactive than ethane. (2 marks)

Examiner’s Tip: Make sure you show BOTH stages of bromine addition. For the test, describe what you observe, not just the name of the test. Mention that ethyne has a triple bond with exposed pi electrons.

3. (a) Explain why ethyne burns with a sooty flame. (2 marks)
(b) State THREE important uses of ethyne. (3 marks)
(c) Write an equation showing the polymerization of ethyne to benzene. (2 marks)
(d) Why is ethyne used in welding and cutting metals? (2 marks)

Examiner’s Tip: For part (a), mention incomplete combustion and production of carbon particles. For part (d), emphasize the high temperature (3500°C) produced when ethyne burns in oxygen.

4. (a) State the homologous series to which ethyne belongs. (1 mark)
(b) What is the general formula for this homologous series? (1 mark)
(c) Name the next two members of the series after ethyne. (2 marks)
(d) Write the equation for the reaction between ethyne and water in the presence of H₂SO₄ and HgSO₄. (2 marks)
(e) Name the product formed in (d) above. (1 mark)

Examiner’s Tip: The general formula for alkynes is CₙH₂ₙ₋₂. The next members are propyne (C₃H₄) and butyne (C₄H₆). The product in (d) is ethanal (acetaldehyde), CH₃CHO.

Memory Aids

For Ethyne Formula: “Two Carbon, Two Hydrogen – That’s Ethyne!” (C₂H₂)

For Ethyne Preparation: “Calcium Carbide Caught Water” (CaC₂ + Water → Ethyne)

For Ethyne Uses: “WRAP”

• Welding and cutting metals
• Ripening fruits artificially
• Acetaldehyde and chemical production
• PVC plastic manufacturing

For Distinguishing Alkynes: “Triple trouble means sooty bubble” (Triple bond causes sooty flame when burning)

For Bond Angle: “Linear ethyne = 180 line” (Ethyne is linear with 180° bond angle)

Related Topics

• Alkanes and alkenes (homologous series comparison)
• Hydrocarbons and organic chemistry
• Addition and substitution reactions
• Combustion reactions and flame types
• Industrial organic chemistry