Quick Summary
- Alkynes contain triple bonds (C≡C) between carbon atoms
- General formula: CnH2n-2
- They are more reactive than alkanes and alkenes
- Ethyne (acetylene) is the most common alkyne in Nigeria, used for welding
- Alkynes burn with a sooty flame due to high carbon content
Understanding Alkynes
Alkynes belong to the hydrocarbon family. Hydrocarbons are compounds made only of carbon and hydrogen atoms. What makes alkynes special is the triple bond between carbon atoms.
Think of chemical bonds like hands holding. A single bond is like holding one hand. A double bond is like holding two hands. A triple bond is like holding three hands – very strong and close together.
The Alkyne Family
The alkyne family starts from ethyne. You cannot have an alkyne with just one carbon atom. You need at least two carbon atoms to form a triple bond.
Here are the first five members:
| Name | Molecular Formula | Structural Formula | Common Use |
|---|---|---|---|
| Ethyne | C2H2 | H-C≡C-H | Welding torches |
| Propyne | C3H4 | CH3-C≡C-H | Chemical synthesis |
| Butyne | C4H6 | CH3CH2-C≡C-H | Laboratory research |
| Pentyne | C5H8 | CH3CH2CH2-C≡C-H | Organic synthesis |
| Hexyne | C6H10 | CH3(CH2)3-C≡C-H | Industrial chemicals |
How to Name Alkynes
Naming alkynes follows IUPAC rules. IUPAC stands for International Union of Pure and Applied Chemistry. They create naming rules for chemicals worldwide.
Follow these steps:
- Find the longest carbon chain with the triple bond
- Number the chain from the end nearest the triple bond
- Use the number where the triple bond starts
- Replace “-ane” ending with “-yne”
Example: If you have a five-carbon chain with a triple bond starting at carbon 2, the name is pent-2-yne.
Physical Properties of Alkynes
Alkynes share some properties with other hydrocarbons. But they have unique features too.
State at room temperature: The first three alkynes (ethyne, propyne, butyne) are gases. Medium-sized alkynes are liquids. Large alkynes are solids.
Smell: Lower alkynes have a strong, unpleasant smell. If you’ve been near a welding shop in Lagos or Kano, you’ve smelled ethyne.
Solubility: Alkynes do not dissolve in water. They dissolve in organic solvents like benzene or ether.
Density: Alkynes are less dense than water. If you pour an alkyne liquid into water, it will float.
Boiling points: As alkynes get bigger (more carbon atoms), their boiling points increase. Ethyne boils at -84°C. Hexyne boils at 71°C.
Chemical Properties of Alkynes
Alkynes are reactive compounds. The triple bond makes them want to react with other substances.
Combustion (Burning): When alkynes burn in air, they produce carbon dioxide and water. But if there is not enough oxygen, they produce a sooty, yellow flame. This is because alkynes have high carbon content.
Equation: 2C2H2 + 5O2 → 4CO2 + 2H2O
The flame from burning ethyne can reach 3,000°C. This is why welders use it to cut through metal.
Addition reactions: Alkynes can add hydrogen, halogens, or hydrogen halides. The triple bond becomes a double bond, then a single bond.
With hydrogen: C2H2 + H2 → C2H4 (ethene)
With more hydrogen: C2H4 + H2 → C2H6 (ethane)
Polymerization: Ethyne can join together to form larger molecules. This process is called polymerization. It creates plastics and synthetic materials.
Test for alkynes: Alkynes decolorize bromine water. If you add red-brown bromine water to an alkyne, the color disappears. This is a common lab test.
Preparation of Alkynes
In the laboratory, you can make alkynes in several ways.
From calcium carbide: This is the most common method in Nigeria. Add water to calcium carbide (CaC2), and you get ethyne gas.
CaC2 + 2H2O → C2H2 + Ca(OH)2
Welders buy calcium carbide from chemical shops in Onitsha, Aba, or other industrial cities.
From dihalides: Remove two halogen atoms from a dihalide using alcoholic potassium hydroxide. This creates a triple bond.
Uses of Alkynes in Nigeria
Alkynes have important uses in daily life and industry.
Welding and cutting: Ethyne mixed with oxygen creates oxyacetylene flames. These flames are hot enough to weld steel. You will find this at construction sites, car repair shops, and metal fabrication workshops across Nigeria.
Making plastics: Industries convert ethyne into PVC (polyvinyl chloride). PVC is used for water pipes, electrical cables, and roofing sheets.
Ripening fruits: Some traders use calcium carbide to produce ethyne gas. The gas helps ripen fruits like mangoes, plantains, and bananas quickly. However, NAFDAC warns against this practice because it is unsafe.
Chemical industry: Alkynes are starting materials for making other chemicals. They help create medicines, pesticides, and industrial solvents.
Lighting: Before electricity became common, people used carbide lamps. These lamps mixed calcium carbide with water to produce ethyne. The gas burned to give light. Some miners and rural communities still use these lamps.
Common Exam Mistakes
WAEC examiners report these common errors about alkynes:
- Wrong general formula: Students write CnH2n instead of CnH2n-2. Remember, alkynes have FEWER hydrogen atoms than alkenes.
- Starting from methyne: Some students think there is a methyne (CH). This does not exist. Alkynes start from ethyne (C2H2).
- Confusing alkanes, alkenes, and alkynes: Learn the differences clearly. Alkanes have single bonds (C-C), alkenes have double bonds (C=C), alkynes have triple bonds (C≡C).
- Not balancing equations: When writing combustion equations, students forget to balance oxygen, carbon dioxide, and water.
- Poor explanations: Examiners say students write “alkynes are unsaturated” without explaining what unsaturated means. Always explain that unsaturated means the compound has double or triple bonds and can add more atoms.
Practice Questions
Multiple Choice Questions
1. What is the general molecular formula for alkynes?
a) CnH2n+2
b) CnH2n
c) CnH2n-2 ✓
d) CnHn
2. Which of the following is NOT a property of alkynes?
a) They burn with a sooty flame
b) They decolorize bromine water
c) They dissolve easily in water ✓
d) They undergo addition reactions
3. The IUPAC name for C2H2 is:
a) Methyne
b) Ethyne ✓
c) Propyne
d) Acetylene
4. When calcium carbide reacts with water, the product is:
a) Methane and calcium hydroxide
b) Ethyne and calcium hydroxide ✓
c) Ethene and calcium oxide
d) Propyne and water
Essay Questions
5. (a) State THREE physical properties of alkynes. (3 marks)
(b) Write a balanced equation for the complete combustion of ethyne. (3 marks)
(c) Explain why alkynes are described as unsaturated hydrocarbons. (2 marks)
Tips: For (a), mention state, solubility, and smell. For (b), balance carefully – count atoms on both sides. For (c), define unsaturated and mention the triple bond can add more atoms.
6. (a) Describe how you would prepare ethyne in the laboratory. (4 marks)
(b) State TWO uses of alkynes in Nigeria. (2 marks)
(c) Why do alkynes burn with a sooty flame? (2 marks)
Tips: For (a), mention calcium carbide, water, and the apparatus. For (b), give specific uses like welding or fruit ripening. For (c), explain that high carbon content leads to incomplete combustion.
7. Distinguish between alkanes, alkenes, and alkynes based on their structural formulas and reactivity. (8 marks)
Tips: Create a table or write in paragraphs. Mention bond types (single, double, triple), general formulas, and how reactive each group is. Give examples for each.
Memory Aids
Remember the hydrocarbon families:
Triple Y, Double E, Single A
– AlkYnes = Triple bonds
– AlkEnes = Double bonds (E comes before Y)
– AlkAnes = Single bonds
General formulas:
“Add 2, Same, Minus 2”
– Alkanes: CnH2n+2 (add 2)
– Alkenes: CnH2n (same)
– Alkynes: CnH2n-2 (minus 2)
Naming:
EPB = Ethyne, Propyne, Butyne (first three members)
Related Topics
- Alkanes – saturated hydrocarbons with single bonds
- Alkenes – unsaturated hydrocarbons with double bonds
- Hydrocarbons – compounds of carbon and hydrogen
- Isomerism in organic chemistry – different structures, same formula
- Addition reactions – how unsaturated compounds gain atoms