CHEMISTRY CLASS NOTES: HYDROCARBONS (ALKANES, ALKENES, AND ALKYNES)

Subject: Chemistry Class: Senior Secondary School 1 (SS 1) Topic: Structures, Properties, and Examples of Alkanes, Alkenes, and Alkynes Duration: 80 Minutes (Double Period)

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1. INTRODUCTION TO HYDROCARBONS

Imagine your daily routine in Nigeria: waking up to a breakfast cooked on a gas cylinder, taking a "yellow bus" (Danfo) or a Keke Napep to school, and perhaps seeing a welder working on a gate along the road. All these activities are powered by a specific group of organic compounds known as Hydrocarbons.

Definition: Hydrocarbons are the simplest organic compounds, consisting entirely of only two elements: Carbon (C) and Hydrogen (H). They are the primary constituents of petroleum (crude oil) and natural gas, which are the mainstays of the Nigerian economy.

The Two Main Families

1. Saturated Hydrocarbons (Alkanes): These contain only single covalent bonds between carbon atoms. They are called "saturated" because the carbon atoms are bonded to the maximum possible number of hydrogen atoms. Think of a "saturated" sponge that cannot hold any more water; a saturated hydrocarbon cannot hold any more hydrogen.
2. Unsaturated Hydrocarbons (Alkenes & Alkynes): These contain double or triple bonds between carbon atoms. Because of these multiple bonds, they have fewer hydrogen atoms than they could potentially hold. This makes them more "eager" to react with other substances.

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2. ALKANES: THE SATURATED GIANTS

Alkanes are often called Paraffins (from the Latin parum affinis, meaning "little affinity"), because they are generally stable and do not react easily with other chemicals at room temperature.

Structure and Naming

Alkanes follow the general molecular formula: $C_nH_{2n+2}$, where $n$ is the number of carbon atoms.

• Bonding: Every carbon atom in an alkane is $sp^3$ hybridized, forming a tetrahedral shape with bond angles of approximately 109.5°.
• Naming (IUPAC): We use a prefix based on the number of carbons and add the suffix "-ane".

Properties of Alkanes

• Physical Properties:
  • The first four members ($C_1$ to $C_4$) are gases (used as cooking gas).
  • Members from $C_5$ to $C_{17}$ are liquids (like petrol and kerosene).
  • Members with 18 or more carbons are waxy solids (like candle wax/paraffin wax).
  • They are non-polar, meaning they do not dissolve in water but dissolve in organic solvents like benzene.
• Chemical Properties:
  • Combustion: This is their most vital reaction. They burn in oxygen to produce Carbon(IV) Oxide, water vapor, and a massive amount of heat.
  • Equation: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{Energy}$
  • Substitution Reaction: They can react with halogens (like Chlorine) under ultraviolet light, where a hydrogen atom is "swapped" for a chlorine atom.

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3. ALKENES: THE DOUBLE-BONDED BUILDERS

Alkenes are unsaturated hydrocarbons that contain at least one carbon-to-carbon double bond ($C=C$). They are also known as Olefins (oil-forming).

Structure and Naming

Alkenes follow the general formula: $C_nH_{2n}$. Note: The series starts with $n=2$ because you need at least two carbons to form a double bond.

• Naming: Use the prefix for the number of carbons and the suffix "-ene".

Properties of Alkenes

• Physical Properties: Similar to alkanes, the lower members are gases, and they are insoluble in water. However, they have slightly lower boiling points than alkanes with the same number of carbons.
• Chemical Properties:
  • Addition Reactions: Because of the double bond, alkenes are very reactive. The double bond "opens up" to allow other atoms (like Hydrogen or Bromine) to join the molecule.
  • Polymerization: This is the process where thousands of small alkene molecules (monomers) join together to form a giant molecule (polymer). For example, Ethene becomes Polythene.

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4. ALKYNES: THE TRIPLE-BONDED POWERHOUSES

Alkynes are highly unsaturated hydrocarbons containing at least one carbon-to-carbon triple bond ($C \equiv C$).

Structure and Naming

They follow the general formula: $C_nH_{2n-2}$.

• Naming: Use the prefix for the number of carbons and the suffix "-yne".

Properties of Alkynes

• Physical Properties: They are colorless gases or liquids. Ethyne has a characteristic "garlic-like" smell when impure.
• Chemical Properties:
  • Addition Reactions: They are even more reactive than alkenes because they can add two sets of atoms to break the triple bond down to a single bond.
  • Combustion: When burned in a controlled supply of pure oxygen, Ethyne produces a very intense, high-temperature flame (about 3,000°C) known as the Oxy-acetylene flame.

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5. REAL-WORLD APPLICATIONS IN NIGERIA

1. Cooking (LPG): The gas in your "Camp Gas" or large cylinders is Liquefied Petroleum Gas, mostly Propane and Butane. Its ability to burn cleanly makes it ideal for indoor cooking.
2. Transportation: Petrol (Gasoline) is a mixture of liquid alkanes (mostly Octane). It provides the energy needed to move vehicles across the country.
3. The "Pure Water" Sachet: Every time you drink "pure water," the sachet you hold is made of Poly-ethene. This plastic is produced by the polymerization of Ethene gas.
4. Fruit Ripening: Local sellers often use "Carbide" to ripen mangoes or plantains. Carbide reacts with moisture to release Ethyne gas, which mimics the natural ripening hormone (Ethene) in plants.
5. Construction: Welders use Ethyne (Acetylene) to cut through thick iron or join metal parts together for gates and building structures.

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6. PROJECT-BASED LEARNING: "THE MOLECULAR ARCHITECT"

Goal: To visualize the 3D geometry of hydrocarbons using local materials.

Materials:

• Hand-rolled clay balls or "Local Plasticine" (Black for Carbon, White for Hydrogen).
• Broomsticks or toothpicks (to represent bonds).
• A piece of cardboard.

Procedure:

1. Methane ($CH_4$): Take 1 black ball and 4 white balls. Stick 4 broomsticks into the black ball so they point towards the corners of a pyramid (tetrahedron). Attach a white ball to each. Note how the "single bonds" allow the hydrogens to be far apart.
2. Ethene ($C_2H_4$): Take 2 black balls. Connect them with two broomsticks (double bond). Attach 2 white balls to each carbon. Note that the molecule is now "flat" (planar).
3. Ethyne ($C_2H_2$): Connect 2 black balls with three broomsticks (triple bond). Attach only 1 white ball to each end. Note that the molecule is a straight line (linear).

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7. HOME PRACTICE ACTIVITIES

Experiment: Testing for Saturation

Objective: To see the difference in reactivity between saturated and unsaturated fats.

• Materials: Vegetable oil (unsaturated), Shea butter or Margarine (mostly saturated), and Iodine solution (tincture of iodine from a first-aid kit).
• Procedure:
  1. Put 2 tablespoons of vegetable oil in a clear glass.
  2. Put 2 tablespoons of melted shea butter in another glass.
  3. Add 3-5 drops of Iodine to each and stir.
• Observation: The purple/brown color of the Iodine will disappear in the vegetable oil but remain in the shea butter.
• Conclusion: The vegetable oil (unsaturated) "added" the iodine to its double bonds, causing the color to vanish. This proves that unsaturated hydrocarbons are more reactive than saturated ones.

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8. LIFE SKILLS & CAREER CONNECTION

• Career Path: This topic is the gateway to Petroleum Engineering, Industrial Chemistry, and Environmental Science. As a leading oil producer, Nigeria needs experts who understand how to refine these hydrocarbons into high-value products like fertilizers and medicines.
• Safety Skill: Alkanes are odorless. The "smell" you perceive from a leaking gas cylinder is an added chemical (Mercaptan). If you smell it, never flip a light switch or use a phone, as the tiny spark can trigger a combustion reaction (explosion). Always open windows for ventilation.
• Environmental Literacy: Understanding that plastics come from alkenes helps us realize why they don't decay easily. Burning "nylon" (polymers) releases toxic fumes; recycling is a better way to manage our environment.

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9. STUDENT REFLECTION & ASSESSMENT

1. The Mystery Formula: A hydrocarbon has 10 Hydrogen atoms and is an Alkane. Using the general formula, calculate how many Carbon atoms it has.
2. The Application Challenge: Why does a welder use Ethyne ($C_2H_2$) instead of Methane ($CH_4$) for cutting steel? (Hint: Think about bond energy and flame temperature).
3. Categorization: Classify the following into Alkanes, Alkenes, or Alkynes and give their IUPAC names:
   • $C_3H_4$
   • $C_2H_6$
   • $C_4H_8$
4. Creative Writing: Imagine you are a Carbon atom. Write a three-sentence story about your life when you are part of a double bond (Alkene) versus when you are part of a single bond (Alkane). Which life is more "exciting" (reactive)?