Hello Future Chemist! Understanding Organic Chemistry
Welcome to the exciting world of Organic Chemistry! This chapter might sound complicated, but it is simply the study of compounds based on the element carbon. Carbon is special because it can form millions of different structures, making up everything from fuels to plastics, and even the molecules inside you!
Don't worry if this seems tricky at first. We will break down the big concepts—like hydrocarbons and polymers—into simple, manageable steps. By the end of these notes, you’ll understand why carbon is the ultimate building block!
Quick Review: What Makes Carbon So Special?
- Carbon (C) atoms can form four strong covalent bonds.
- They can bond easily with other carbon atoms, forming long chains, rings, or complex branched structures.
Section 1: The Basics – Hydrocarbons and Homologous Series
1.1 Organic Compounds and Hydrocarbons
An Organic Compound is a chemical compound containing carbon, usually bonded to hydrogen, oxygen, nitrogen, or other elements.
The simplest organic compounds are called hydrocarbons.
- Key Definition: A hydrocarbon is a compound that contains hydrogen (H) atoms and carbon (C) atoms only.
- Real-World Example: Petrol, natural gas (methane), and diesel are all mixtures composed mainly of hydrocarbons.
1.2 What is a Homologous Series?
Imagine organizing chemicals into families. A homologous series is a family of organic compounds that share three key features:
- They share the same general formula.
- Each member differs from the next by a fixed unit (usually a \(CH_2\) group).
- They have similar chemical properties and show a gradual change in physical properties (like boiling point) as the chain gets longer.
We will look at two important families: the Alkanes and the Alkenes.
Quick Tip for Naming
The beginning of the name (the prefix) tells you how many carbons are in the chain. You MUST memorize the first four:
- C1: Meth- (e.g., Methane)
- C2: Eth- (e.g., Ethane)
- C3: Prop- (e.g., Propane)
- C4: But- (e.g., Butane)
Memory Aid (Mnemonic): Monkeys Eat Peeled Bananas (M, E, P, B = 1, 2, 3, 4 carbons)
Section 2: Alkanes – The Saturated Family
2.1 Structure and Formulae
Alkanes are the simplest family of hydrocarbons. They only have single covalent bonds between the carbon atoms.
- Suffix: All alkanes end in -ane.
- General Formula: \(C_n H_{2n+2}\) (Where 'n' is the number of carbon atoms).
If you know the number of carbons (n), you can calculate the number of hydrogens (H).
| Alkane Name | n (Carbons) | Molecular Formula (\(C_n H_{2n+2}\)) |
|---|---|---|
| Methane | 1 | \(C_1 H_{(2\times 1)+2}\) = \(CH_4\) |
| Ethane | 2 | \(C_2 H_{(2\times 2)+2}\) = \(C_2 H_6\) |
| Propane | 3 | \(C_3 H_{(2\times 3)+2}\) = \(C_3 H_8\) |
| Butane | 4 | \(C_4 H_{(2\times 4)+2}\) = \(C_4 H_{10}\) |
2.2 Saturated Compounds
Alkanes are described as saturated.
- Analogy: Imagine a sponge that is completely full of water—it cannot absorb any more. In chemistry, a saturated compound is full of hydrogen atoms.
- Meaning: Every carbon atom is bonded to the maximum possible number of other atoms (only by single bonds). There are no double or triple bonds.
2.3 Properties and Uses of Alkanes
Alkanes are generally quite unreactive because the C-C and C-H single bonds are very strong. Their main chemical reaction is combustion (burning).
- Small alkanes (C1 to C4) are gases at room temperature (e.g., Methane is natural gas).
- Medium alkanes are liquids (e.g., Petrol, paraffin).
- Large alkanes are solids (e.g., Waxes).
They are hydrocarbons, they are saturated (single bonds only), and they follow the formula \(C_n H_{2n+2}\). They are primarily used as fuels.
Section 3: Alkenes – The Unsaturated Family
3.1 Structure and Formulae
Alkenes are the second family we study. They are different from alkanes because they contain at least one carbon-carbon double bond (\(C=C\)).
- Suffix: All alkenes end in -ene.
- General Formula: \(C_n H_{2n}\) (This is because the double bond means they have two fewer hydrogens than the corresponding alkane).
Note: Because you need at least two carbon atoms to form a double bond, there is no such thing as 'Methene' (n=1).
| Alkene Name | n (Carbons) | Molecular Formula (\(C_n H_{2n}\)) |
|---|---|---|
| Ethene | 2 | \(C_2 H_{(2\times 2)}\) = \(C_2 H_4\) |
| Propene | 3 | \(C_3 H_{(2\times 3)}\) = \(C_3 H_6\) |
| Butene | 4 | \(C_4 H_{(2\times 4)}\) = \(C_4 H_8\) |
3.2 Unsaturated Compounds
Alkenes are described as unsaturated.
- Analogy: The sponge is NOT full! The double bond (\(C=C\)) is weaker than two single bonds, making the molecule ready to break open and bond with other atoms.
- Meaning: The compound contains one or more double or triple bonds, meaning it could potentially add more hydrogen atoms (or other atoms) if the double bond were to break.
3.3 Testing for Unsaturation (Alkenes vs. Alkanes)
Because of their double bond, alkenes are much more reactive than alkanes. This allows us to use a simple chemical test to tell them apart: the Bromine Water Test.
Bromine water is naturally an orange-brown colour.
The Bromine Water Test
Step 1: Add a few drops of orange-brown Bromine water to your hydrocarbon sample.
Observation & Conclusion:
- If the sample is an Alkene (unsaturated), the double bond breaks and the bromine atoms add across the carbons. The orange-brown colour disappears (it becomes colourless).
- If the sample is an Alkane (saturated), the bromine cannot react easily, and the orange-brown colour remains.
Students often confuse the colour change. Remember: Alkenes react, and the bromine colour goes away (decolourises). Alkanes do not react, and the colour stays.
Section 4: Key Reactions – Combustion
Hydrocarbons (like methane or petrol) are essential fuels because they release a lot of energy when burned (exothermic reaction). This reaction is called combustion.
4.1 Complete Combustion
This happens when there is an excess (plenty) of oxygen. It is the most efficient way to burn fuels.
Products: The products are always carbon dioxide and water.
Equation (General):
Hydrocarbon + Oxygen \(\longrightarrow\) Carbon Dioxide + Water
Example (Methane):
\(CH_4 + 2O_2 \longrightarrow CO_2 + 2H_2O\)
4.2 Incomplete Combustion
This happens when the supply of oxygen is limited (not enough oxygen). This is bad because it wastes fuel and creates dangerous products.
Products: This reaction produces carbon monoxide (CO) and/or solid carbon (soot), along with water.
Equation (Partial):
Hydrocarbon + Limited Oxygen \(\longrightarrow\) Carbon Monoxide + Water (+ Soot)
The Danger of Carbon Monoxide (CO)
Carbon monoxide is a poisonous gas.
- It is colourless and odourless (you can't see or smell it).
- When breathed in, it replaces oxygen in your bloodstream, preventing oxygen from reaching your body’s organs. It can cause headaches, dizziness, and even death.
The amount of soot (black smoke) produced by a diesel engine is a good indicator of incomplete combustion. More soot means less oxygen was available during burning, meaning less energy released per litre of fuel.
Section 5: Making Plastics – Polymers
5.1 Monomers and Polymers
Many useful materials, especially plastics, are giant molecules built from small repeating units.
- Monomer: A small, basic molecule (like a single LEGO block). E.g., Ethene.
- Polymer: A very large molecule made up of thousands of monomers joined together (like a long LEGO chain).
The process of joining many monomers together to form a polymer is called polymerisation.
5.2 Addition Polymerisation
In the Single Award curriculum, you need to understand the simplest type of polymerisation, called addition polymerisation. This process works with alkenes because they have a double bond (\(C=C\)).
Step-by-Step Process:
- We start with alkene monomers (such as Ethene, \(C_2 H_4\)).
- Under specific conditions (high pressure and heat, often with a catalyst), the double bond in each monomer breaks open.
- The broken bonds allow the monomers to link up end-to-end, forming a very long chain called a polymer.
Example: Making Poly(ethene) (Polythene)
The monomer is Ethene.
The polymer is Poly(ethene) (often called Polythene).
Use: Polythene is used everywhere—carrier bags, bottles, and plastic sheeting.
Addition polymers are made when unsaturated molecules (monomers) link up by breaking their double bonds. No small atoms are lost in this process—the atoms in the polymer chain are exactly the same as the atoms in all the monomers combined.
Quick Chapter Review
- Organic Chemistry studies compounds based on Carbon.
- Hydrocarbons contain only Carbon and Hydrogen.
- Alkanes are saturated (\(C_n H_{2n+2}\), single bonds, unreactive).
- Alkenes are unsaturated (\(C_n H_{2n}\), contain \(C=C\) double bonds, more reactive).
- Test for Alkenes: They decolourise (turn colourless) Bromine water.
- Complete combustion produces safe products (\(CO_2\) and \(H_2O\)).
- Incomplete combustion produces dangerous Carbon Monoxide (CO) and soot.
- Plastics are polymers made from smaller units called monomers via addition polymerisation.
You’ve got this! Understanding the difference between saturated and unsaturated molecules is the most important step in Organic Chemistry. Keep practising those formulae!