Combined Science (9204) | Organic chemistry

🔬 Comprehensive Study Notes: Organic Chemistry (Combined Science 9204) 🏹

Hello future scientist! Welcome to the world of Organic Chemistry. Don't worry if the name sounds complicated—it’s simply the chemistry of carbon, and carbon is one of the most flexible and exciting elements on the Periodic Table!

Understanding this chapter is vital because organic compounds make up the fuels we use (petrol, gas), the clothes we wear, and the plastics that surround us. Let’s dive into how carbon builds molecules!

1. Introduction to Organic Chemistry

What Makes Carbon Special?

Organic Chemistry is defined as the study of compounds containing carbon atoms, usually bonded to hydrogen atoms, but sometimes also oxygen, nitrogen, and others.

• The Foundation: Carbon (C) is unique because it can form four strong covalent bonds.
• Building Blocks: Carbon atoms can link together almost endlessly to form long chains, rings, and complex structures. This ability is called catenation.

⚠ Quick Fact: Although carbon dioxide (\(CO_2\)) contains carbon, it is usually classified as inorganic because it doesn't fit the typical structure of chains or rings.

2. Hydrocarbons: The Basics

The simplest organic compounds are called hydrocarbons. These are molecules that contain only carbon and hydrogen atoms.

Homologous Series

Hydrocarbons belong to groups called homologous series. A homologous series is a family of compounds that share the same general formula and have similar chemical properties.

• Each member differs from the next by a constant unit, usually a methyl group (\(CH_2\)).
• As the molecules get bigger (more carbon atoms), their boiling points increase, and they become less volatile (less easily evaporated).

💭 Memory Aid for Naming: All organic names start with a prefix indicating the number of carbon atoms in the main chain. You MUST know the first four:

Monkeys
Eat
Peeled
Bananas

C1: Meth- (Methane)
C2: Eth- (Ethane)
C3: Prop- (Propane)
C4: But- (Butane)

3. The Alkanes: Saturated Hydrocarbons

Alkanes are the most basic homologous series. They are the simplest, stable components of crude oil and natural gas.

Structure and Formula

• Bonding: Alkanes only contain carbon-carbon single bonds (\(C-C\)).
• Saturated: Because they only have single bonds, the molecule contains the maximum possible number of hydrogen atoms. We call them saturated.
• General Formula: The formula for any alkane is: \(C_nH_{2n+2}\) (where n is the number of carbon atoms).
• Suffix: Names always end in -ane.

Examples of Alkanes

C1: Methane (\(CH_4\)) - Natural gas
C2: Ethane (\(C_2H_6\))
C3: Propane (\(C_3H_8\)) - Used in portable stoves
C4: Butane (\(C_4H_{10}\)) - Lighter fuel

Properties of Alkanes

Alkanes are generally unreactive because the strong single bonds require a lot of energy to break. Their main chemical reaction is combustion (burning).

4. The Alkenes: Unsaturated Hydrocarbons

Alkenes are the next homologous series. They are much more reactive than alkanes and are very important in making plastics.

Structure and Formula

• Bonding: Alkenes MUST contain at least one carbon-carbon double bond (\(C=C\)).
• Unsaturated: Because they have a double bond, they could theoretically add more atoms (like hydrogen or bromine) across that bond. We call them unsaturated.
• General Formula: The formula for any alkene is: \(C_nH_{2n}\) (Note: n must be 2 or greater, as you need two carbons to form a double bond).
• Suffix: Names always end in -ene.

Examples of Alkenes

C2: Ethene (\(C_2H_4\)) - The monomer for polythene
C3: Propene (\(C_3H_6\))
C4: Butene (\(C_4H_8\))

The Test for Unsaturation (Alkenes vs. Alkanes)

This is a crucial test you need to know. Since alkenes are unsaturated, the double bond allows them to react readily with certain substances.

The Bromine Water Test:

1. Add a few drops of aqueous bromine water (which is an orange/brown colour) to the hydrocarbon sample.
2. If the substance is an Alkene (unsaturated): The bromine reacts across the double bond, causing the orange/brown colour to quickly disappear (decolourise).
3. If the substance is an Alkane (saturated): The single bonds do not react easily, so the bromine water remains orange/brown.

Analogy: Think of saturation like a sponge that is full of water (no space for new atoms). Unsaturated is like a dry sponge (it has space to absorb the bromine).

5. Reactions of Hydrocarbons: Combustion

The primary use of hydrocarbons (like those found in petrol and natural gas) is as fuels. When they burn, they release a large amount of energy (exothermic reaction).

1. Complete Combustion (Ideal Burning)

This happens when there is a plentiful supply of oxygen (air).

Hydrocarbon + Oxygen \(\rightarrow\) Carbon Dioxide + Water

Example (Methane):

\(CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O\)

The goal is always complete combustion because it produces the maximum amount of energy and the products (\(CO_2\) and \(H_2O\)) are less harmful than those from incomplete combustion.

2. Incomplete Combustion (Dirty Burning)

This happens when there is a limited supply of oxygen.

Hydrocarbon + Limited Oxygen \(\rightarrow\) Carbon Monoxide + Carbon (Soot) + Water

⚠ Danger Point: Carbon Monoxide (CO)

• Carbon monoxide is a colourless, odourless, and extremely poisonous gas.
• It binds to the haemoglobin in your blood instead of oxygen, leading to suffocation.
• Common Mistake to Avoid: Always ensure equations balance to show \(CO_2\) if the combustion is complete, and \(CO\) or \(C\) (soot) if it is incomplete.

6. Crude Oil and Its Uses

Crude oil is a finite (non-renewable) resource formed from the remains of ancient sea creatures under heat and pressure over millions of years.

It is a complex mixture of mostly alkanes (hydrocarbons) of varying chain lengths.

Separation: Fractional Distillation

Before crude oil can be useful, it must be separated into different parts (fractions).

Principle: Separation is based on the different boiling points of the components.

Step-by-Step Process:

1. Crude oil is heated until most of it vaporises.
2. The hot vapour enters the bottom of a tall column (the fractionating column).
3. As the vapour rises, it cools down.
4. Different fractions condense (turn back into liquid) at different temperatures/heights.
5. At the bottom (Hot): Large molecules (high boiling points, e.g., bitumen) condense.
6. At the top (Cool): Small molecules (low boiling points, e.g., refinery gases) condense and are collected.

Analogy: Imagine a staircase. The heavy, lazy molecules (bitumen) only make it up the first step. The small, light, energetic molecules (gases) reach the very top.

Fraction Uses (from Top to Bottom of Column)

• Refinery Gas (Smallest C chains): Bottled gas, fuel for heating.
• Gasoline/Petrol: Fuel for cars.
• Naphtha: Used to make other chemicals (petrochemicals).
• Kerosene/Paraffin: Jet fuel, heating oil.
• Diesel/Gas Oil: Fuel for diesel engines.
• Lubricating Oil: Oils, waxes, polishes.
• Fuel Oil: Ships, power stations.
• Bitumen (Largest C chains): Roofing, road surfacing.

Cracking

There is a high demand for short-chain hydrocarbons (like petrol and naphtha) but crude oil produces far too much of the less useful, longer-chain fractions (like heavy fuel oil).

Cracking is the process of breaking down large, long-chain hydrocarbons into smaller, more useful molecules.

Products of Cracking: Cracking always produces a mixture of a smaller alkane and an alkene.

Large alkane \(\rightarrow\) Smaller alkane + Alkene

💡 Importance: Cracking increases the supply of useful fuels (like petrol) and also provides alkenes (like ethene and propene) which are essential for making plastics.

7. Polymers and Polymerisation

Polymers are huge molecules made up of many small, repeating units joined together. Think of them as giant chemical necklaces.

Monomers and Polymers

• Monomer: The small molecule that serves as the basic repeating unit. (One bead)
• Polymer: The long chain molecule formed by linking monomers together. (The whole necklace)

Addition Polymerisation

The most common type of polymerisation in IGCSE is addition polymerisation. This is the process where many unsaturated monomers (alkenes) add together across their double bond to form a long chain polymer.

The Key Example: Polythene

The monomer is ethene (\(C_2H_4\)). The polymer is poly(ethene), commonly known as Polythene or Polyethylene.

During the reaction, the double bond in each ethene molecule breaks, allowing the carbon atoms to link up repeatedly to form a massive single chain.

$$ \text{n} \quad [\text{Ethene monomer}] \quad \xrightarrow{\text{Heat/Pressure}} \quad [\text{Poly(ethene) polymer}] $$

Uses: Polythene is used everywhere—plastic bags, bottles, washing-up bowls, and food wrapping.

The Problem with Polymers

Most addition polymers are inert (unreactive) because their backbones are essentially long alkane chains (only single C-C bonds). This means they do not easily break down naturally when thrown away, making them non-biodegradable and leading to landfill and pollution issues.

You’ve covered the fundamentals of organic chemistry! Keep practicing those formulas and reactions, and you’ll master this chapter in no time!