An Introduction to Organic Chemistry
AN INTRODUCTION TO ORGANIC CHEMISTRY
Complete Study Notes for Cambridge IGCSE Chemistry
SECTION 16.1: Understanding Organic Chemistry Basics
What Are Organic Compounds?
Organic compounds are special chemicals that contain the element carbon. You can find them everywhere in your daily life! The food you eat (like bread, rice, and meat), your hair, plastic bottles, soap, and even medicines are all made from organic compounds.
There are two types of organic compounds:
- Natural organic compounds - These come from nature, like the proteins in your hair or the sugar in fruits
- Synthetic organic compounds - These are man-made in factories, like plastics, detergents, and modern medicines
What Are Hydrocarbons?
Hydrocarbons are the simplest type of organic compounds. They contain only two elements:
- Hydrogen (H)
- Carbon (C)
Think of hydrocarbons like LEGO blocks - they're made of just two types of pieces! Examples include methane (CH₄) in natural gas and propane (C₃H₈) in camping fuel.
Understanding Homologous Series
Imagine you have a family where all the children have the same nose shape - that's similar to a homologous series!
A homologous series is a family of similar organic compounds that share these things:
- The same general formula (a pattern for how many atoms they have)
- The same functional group (a special group of atoms that makes them act similarly)
- Similar chemical properties (they react in similar ways)
- Each member differs from the next by a -CH₂- unit (like adding one more LEGO block)
The four main homologous series you need to know are:
- Alkanes (like methane, ethane, propane)
- Alkenes (like ethene, propene)
- Alcohols (like methanol, ethanol)
- Carboxylic acids (like methanoic acid, ethanoic acid)
General Formulas: The Pattern for Each Family
Each homologous series has a mathematical pattern called a general formula:
| Homologous Series | General Formula | Example |
|---|---|---|
| Alkanes | CnH2n+2 | CH₄, C₂H₆, C₃H₈ |
| Alkenes | CnH2n | C₂H₄, C₃H₆, C₄H₈ |
| Alcohols | CnH2n+1OH | CH₃OH, C₂H₅OH |
| Carboxylic acids | CnH2n+1COOH | HCOOH, CH₃COOH |
CnH2n+2
Where n = number of carbon atoms
Example: If n = 4
C₄H(2×4)+2 = C₄H₁₀ (Butane)
What Are Functional Groups?
A functional group is like a special badge that determines how a compound behaves. It's an atom or group of atoms that gives compounds their chemical properties.
Here are the main functional groups:
Characteristics of a Homologous Series
Members of the same homologous series are like brothers and sisters in a family - they share common features:
- Same general formula - They follow the same mathematical pattern
- Same functional group - They have the same "badge"
- Similar chemical reactions - They behave similarly in experiments
- Differ by -CH₂- - Each member has one more CH₂ unit than the previous one
- Gradual change in physical properties - As you go down the series, properties like boiling point and thickness change gradually
• Methanol (CH₃OH) boils at 65°C
• Ethanol (C₂H₅OH) boils at 78°C
• Propanol (C₃H₇OH) boils at 97°C
• Butanol (C₄H₉OH) boils at 117°C
Notice how the boiling point increases as the molecule gets bigger!
Saturated vs Unsaturated Compounds
Think of saturated compounds like a full parking lot - no more cars can fit!
Saturated compounds have molecules where all carbon-carbon bonds are single bonds (C-C). Examples:
- Propane: C₃H₈
- Butanoic acid: C₄H₈O₂
Unsaturated compounds have molecules with one or more carbon-carbon double bonds (C=C) or triple bonds (C≡C). Examples:
- Propene (C₃H₆) - has one C=C bond
- Butadiene (C₄H₆) - has two C=C bonds
- Ethyne/Acetylene (C₂H₂) - has one C≡C bond
SECTION 16.2: Structural Formulas and Isomers
Different Ways to Show Molecules
Scientists use different "languages" to describe the same molecule. Let's learn three important ones:
| Type of Formula | What It Shows | Example |
|---|---|---|
| Molecular Formula | Number and type of each atom, but NOT how they're connected | C₂H₆O |
| Displayed Formula | ALL the atoms and ALL the bonds | Shows complete structure with all bonds drawn |
| Structural Formula | How atoms are arranged without drawing every bond | CH₃CH₂OH or C₂H₅OH |
What Are Structural Isomers?
Here's where chemistry gets really interesting! Structural isomers are compounds that have the same molecular formula but different structural formulas.
Think of it like building with LEGO blocks - you can use the same pieces to build different things!
SECTION 16.3: Naming Organic Compounds
How to Name Alkanes
Alkanes are hydrocarbons with only single bonds. Their names always end in -ane.
The first part of the name tells you how many carbon atoms there are:
| Number of Carbons | Prefix | Alkane Name | Molecular Formula | Structural Formula |
|---|---|---|---|---|
| 1 | Meth- | Methane | CH₄ | CH₄ |
| 2 | Eth- | Ethane | C₂H₆ | CH₃CH₃ |
| 3 | Prop- | Propane | C₃H₈ | CH₃CH₂CH₃ |
| 4 | But- | Butane | C₄H₁₀ | CH₃CH₂CH₂CH₃ |
How to Name Alkenes
Alkenes are hydrocarbons with at least one double bond (C=C). Their names end in -ene.
| Number of Carbons | Alkene Name | Molecular Formula | Structural Formula |
|---|---|---|---|
| 2 | Ethene | C₂H₄ | CH₂=CH₂ |
| 3 | Propene | C₃H₆ | CH₂=CHCH₃ |
| 4 | Butene | C₄H₈ | CH₂=CHCH₂CH₃ (but-1-ene) CH₃CH=CHCH₃ (but-2-ene) |
How to Name Alcohols
Alcohols contain the hydroxyl group (-OH). Their names end in -ol.
| Number of Carbons | Alcohol Name | Molecular Formula | Structural Formula |
|---|---|---|---|
| 1 | Methanol | CH₄O | CH₃OH |
| 2 | Ethanol | C₂H₆O | CH₃CH₂OH |
| 3 | Propanol | C₃H₈O | CH₃CH₂CH₂OH (propan-1-ol) CH₃CHOHCH₃ (propan-2-ol) |
| 4 | Butanol | C₄H₁₀O | CH₃CH₂CH₂CH₂OH (butan-1-ol) CH₃CHOHCH₂CH₃ (butan-2-ol) |
How to Name Carboxylic Acids
Carboxylic acids contain the carboxyl group (-COOH). Their names end in -oic acid.
| Number of Carbons | Acid Name | Molecular Formula | Structural Formula |
|---|---|---|---|
| 1 | Methanoic acid | CH₂O₂ | HCOOH |
| 2 | Ethanoic acid | C₂H₄O₂ | CH₃COOH |
| 3 | Propanoic acid | C₃H₆O₂ | CH₃CH₂COOH |
| 4 | Butanoic acid | C₄H₈O₂ | CH₃CH₂CH₂COOH |
SECTION 16.4: Fossil Fuels and Petroleum
What Are Fossil Fuels?
Fossil fuels are substances that burn easily in air to produce energy. They formed from dead plants and animals that lived millions of years ago.
The three main fossil fuels are:
- Coal - A solid fuel made mostly of carbon
- Natural gas - A gaseous fuel made mostly of methane (CH₄)
- Petroleum (crude oil) - A liquid fuel that is a mixture of many different hydrocarbons
What Is Petroleum?
Petroleum (also called crude oil) is a dark brown, thick, foul-smelling liquid. It's not very useful in its natural form, so we must separate it into different parts called fractions.
Petroleum is valuable because it's used to make:
- Fuels (gasoline, diesel, jet fuel)
- Plastics
- Detergents
- Medicines
- Synthetic rubber
Fractional Distillation: Separating Petroleum
Fractional distillation is the process used to separate petroleum into useful fractions. It works because different hydrocarbons have different boiling points.
Step 1: Petroleum is heated in a furnace until it turns into vapor (gas)
Step 2: The vapor rises up a tall tower called a fractionating column
Step 3: As the vapor rises and cools, different fractions condense (turn back to liquid) at different heights
Step 4: Lighter fractions (smaller molecules) are collected at the TOP because they have lower boiling points
Step 5: Heavier fractions (bigger molecules) are collected at the BOTTOM because they have higher boiling points
Properties of Petroleum Fractions
As you move from the BOTTOM to the TOP of the fractionating column:
| Property | Bottom → Top |
|---|---|
| Chain length (number of carbons) | Decreases (C₇₀ → C₁) |
| Boiling point | Decreases (350°C → below 40°C) |
| Volatility (how easily it evaporates) | Increases |
| Viscosity (thickness) | Decreases (thick → runny) |
| Ease of burning | Increases |
| Color | Darker → Lighter |
Petroleum Fractions and Their Uses
Here are the main fractions collected from petroleum, from top to bottom of the column:
| Fraction | Carbon Atoms | Boiling Point | Uses |
|---|---|---|---|
| Refinery gas | C₁ to C₄ | Below 40°C | Gas for heating and cooking |
| Petrol (Gasoline) | C₅ to C₁₀ | 40-75°C | Fuel for cars |
| Naphtha | C₇ to C₁₄ | 90-150°C | Chemical feedstock (to make plastics, detergents, medicines) |
| Kerosene (Paraffin) | C₁₀ to C₁₆ | 150-240°C | Jet fuel for airplanes |
| Diesel oil | C₁₅ to C₂₅ | 220-250°C | Fuel for diesel engines (trucks, buses) |
| Fuel oil | C₂₀ to C₃₀ | 250-320°C | Fuel for ships and home heating |
| Lubricating oil | C₂₀ to C₃₅ | 300-350°C | Lubricants, waxes, and polishes |
| Bitumen | C₇₀+ | Above 350°C | Making roads |
Why Do We Use Different Fractions for Different Jobs?
The properties of each fraction make it suitable for specific uses:
- Lighter fractions (like petrol) burn easily and cleanly, making them good fuels for vehicles
- Heavier fractions (like lubricating oil) are thick and don't evaporate easily, making them good for reducing friction in engines
- Very heavy fractions (like bitumen) are very thick and sticky, perfect for making roads
Answer: Kerosene has a higher energy content per volume and is less volatile (safer to store), making it better for airplanes that need to carry lots of fuel.
Conserving Petroleum
Since petroleum takes millions of years to form and we're using it faster than nature can replace it, we need to conserve it! Here's how:
- Reduce usage - Use public transportation instead of driving individual cars
- Use alternatives - Solar energy, wind energy, and biofuels (fuels made from plants and animals)
- Improve efficiency - Design better engines and power stations that use less fuel
KEY VOCABULARY REVIEW
| Term | Definition |
|---|---|
| Organic compound | A compound containing carbon (except CO₂ and carbonates) |
| Hydrocarbon | A compound containing only hydrogen and carbon |
| Homologous series | A family of similar compounds with the same functional group |
| Functional group | An atom or group of atoms that determines chemical properties |
| General formula | A pattern showing the ratio of atoms in a homologous series |
| Saturated | Contains only single carbon-carbon bonds |
| Unsaturated | Contains double or triple carbon-carbon bonds |
| Structural isomers | Compounds with the same molecular formula but different structures |
| Fossil fuel | Fuels formed from dead organisms millions of years ago |
| Fractional distillation | Separating petroleum into fractions based on boiling points |
| Volatile | Evaporates easily at low temperatures |
| Viscous | Thick and doesn't flow easily |
IMPORTANT FORMULAS SUMMARY
CnH2n+2
Where n = number of carbon atoms
Examples: Methane (CH₄), Ethane (C₂H₆), Propane (C₃H₈), Butane (C₄H₁₀)
2. ALKENES - General Formula:
CnH2n
Where n = number of carbon atoms (n ≥ 2)
Examples: Ethene (C₂H₄), Propene (C₃H₆), Butene (C₄H₈)
3. ALCOHOLS - General Formula:
CnH2n+1OH
Where n = number of carbon atoms
Examples: Methanol (CH₃OH), Ethanol (C₂H₅OH), Propanol (C₃H₇OH)
4. CARBOXYLIC ACIDS - General Formula:
CnH2n+1COOH
Where n = number of carbon atoms (counting the one in COOH)
Examples: Methanoic acid (HCOOH), Ethanoic acid (CH₃COOH), Propanoic acid (C₂H₅COOH)
PRACTICE QUESTIONS
Basic Understanding:
- What elements are present in hydrocarbons?
- Name the four homologous series you studied in this chapter.
- What is the general formula for alkanes?
- Which functional group is found in alcohols?
- Name three fossil fuels.
Application:
- An alkane has 5 carbon atoms. What is its molecular formula? What is its name?
- Draw the displayed formula of propene.
- Identify the functional groups in this molecule: CH₃CH₂COOH
- Are these compounds isomers? C₂H₆O (CH₃CH₂OH) and C₂H₆O (CH₃OCH₃)
- Which petroleum fraction would you use to fuel a car? A jet plane? To make roads?
Higher-Order Thinking:
- Explain why members of a homologous series have similar chemical properties.
- Why is petroleum referred to as "black gold"?
- Suggest two reasons why we should conserve petroleum.
- Explain why lighter petroleum fractions are collected at the top of the fractionating column.
Summary
In this chapter, you learned that:
- Organic compounds contain carbon and are found naturally or made synthetically
- Hydrocarbons contain only hydrogen and carbon
- Organic compounds are grouped into homologous series based on their functional groups
- The four main series are alkanes, alkenes, alcohols, and carboxylic acids
- Each series has a general formula and specific functional group
- Structural isomers have the same molecular formula but different structures
- Fossil fuels (coal, natural gas, petroleum) formed from ancient organisms
- Petroleum is separated by fractional distillation into useful fractions
- Different fractions have different properties and uses based on their molecular size
