📋 What You'll Learn
Organic Formulae
How to write and understand displayed, structural, and molecular formulas. Recognising structural isomers.
Homologous Series
What bonds families of compounds together—the functional group. Using general formulae to predict any member.
Saturated vs Unsaturated
The difference between compounds with only single bonds and those with carbon-carbon double bonds.
Naming Rules
The systematic naming system that lets you read a name and draw the structure—or vice versa.
🧪 Part 1: Organic Formulae
What is Organic Chemistry?
Organic chemistry is the scientific study of the structure, properties, and reactions of organic compounds—molecules that contain carbon atoms.
A special rule: Metal carbonates, carbon dioxide (CO₂), and carbon monoxide (CO) are not classified as organic compounds, even though they contain carbon. We treat these as inorganic chemistry. This is just a convention, but it's important to remember for exams.
Why carbon? Carbon can form four covalent bonds and link to itself, creating long chains and complex structures. This is why millions of organic compounds exist—far more than any other element can create.
Hydrocarbons
Many of the organic compounds you'll study are hydrocarbons—compounds containing only hydrogen and carbon atoms. They're the simplest organic compounds and the building blocks for understanding more complex molecules.
Three Ways to Represent Organic Compounds
Organic molecules can be shown in three main ways, each giving different levels of detail:
1️⃣ Displayed Formula
A displayed formula shows every single atom and every bond in a molecule, arranged to show their spatial relationships. It's the most detailed representation.
H H H H
| | | |
H—C—C—C—C—H
| | | |
H H H H
This is pentane (C₅H₁₂)
All atoms and all bonds are shown
What can you read from a displayed formula?
- The exact number of carbon atoms (count the C's)
- The exact number of hydrogen atoms (count the H's)
- The types of bonds present (single C–C, double C=C, etc.)
- How atoms are arranged in space
Reading a Displayed Formula
Example:
A displayed formula shows: H–C–C–C–H with all hydrogens attached to each carbon.
Count carbons:
3 carbon atoms → This tells us the compound has "prop" in its name
Count hydrogens:
8 hydrogen atoms
Check bonds:
Only C–C single bonds (no C=C) → This is an
alkane
Result:
C₃H₈ (propane) with name ending in "-ane"
2️⃣ Structural Formula
Extended tier only. A structural formula simplifies the displayed formula by omitting most C–H bonds. It shows only the important bonds (like C=C double bonds) and the overall carbon skeleton.
Golden Rule for Structural Formulae:
👨👩👧👦 Part 2: Homologous Series & Functional Groups
🎯 What to Memorise