In simple terms
A friendly intro before the formal notes — no formulas yet.
The Language of Organic Molecules
Organic chemistry uses a systematic language (IUPAC) to name and classify the vast number of carbon-based compounds. This system is based on identifying the main carbon chain, functional groups, and their positions.
Think of naming an organic molecule like giving a street address. The longest carbon chain is the 'street name' (e.g., Pentane Street), functional groups are 'house numbers' (e.g., No. 2), and any side branches are 'landmarks' (e.g., opposite the Oak tree). The IUPAC rules ensure everyone can find the exact same 'house' (molecule) from its 'address' (name).
- 1
A homologous series is a family of compounds with the same functional group and general formula, where successive members differ by a –CH₂ group.
- 2
Functional groups are specific atoms or groups of atoms, like –OH or –COOH, that determine a molecule's characteristic chemical reactions.
- 3
Apply IUPAC rules by first finding the longest continuous carbon chain, then numbering it to give the lowest possible numbers (locants) to functional groups and substituents.
- 4
A displayed formula shows every single atom and every single bond, whereas a skeletal formula simplifies this by representing carbon chains as zig-zag lines.
Explore the concept
Use the live diagram and synced steps — play it or tap a step card to walk through.
Key formulas
Tap any symbol to reveal exactly what it means and its units.
Full topic notes
Formal explanation with the rigour you need for the exam.
Representing Organic Molecules: Types of Formulae
A single organic compound can be represented in several ways, each providing a different level of detail. It is crucial to be able to interpret and draw each type.
Empirical Formula: The simplest whole-number ratio of atoms of each element in a compound. (e.g., for glucose).
Molecular Formula: The actual number of atoms of each element in one molecule of the compound. (e.g., for glucose).
Structural Formula: Shows the arrangement of atoms in a molecule, indicating which atoms are bonded to which, but in a condensed format. (e.g., for ethanol).
Displayed Formula: Shows every atom and every bond in the molecule. It gives a 2D representation of the connectivity. (e.g., drawing out all C-H, C-C, and C-O bonds for ethanol).
Skeletal Formula: A simplified representation where carbon chains are lines, and carbon/hydrogen atoms are implied. Vertices and ends of lines are carbons. Functional groups are explicitly drawn.
Homologous Series and Functional Groups
To manage the vast number of organic compounds, we group them into 'families' called homologous series. Each series is defined by a specific functional group, which is the reactive part of the molecule. All members of a homologous series share similar chemical properties and can be represented by a general formula.
Homologous Series Characteristics: Same functional group, successive members differ by a unit, can be represented by a general formula (e.g., Alkanes: ), show a gradual change in physical properties, and have similar chemical properties.
Key AS-Level Functional Groups:
Alkane (C-C single bonds)
Alkene (C=C double bond)
Alcohol (-OH)
Halogenoalkane (-F, -Cl, -Br, -I)
Aldehyde (-CHO)
Ketone (C=O within a chain)
Carboxylic Acid (-COOH)
The IUPAC System of Nomenclature
The International Union of Pure and Applied Chemistry (IUPAC) provides a systematic method for naming organic compounds. The name of any compound is built from three parts: a prefix (substituents), a stem (number of carbons in the main chain), and a suffix (principal functional group).
Name = (Prefixes) + (Stem) + (Suffix)
Step 1: Find the Stem. Identify the longest continuous carbon chain that contains the principal functional group. This gives the stem name (meth-, eth-, prop-, but-, pent-, hex-).
Step 2: Find the Suffix. Identify the principal functional group to determine the suffix (e.g., '-ol' for alcohol, '-oic acid' for a carboxylic acid).
Step 3: Find the Prefixes. Identify any side chains (alkyl groups) or less important functional groups (e.g., halogens).
Step 4: Number the Chain. Number the longest chain from the end that gives the principal functional group the lowest possible number. If there's no functional group, give the substituents the lowest possible numbers.
Step 5: Assemble the Name. Write the full name by putting substituents in alphabetical order (ignoring di-, tri-), preceded by their position numbers. The stem and suffix follow. For example, 3-chloro-2-methylpentane.
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
An organic compound has the structural formula . Give its IUPAC name.
- 1
Identify the principal functional group: The -OH group indicates it is an alcohol, so the suffix will be '-ol'.
Draw the skeletal formula for 2,2-dichloro-3-methylbutane and state its molecular formula.
- 1
Analyse the name:
How it all connects
The big idea sits in the middle — tap a linked idea to explore the link.
Tap a linked idea to see how it connects back to the main topic — that connection is what examiners reward.
Glossary
Try to recall each definition before you reveal it.
Quick check
Answer in your head first — then tap to check. No pressure.
Revision flashcards
Flip the card. Test yourself before the exam.
What is a homologous series?
A series of organic compounds with the same functional group and similar chemical properties, in which successive members differ by a group.
Key takeaways
Review these before you close the topic — retrieval beats re-reading.
- ✓
Empirical Formula: The simplest whole-number ratio of atoms of each element in a compound. (e.g., for glucose).
- ✓
Molecular Formula: The actual number of atoms of each element in one molecule of the compound. (e.g., for glucose).
- ✓
Structural Formula: Shows the arrangement of atoms in a molecule, indicating which atoms are bonded to which, but in a condensed format. (e.g., for ethanol).
- ✓
Displayed Formula: Shows every atom and every bond in the molecule. It gives a 2D representation of the connectivity. (e.g., drawing out all C-H, C-C, and C-O bonds for ethanol).
- ✓
Skeletal Formula: A simplified representation where carbon chains are lines, and carbon/hydrogen atoms are implied. Vertices and ends of lines are carbons. Functional groups are explicitly drawn.
Practice — then mark it
The whole point: a real Cambridge question, marked mark-by-mark.
Practice Questions
Practice Questions
Extra simulations & links
PhET, GeoGebra and other curated tools — open in a new tab.
Frequently asked
Checkpoint
One marked question is worth ten re-reads — close the loop before you move on.
Reading it isn’t knowing it — prove it.
Before you move on: do Practice Questions on paper, snap a photo, and get examiner-style feedback on exactly where you win and lose marks.