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).
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A homologous series is a family of compounds with the same functional group and general formula, where successive members differ by a –CH₂ group.
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Functional groups are specific atoms or groups of atoms, like –OH or –COOH, that determine a molecule's characteristic chemical reactions.
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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.
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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.
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Simulations
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1 simulation
- PhETCore9701 13.1
Build a Molecule
Snap C, H and O atoms together into molecules; the sim names what you have built and shows it in 3D.
Try this
- On Playground build ethanol, C₂H₆O, and open its 3D view.
- Take it apart and join the same atoms as C–O–C: methoxymethane.
- Build ethanoic acid — the sim labels it with its common name, acetic acid.
Look for The functional group, not the formula, decides the family: –OH an alcohol, C–O–C an ether, –COOH a carboxylic acid.
Simulation by PhET Interactive Simulations, University of Colorado Boulder · Licensed to MarkScheme (public licence CC BY-NC 4.0 since 2026-03-30)
Key formulas
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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.
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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.
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Analyse the name:
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Glossary
Key terms for this topic — skim now; the Check step will test them.
- Homologous series
A series of organic compounds with the same functional group and similar chemical properties, in which successive members differ by a group.
- Functional group
An atom or group of atoms in a molecule that is responsible for its characteristic chemical reactions.
- Lowest locant rule
When numbering the carbon chain, start from the end that gives the functional groups and substituents the lowest possible numbers.
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Key takeaways
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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.
Complete Fig. 4.1 to: • draw the structure of compound A • name compound B.
Name and classify the three other functional groups in salbutamol in Table 6.1.
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