Look at the labels below. These are real chemical names taken from everyday products. Read each name carefully and think about what patterns you can notice.
1Pattern hunting. Examine the names above. Each begins with a prefix (meth-, eth-, prop-, but-, pent-, hex-). What do these prefixes correspond to? Formulate a hypothesis.
2End-of-name clues. Some names end in -ane, one ends in -ol, and another in -oic acid. Look at the molecular formulae. What might the ending of the name tell us about the molecule?
Organic chemistry is built on the unique bonding properties of carbon. Review the concept map below, then complete the tasks.
3Fill the gaps. Complete the sentences using the concept map as a guide.
A carbon atom has electrons in its outer shell and therefore forms covalent bonds. Carbon can bond to other atoms to form chains and rings. These chains may be or branched. In addition to carbon, organic molecules typically contain atoms bonded to the chain.
4Quick recall. What type of bond is formed when two atoms share a pair of electrons?
Historically, organic compounds received trivial names based on their origin — for example, formic acid from Latin formica (ant) and acetic acid from Latin acetum (vinegar). As the number of known organic compounds grew to millions, this approach became unworkable. The International Union of Pure and Applied Chemistry (IUPAC) developed a systematic naming convention that encodes the structure of a molecule directly into its name. A correctly written IUPAC name allows any chemist in the world to reconstruct the structural formula of the compound.
There are several ways to represent an organic molecule on paper. Each level of representation reveals different information.
| Formula Type | Definition | Example (Propan-1-ol) | Shows |
|---|---|---|---|
| Molecular | Total count of each atom in the molecule | C₃H₈O | Composition only |
| Empirical | Simplest whole-number ratio of atoms | C₃H₈O | Ratio of atoms |
| Structural | Shows how atoms are grouped, without individual bonds | CH₃CH₂CH₂OH | Groups of atoms |
| Displayed | Shows every bond and every atom | (see diagram below) | All bonds |
| Skeletal | Carbon backbone as zig-zag lines; C and H atoms implied | (see diagram below) | Shape of backbone |
5Identify. What type of formula is each of the following?
6Convert. Write the structural formula for butane (C₄H₁₀), showing each carbon with its hydrogen atoms.
Every IUPAC name of a simple organic compound consists of three parts that together describe the molecule's structure:
The root of the name tells you how many carbon atoms are in the longest continuous carbon chain.
| Carbon atoms | 1 | 2 | 3 | 4 | 5 | 6 |
|---|---|---|---|---|---|---|
| Root name | meth- | eth- | prop- | but- | pent- | hex- |
The suffix identifies the homologous series (functional group family) to which the molecule belongs.
| Homologous series | Functional group | Suffix | General formula | Example |
|---|---|---|---|---|
| Alkane | C–C single bonds only | -ane | CnH2n+2 | Propane |
| Alkene | C=C double bond | -ene | CnH2n | Propene |
| Alcohol | –OH | -ol | CnH2n+1OH | Propan-1-ol |
| Carboxylic acid | –COOH | -oic acid | CnH2n+1COOH | Propanoic acid |
| Aldehyde | –CHO | -al | CnH2n+1CHO | Propanal |
| Ketone | >C=O (internal) | -one | — | Propanone |
| Haloalkane | C–X (X = F, Cl, Br, I) | prefix: fluoro-, chloro-, bromo-, iodo- | — | 1-chloropropane |
When a functional group can appear at different positions, a locant (number) is used to indicate where it is attached. The chain is always numbered so that the functional group receives the lowest possible locant. For example: butan-1-ol (not butan-4-ol) and but-1-ene (not but-3-ene).
7Classify. Sort each compound into the correct homologous series.
8Name that molecule. Use the three-step naming method to give the IUPAC name for each compound.
| Structural formula | Longest chain | Functional group | IUPAC name |
|---|---|---|---|
| CH₃CH₂CH₂CH₂CH₃ | |||
| CH₃CH₂CH=CH₂ | |||
| CH₃CH₂CH₂OH | |||
| CH₃CH₂CH₂CH₂COOH |
Molecules that share the same molecular formula but differ in the arrangement of atoms are called structural isomers. Because IUPAC nomenclature encodes structure, isomers receive different names. This also means they can have different physical and chemical properties.
When a carbon chain has a branch (substituent), the naming procedure adds a prefix:
1. Find the longest continuous chain — this gives the root name.
2. Number the chain so that the branch gets the lowest locant.
3. Name the branch as an alkyl group (methyl-, ethyl-, propyl-).
4. Combine: locant-branch name + root + suffix.
9Reason. Butane boils at −1 °C while 2-methylpropane boils at −12 °C, yet both have the molecular formula C₄H₁₀. Use your understanding of intermolecular forces to explain why the branched isomer has a lower boiling point.
10Predict & name. Draw (or describe) two structural isomers of C₅H₁₂. Name each one using IUPAC rules.
11Explain. A student writes the name "3-methylbutane" instead of "2-methylbutane". Explain why the student's name is incorrect according to IUPAC rules.
Before the 1890s, organic compounds were named after their source material, their discoverer, or some characteristic property. Urea was named because it was first isolated from urine; morphine took its name from Morpheus, the Greek god of dreams, because of its sedative effects. As the field expanded, different laboratories sometimes gave the same compound different names, while entirely different substances occasionally shared the same trivial name.
In 1892, a congress of chemists met in Geneva to establish the first standardised rules for naming organic compounds. These rules were refined over the following century by IUPAC. The modern system encodes three critical pieces of information into every name: the length of the carbon backbone, the type of functional group present, and the position of that functional group on the chain. As a result, it is now possible to reconstruct a molecule's structure from its IUPAC name alone — something that was impossible with trivial names.
Today, over 200 million organic compounds are registered in the Chemical Abstracts Service database. Without systematic nomenclature, scientific communication about these molecules would be chaotic. The IUPAC system thus serves not merely as a labelling convention, but as a universal structural language for chemistry.
12Main idea. In one sentence, state the central argument of the passage.
13Extract. According to the text, what three pieces of information does a modern IUPAC name encode?
14True or false — based on the text.
a) Before 1892, naming conventions were globally standardised.
b) Trivial names could sometimes cause confusion because different labs used different names for the same compound.
c) The IUPAC name of a compound cannot be used to deduce its structure.
15Connect. The passage describes IUPAC nomenclature as "a universal structural language." Use an example from the lesson to explain why this phrase is accurate.
Use the controls below to build a simple carbon chain. Observe how the name, structural formula, and skeletal formula change as you adjust the chain length and functional group. Record your observations in the fields provided.
Answer the following questions in the style expected by the AQA A-level Chemistry examination. Show all reasoning where required.
Question 1 (2 marks)
Give the IUPAC name of the compound with the structural formula CH₃CH₂CH(CH₃)CH₂CH₃.
Question 2 (2 marks)
Draw the displayed formula of propan-2-ol.
Question 3 (3 marks)
There are three structural isomers of C₅H₁₂. Draw the structural formula of each one and give its IUPAC name.
Question 4 (3 marks)
A student is given the skeletal formula below.
(a) Determine the molecular formula of this compound. (1 mark)
(b) Give the IUPAC name of this compound. (1 mark)
(c) Write the structural formula for this compound. (1 mark)
Question 5 (4 marks)
Explain why it is possible for two compounds to have the same molecular formula but different IUPAC names. Include in your answer the term structural isomer and provide an example using compounds with the formula C₄H₁₀.
H1Conceptual understanding. In your own words, explain the purpose of IUPAC nomenclature and why trivial names are insufficient for modern chemistry. Include at least one specific example.
H2Structural analysis. Complete the table by filling in the missing cells.
| IUPAC Name | Structural Formula | Molecular Formula | Homologous Series |
|---|---|---|---|
| Hexane | |||
| CH₃CH=CHCH₃ | |||
| C₃H₈O | Alcohol | ||
| Pentanoic acid |
H3Cause and effect. Two compounds share the molecular formula C₃H₆O₂. One is propanoic acid and the other is methyl methanoate (an ester). Explain why these two molecules have different IUPAC names despite sharing the same molecular formula. What structural feature distinguishes them?
H4Exam-style. (3 marks) The compound 2-methylpentane is a structural isomer of hexane.
(a) Write the structural formula of 2-methylpentane. (1 mark)
(b) Explain, in terms of intermolecular forces, why 2-methylpentane has a lower boiling point than hexane. (2 marks)
Answer each reflective question briefly. Be honest about what you understand and what still needs reinforcement.
1. AQA A-level Chemistry textbook, Chapter 12: Basic Concepts of Organic Chemistry — review the section on nomenclature with worked examples.
2. Royal Society of Chemistry — Learn Chemistry: interactive nomenclature quizzes and naming practice (freely available online).
3. IUPAC recommendations 2013 (Blue Book) — for those wishing to explore advanced naming rules beyond the A-level specification.
A running notebook for the entire lesson. Write or draw.