3.2.4Additional lessons
IUPAC Nomenclature — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 46 of 98
Part 1 of 6Building a Name
A systematic name is assembled in a fixed order so that every compound has one unambiguous name.
1
Find the longest continuous carbon chain that contains the principal functional group; its length gives the stem: meth, eth, prop, but, pent or hex.
2
Identify the suffix set by the highest-priority functional group, such as -ane, -ene, -ol, -al, -one, -oic acid or -amine.
3
Number the chain from the end that gives the principal functional group the lowest possible number.
4
Name each branch as an alkyl group (methyl, ethyl, propyl) or a halogen prefix (chloro, bromo) and give it the number of the carbon carrying it.
5
Arrange the prefixes alphabetically, use di, tri or tetra for repeats, and separate numbers with commas and numbers from words with hyphens.
Part 2 of 6Stems and Suffixes
Alkane stem
- Counting carbons gives meth (1), eth (2), prop (3), but (4), pent (5) and hex (6), with -ane for single bonds only.
Alkene and alkyne
- A C=C bond gives the ending -ene and a C≡C bond gives -yne, with a number showing where the multiple bond starts.
Alcohol
- The –OH group is shown by the ending -ol, as in propan-2-ol, where the number locates the carbon bearing the hydroxyl.
Aldehyde and ketone
- An aldehyde ends in -al and is always carbon 1, whereas a ketone ends in -one and needs a number when the chain is long enough.
Acid and amine
- A carboxylic acid ends in -oic acid with the acid carbon as carbon 1, and a primary amine ends in -amine.
| Group | Suffix or prefix | Example |
|---|---|---|
| Alkane | -ane | butane |
| Alkene | -ene | but-1-ene |
| Alcohol | -ol | pentan-2-ol |
| Aldehyde | -al | propanal |
| Ketone | -one | butanone |
| Carboxylic acid | -oic acid | ethanoic acid |
| Halogenoalkane | chloro-, bromo- | 2-chloropropane |
| Amine | -amine | ethanamine |
Part 3 of 6Branches and Numbering
Branches and Numbering
Alkyl branches
- A branch that is a carbon chain missing one hydrogen is named from its stem with -yl, for example CH₃ is methyl and C₂H₅ is ethyl.
Lowest locants
- When a name has several substituents, choose the numbering direction that gives the smaller number at the first point of difference.
Repeated groups
- Identical branches are counted with di, tri or tetra, and each one still needs its own locant, as in 2,2-dimethylbutane.
Alphabetical order
- Prefixes are listed alphabetically ignoring di and tri, so ethyl comes before methyl in 3-ethyl-2-methylpentane.
Part 4 of 6Reading a Name Backwards
1
Start with the stem and suffix to draw the main chain and put the functional group in place, for example hex-2-ene has six carbons with a double bond between C2 and C3.
2
Add each branch at its stated position, remembering that locants refer to the numbered carbons of the main chain.
3
Fill the remaining bonds of every carbon with hydrogen atoms so each carbon has four bonds in total.
4
Check the name you would give your drawing; if it differs from the original, a longest chain or a numbering step has gone wrong.
Part 5 of 6Worked Example
Question Name the compound CH₃CH(CH₃)CH₂CH(OH)CH₃.
- The longest chain containing the carbon bearing OH has five carbons, so the stem is pentane and the group is an alcohol (-ol).
- Number from the end nearer the OH group: counting from the right-hand CH₃ the OH carbon is C2, whereas counting from the left it would be C4.
- With this numbering the methyl branch sits on C4, so the substituent is 4-methyl.
- Combine the parts in orderbranch prefix, stem, locant and suffix.
Answer 4-methylpentan-2-ol
Part 6 of 6Exam Checklist & Tips
3.2.4 – Key Points for the Exam
- Longest chain
- The longest unbranched carbon chain containing the principal functional group;
- it sets the stem of the name.
- The longest unbranched carbon chain containing the principal functional group;
- Locant A number showing which carbon of the main chain carries a substituent or functional group.
- Substituent An atom or group attached to the main chain in place of a hydrogen, such as methyl or chloro.
- Homologous series A family of compounds with the same functional group and general formula, differing by CH₂.
- Principal functional group The group of highest priority, which determines the suffix and the direction of numbering.
Common Mistakes
- Choosing the longest visible straight line instead of the longest continuous chain, which may bend around corners in a drawing.
- Numbering from the wrong end so that the functional group does not receive the lowest number.
- Listing prefixes by size or number of carbons instead of alphabetically.
- Omitting commas between locants or writing a number next to a word without a hyphen.
Last updated:
\n