Topic 4 · page 4 of 17
Naming organic compounds
IUPAC names are built from a stem for the longest carbon chain, plus the names and positions of whatever is attached to it. The same rules let you draw a molecule from its name.
Read first: Ways of writing a formula
Stems and prefixes up to ten carbons 4.54.5(i)
The International Union of Pure and Applied Chemistry (IUPAC) sets one system of names, so that every compound has one name that tells you its structure. The stem shows how many carbons are in the longest chain. You must know the stems up to ten carbons.
| Carbons | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
|---|---|---|---|---|---|---|---|---|---|---|
| Stem | meth- | eth- | prop- | but- | pent- | hex- | hept- | oct- | non- | dec- |
| Alkane | methane | ethane | propane | butane | pentane | hexane | heptane | octane | nonane | decane |
| Formula | CH₄ | C₂H₆ | C₃H₈ | C₄H₁₀ | C₅H₁₂ | C₆H₁₄ | C₇H₁₆ | C₈H₁₈ | C₉H₂₀ | C₁₀H₂₂ |
The ending shows the family: -ane for an alkane, -ene for an alkene, -ol for an alcohol. The first prefix a branch can take is also built on the stem:
| Group | Name | Formula |
|---|---|---|
| a one-carbon branch | methyl | –CH₃ |
| a two-carbon branch | ethyl | –C₂H₅ |
| a halogen atom | fluoro, chloro, bromo, iodo | –F, –Cl, –Br, –I |
A branch made of carbon and hydrogen is an alkyl group. It is named from the alkane with the last -ane changed to -yl.
The five naming rules 4.5(i)
- Find the longest carbon chain. It gives the stem, and it is the main chain.
- Identify the groups attached to it (methyl, ethyl, chloro, bromo and so on).
- Number the main chain from the end that gives the attached groups the lowest numbers.
- Use di-, tri-, tetra- when the same group appears more than once, and give each its own number. List different groups in alphabetical order.
- Write commas between numbers and hyphens between numbers and letters. The name is one word.
Worked exampleNaming a branched alkane
Name the molecule drawn in the figure above.
- The longest chain has 5 carbons, so the stem is pent- and the main chain is pentane.
- There is one group, a methyl (–CH₃).
- Numbering from the left puts the methyl on carbon 2. Numbering from the right puts it on carbon 4. Choose the lower number: 2.
- Name: 2-methylpentane.
Worked exampleTwo different groups
Name the molecule below.
- The longest chain has 3 carbons: propane.
- The groups are bromo and methyl, both on carbon 2.
- List them alphabetically: b comes before m, so bromo goes first. Repeat the number for each group: 2-bromo-2-methyl.
- Name: 2-bromo-2-methylpropane.
When there are two of the same group, use the prefix and a number for each: 2,2-dimethylbutane has two methyl groups, both on carbon 2. The prefix di- is ignored when you put groups in alphabetical order, so 3-ethyl-2,2-dimethylpentane lists ethyl first (e before m).
Cycloalkanes and halogenoalkanes
- A ring gets the prefix cyclo-: cyclopropane, cyclohexane.
- A ring with one attached group needs no number: methylcyclopropane.
- A ring with two or more attached groups is numbered so that the groups get the lowest numbers: 1,2-dimethylcyclopropane, 1,1-dimethylcyclopropane.
- A halogenoalkane is named as an alkane with a halogen prefix: 1-bromobutane, 2-chloropropane. With a single carbon, no number is needed: chloromethane, trichloromethane.
Drawing a compound from its name 4.5(ii)
The rules work backwards. Always finish by naming what you have drawn and checking it matches.
MethodDrawing from a name
- Draw the main chain from the stem: for example, pent- is five carbons. Number them.
- Attach each group named in the name to the carbon whose number is given.
- Complete the formula. In a displayed formula, give every carbon four bonds with hydrogens. In a skeletal formula, hydrogens are left out.
- Check: count the carbons, make sure each has four bonds, and name your drawing again.
Worked exampleDraw 1,2-dibromoethane
- eth- is two carbons: C–C.
- dibromo means two bromine atoms. The numbers 1,2 put one on each carbon.
- Each carbon has one Br and one C, so two hydrogens complete it: BrCH₂CH₂Br.
- Displayed formula: each carbon has two C–H bonds, one C–Br bond and the C–C bond. Skeletal formula: a single line with a Br at each end.
Check yourself
- Give the names and molecular formulae of the straight-chain alkanes with 7, 8, 9 and 10 carbon atoms.
- Name the structures A to D in the figure.
- A student calls CH₃CH₂CH₂CH(CH₃)CH₃ "4-methylpentane". Explain why this is wrong and give the correct name.
- Name CH₃CH(Br)CH₂CH₃ and CH₃C(CH₃)₂Br.
- Draw the skeletal formula of 2,3-dimethylbutane, the displayed formula of 1,2-dibromoethane and the skeletal formula of 2-chlorobutane.
- Name the ring that has four carbons in the ring and two methyl groups on adjacent ring carbons.
Answers cover these until you have tried the questions
- Heptane C₇H₁₆; octane C₈H₁₈; nonane C₉H₂₀; decane C₁₀H₂₂.
- A: 3-methylpentane (five-carbon chain, methyl on carbon 3). B: 2,2-dimethylbutane (four-carbon chain, two methyls on carbon 2). C: 1-bromobutane (four carbons, bromine on carbon 1). D: trichloromethane (one carbon with three chlorines).
- The five-carbon chain must be numbered from the end that gives the methyl the lowest number. Numbered from the left, as written, the methyl is on carbon 4; numbered from the right it is on carbon 2. The correct name is 2-methylpentane.
- CH₃CH(Br)CH₂CH₃ is a four-carbon chain with bromine on carbon 2: 2-bromobutane. CH₃C(CH₃)₂Br is a three-carbon chain with a bromine and a methyl on carbon 2: 2-bromo-2-methylpropane.
- See the figure below. a: a four-carbon zigzag with a methyl on carbon 2 and a methyl on carbon 3. b: the displayed formula of BrCH₂CH₂Br, with one Br on each carbon and two C–H bonds on each carbon. c: a four-carbon zigzag with Cl on carbon 2.
- 1,2-dimethylcyclobutane. The ring is cyclobutane; two methyl groups on adjacent carbons take the numbers 1 and 2.
