Coaching Centre
Naming hydrocarbons
Why carbon forms so many compounds, homologous series, and the IUPAC rules for naming alkanes, alkenes, alkynes and halogenated hydrocarbons up to C8, including methyl and ethyl branches, with worked examples.
Every bowser in Australia shows an octane number: 91, 95, 98. The 100 on that scale belongs to one compound, 2,2,4-trimethylpentane. It has the same molecular formula as octane, C₈H₁₈, yet octane itself knocks so badly in an engine that it rates below zero. The name tells you exactly why they differ: a five-carbon chain with three methyl branches, packed into a compact, bushy shape. In organic chemistry the name is the structure, written in words.
Why carbon makes so many compounds
A carbon atom has four valence electrons, so it forms four covalent bonds. Those bonds are strong, and carbon can bond to other carbon atoms as easily as to hydrogen, oxygen or nitrogen. That lets it build long chains, branches and rings (catenation). Between two carbons it can form a single bond, a double bond or a triple bond. Hydrocarbons are compounds of carbon and hydrogen only. They are saturated if every C–C bond is single, and unsaturated if there is at least one C=C or C≡C.
Homologous series
A homologous series is a family of compounds with the same functional group and the same general formula, in which each member differs from the next by a CH₂ unit. Members of a series have similar chemical reactions and physical properties that change gradually with chain length.
| Series | General formula | Functional group | Suffix | Example (C₄) |
|---|---|---|---|---|
| Alkanes | $\mathrm{C}_n\mathrm{H}_{2n+2}$ | only C–C single bonds | -ane | butane, C₄H₁₀ |
| Alkenes | $\mathrm{C}_n\mathrm{H}_{2n}$ | C=C | -ene | but-1-ene, C₄H₈ |
| Alkynes | $\mathrm{C}_n\mathrm{H}_{2n-2}$ | C≡C | -yne | but-1-yne, C₄H₆ |
The first part of every name, the stem, counts the carbons in the main chain. Learn these eight until they are automatic.
| Carbons | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
|---|---|---|---|---|---|---|---|---|
| Stem | meth- | eth- | prop- | but- | pent- | hex- | hept- | oct- |
| Branch name | methyl | ethyl | (longer branches are beyond the HSC course) | |||||
The naming rules
- Find the parent chain. It is the longest continuous chain of carbons. If there is a C=C or C≡C, the parent chain must contain it. Chains bend on paper, so trace every route, not just the horizontal line. If two chains are equally long, pick the one with more branches.
- Number the chain from the end that gives the multiple bond the lowest number. If there is no multiple bond, number from the end that gives the branches the lowest set of numbers (compare at the first point of difference: 2,2,4 beats 2,4,4).
- Name the branches (methyl, ethyl, fluoro, chloro, bromo, iodo) with their positions. Use di-, tri-, tetra- for repeats and list the branches in alphabetical order, ignoring di- and tri-. If the numbering is still tied, the branch that comes first in the alphabet gets the lower number.
- Write it out: numbers are separated from each other by commas and from letters by hyphens, and there are no spaces. The position of the multiple bond goes just before -ene or -yne, as in pent-2-ene.
Name the alkane CH₃C(CH₃)₂CH₂CH(CH₃)CH₃.
Name CH₃CH=CHCH(CH₃)CH₃.
Halogenated hydrocarbons
Replace a hydrogen with a halogen and you get a haloalkane (or haloalkene). Halogens are named as branches: fluoro, chloro, bromo, iodo. They are numbered and alphabetised exactly like alkyl branches.
- CH₃CHBrCH(CH₃)CH₃ is 2-bromo-3-methylbutane. Both numbering directions give {2,3}, so the tie goes to bromo, which comes first in the alphabet.
- CCl₂F₂, the old refrigerant CFC-12, is dichlorodifluoromethane. Chlorine and fluorine radicals released from CFCs in the stratosphere destroyed ozone until the 1987 Montreal Protocol phased them out.
- CH₂=CHCl is chloroethene (vinyl chloride), the monomer for PVC (7L).
Rings and benzene
Carbon chains can also close into rings. Cyclohexane, C₆H₁₂, is saturated but has the same formula as hexene, because closing a ring removes two hydrogens just as a double bond does. Benzene, C₆H₆, is a flat hexagon in which six electrons are shared evenly around the whole ring (delocalised). That makes it far less reactive than its three "double bonds" suggest: it does not decolourise bromine water quickly. Rings appear in the polymers PS and PET later in this module, but the HSC naming questions stick to chains.
- Puzzle 1 is drawn with a bend. Tap the carbons of the longest chain in order. How long is it, and what would you have called it if you only read the straight line?
- In each puzzle, after the chain is found, tap the carbon that should be C1. Check whether the sim agrees, and say which rule decided it.
- Puzzle 5 has two chains of the same length. Which one does the sim want, and why?
- Puzzle 7 is an alkene. Try tracing a longer chain that misses the C=C. What does the sim tell you?
Test yourself
What is the IUPAC name of CH₃CH₂CH(CH₃)CH₂CH₂CH₃?
Six-carbon chain; numbering from the left puts the methyl on C3, which is lower than C4.
Which formula belongs to an alkyne?
Alkynes are CₙH₂ₙ₋₂: for n = 5 that is C₅H₈.
The correct name of CH₂=CHCH(CH₃)CH₃ is
Number from the double-bond end: the C=C starts at C1, so the methyl is on C3.
Which name follows IUPAC rules?
Branches go in alphabetical order ignoring di-, and the lowest set of locants is {2,2,3}.
CH₃CHClCH₂CH₂Br is named
Numbering from the Br end gives {1,3}; from the other end {2,4}. {1,3} is lower, and bromo is written before chloro because b comes before c.
Label the parts of the name on the structure.
Sort each open-chain hydrocarbon by its molecular formula.
A family of compounds that differ by CH₂ is a series. A hydrocarbon with only single bonds is . The parent chain is the continuous chain of carbon atoms. Branches are listed in order. A CH₃ branch is called a group.
A student named a compound 2-ethyl-3-methylbutane. Draw (or describe) the structure the student meant, explain why the name is incorrect, and give the correct IUPAC name.
The student meant a four-carbon chain with an ethyl group (–CH₂CH₃) on C2 and a methyl group on C3. The ethyl group's two carbons join onto C2, so the longest continuous chain runs from the end of the ethyl group through C2, C3 and C4: that is five carbons, not four. The parent chain is therefore pentane, and the original C1 and the methyl group on C3 become branches. Numbering the five-carbon chain from the end nearer the branches puts methyl groups on C2 and C3. The correct name is 2,3-dimethylpentane (C₇H₁₆).