Organic Chemistry – Some Basic Principles and Techniques · Lesson 4 of 15
Naming Hydrocarbons
“Build systematic names by choosing, numbering, and describing the correct carbon framework.”
• Distinguish common names from systematic names linked to structure. • Select and number parent chains and simple carbon rings. • Name alkyl substituents, including repeated and branched groups. • Explain alphabetical ordering and lowest-locant decisions.
Names should let us recover a structure
A useful chemical name must distinguish one compound from another and ideally let us draw its structure. Common names often record a source, property, or historical association. Formic acid recalls the Latin name for an ant, while citric acid refers to citrus fruits. Buckminsterfullerene recalls the resemblance of its carbon cage to architectural domes. These names remain useful, but they do not all follow one structural recipe.
The IUPAC system builds a name from a parent framework, substituents, and any characteristic functional groups. In this lesson the main framework is a saturated hydrocarbon: a compound containing carbon and hydrogen with only carbon–carbon single bonds. Such an open-chain compound is an alkane. Paraffin is an older name for this family. Unsaturated hydrocarbons contain at least one carbon–carbon double or triple bond.
| Formula | Common or retained name | Systematic counterpart where useful |
|---|---|---|
| Methane | Methane | |
| n-Butane | Butane | |
| Isobutane | 2-Methylpropane | |
| Neopentane | 2,2-Dimethylpropane | |
| n-Propyl alcohol | Propan-1-ol | |
| Formaldehyde | Methanal | |
| Acetone | Propanone | |
| Chloroform | Trichloromethane | |
| Acetic acid | Ethanoic acid | |
| Benzene | Benzene | |
| Anisole | Methoxybenzene | |
| Aniline | Aminobenzene | |
| Acetophenone | 1-Phenylethan-1-one | |
| Ethyl methyl ether | Methoxyethane |
Learn the parent names and substituent building blocks
For an unbranched alkane, the parent name identifies the number of carbons and ends in “ane”. Knowing these names avoids treating each later name as a new word. An alkyl substituent is obtained conceptually by removing one hydrogen from an alkane; changing “ane” to “yl” gives its group name. The missing hydrogen leaves the point through which the group attaches to the parent.
| Carbons | Alkane | Molecular formula | Corresponding straight-chain alkyl group |
|---|---|---|---|
| 1 | Methane | Methyl | |
| 2 | Ethane | Ethyl | |
| 3 | Propane | Propyl | |
| 4 | Butane | Butyl | |
| 5 | Pentane | Pentyl | |
| 6 | Hexane | Hexyl | |
| 7 | Heptane | Heptyl | |
| 8 | Octane | Octyl | |
| 9 | Nonane | Nonyl | |
| 10 | Decane | Decyl | |
| 20 | Icosane | Icosyl | |
| 30 | Triacontane | Triacontyl |
Me, Et, Pr, and Bu abbreviate methyl, ethyl, propyl, and butyl. The point of attachment matters: propan-1-yl and propan-2-yl are different groups even though both contain three carbons. Common branched names include isopropyl, sec-butyl, isobutyl, tert-butyl, and neopentyl. Learn their connectivity rather than remembering only their carbon count.
| Common group name | Condensed group, with attachment shown at the left |
|---|---|
| Isopropyl | |
| sec-Butyl | |
| Isobutyl | |
| tert-Butyl | |
| Neopentyl |
| Straight-chain group | Condensed structure with attachment at left |
|---|---|
| Methyl | |
| Ethyl | |
| Propyl | |
| Butyl | |
| Decyl |
Choose the parent before numbering
First trace the longest continuous carbon chain. It need not run horizontally in the drawing, and it cannot jump across a branch or count one atom twice. If equally long chains are possible in the cases considered here, select the one carrying more substituents. Only after fixing the parent should you number it. Otherwise a correct-sounding number may describe the wrong skeleton.
Number from the end giving the lowest set of substituent positions. Compare the two sequences at their first point of difference; do not add the numbers. Record each branch position, identify its alkyl name, and write substituents alphabetically before the parent name. Separate numbers by commas and numbers from words by hyphens. Do not place spaces inside a simple systematic name.
Problem
Name .
- 1.The longest chain has nine carbons, so the parent is nonane. Numbering from the left gives branches at 2 and 6; from the right it gives 4 and 8.
- 2.Use 2 and 6 because 2 is lower than 4 at the first comparison. The branches are methyl at 2 and ethyl at 6.
- 3.List ethyl before methyl alphabetically. The name is 6-ethyl-2-methylnonane. Alphabetical listing does not reverse the numbering already chosen.
When identical substituents occur repeatedly, use di, tri, tetra, penta, or hexa and retain a position number for each one. Thus 2,2,4-trimethylpentane has three methyl groups, including two on carbon 2. Ignore these multiplying prefixes when alphabetising: ethyl precedes dimethyl. If two numbering directions give the same set of locants, give the lower locant to the substituent that comes first alphabetically in these simple cases.
Problem
Choose between 3-ethyl-6-methyloctane and 6-ethyl-3-methyloctane for the same skeleton.
- 1.Both directions give the locant set 3,6. The ordinary first-point comparison cannot distinguish them.
- 2.Ethyl precedes methyl alphabetically, so assign ethyl the lower number. The correct name is 3-ethyl-6-methyloctane.
- 3.Do not use this tie-break before choosing the correct parent or before comparing unequal locant sets.
Branched groups and rings use the same logic
A complex branch can itself be named like a small branched chain. Start its numbering at the carbon directly attached to the main parent, and enclose its full name in parentheses. For example, 5-(2-ethylbutyl)-3,3-dimethyldecane contains a decane parent and a numbered branched substituent at carbon 5. In common substituent names, iso and neo are counted for alphabetising, while sec and tert are not.
For a saturated monocyclic carbon ring, prefix cyclo to the alkane name with the same number of ring carbons. Apply low-locant and alphabetical decisions to ring substituents. A ring has no permanent “left end”: the starting point and direction must be chosen together. A single substituent on a simple otherwise unsubstituted ring normally does not need a locant.
Problem
Explain 3-ethyl-1,1-dimethylcyclohexane rather than 1-ethyl-3,3-dimethylcyclohexane.
- 1.Both names describe candidates for numbering the same substituted ring. List all substituent locants, including repeated ones.
- 2.The first gives 1,1,3; the second gives 1,3,3. The first difference is the second number: 1 is lower than 3.
- 3.Choose 1,1,3. Ethyl is still listed before dimethyl in the written name; alphabetical listing and ring numbering are separate decisions.
The visually straight chain is not necessarily the longest chain. “Lowest locants” does not mean the lowest sum, and di or tri does not decide alphabetical order. Every repeated substituent still needs its own locant.
Quiz
What is the parent name of an unbranched eight-carbon alkane?
Which locant sequence is preferred: 2,5,6 or 3,4,7?
Which substituent is written first alphabetically?
Where does numbering begin inside a branched substituent?
How many ring carbons occur in cyclopentane?
Practice Problems
- Name and justify the numbering.
- Draw 2,2,4-trimethylpentane and 3-ethyl-4,4-dimethylheptane, showing the parent chains.
- Explain why 2,5,6-trimethyloctane is preferred to 3,4,7-trimethyloctane.
- Draw and distinguish isobutyl, sec-butyl, tert-butyl, and neopentyl groups, marking the attachment point.
- Draw 1-methyl-3-propylcyclohexane and explain its numbering.
- Explain how you choose between two equally long candidate parent chains in the examples studied here.
Key Takeaways
• Choose the correct parent before assigning locants. • Compare locant sequences at their first difference. • Alphabetical order controls listing; multiplying prefixes are ignored. • Branched substituents and carbon rings require explicit numbering decisions.