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Alkanes are saturated hydrocarbons that contain only single bonds between carbon atoms. Their general formula is \(C_nH_{2n+2}\), where \(n\) is the number of carbon atoms.
Nomenclature of linear alkanes
For linear alkanes, the following prefixes are used according to the number of carbon atoms:
| Carbons | Prefix | Name | Formula |
|---|---|---|---|
| 1 | Met- | Methane | CH₄ |
| 2 | Et- | Ethane | C₂H₆ |
| 3 | Prop- | Propane | C₃H₈ |
| 4 | But- | Butane | C₄H₁₀ |
| 5 | Pent- | Pentane | C₅H₁₂ |
| 6 | Hex- | Hexane | C₆H₁₄ |
| 7 | Hept- | Heptane | C₇H₁₆ |
| 8 | Oct- | Octane | C₈H₁₈ |
| 9 | Non- | Nonane | C₉H₂₀ |
| 10 | Dec- | Decane | C₁₀H₂₂ |
Nomenclature of branched alkanes
To name branched alkanes, follow these rules:
- Identify the main chain (the longest carbon chain).
- Number the main chain from the end nearest to the first branch.
- Identify and name the branches (alkyl radicals).
- Indicate the positions of the branches with numbers.
- If there are several identical radicals, use prefixes: di-, tri-, tetra-, etc.
- List the branches alphabetically (ignoring di-, tri-, etc. prefixes).
Main alkyl radicals
| Name | Formula |
|---|---|
| Methyl | -CH₃ |
| Ethyl | -CH₂CH₃ |
| Propyl | -CH₂CH₂CH₃ |
| Isopropyl | -CH(CH₃)₂ |
| Butyl | -CH₂CH₂CH₂CH₃ |
Examples
2-methylbutane:A butane with a methyl group at position 2, not at 3 because that would be a higher position.
3,3-dimethylhexane:Main chain: hexane. With two methyl groups attached to the same carbon atom at position C3, forming a quaternary carbon.
2,3-dimethylbutane:It does not matter whether you number from the left or the right because both ends are the same distance from the branches. The sum of the locants (2+3=5) is the same numbering from either end.
2,4,6-trimethylheptane:If we number from the left, the methyls are at positions 2, 4, 6. If we number from the right, they are also at positions 2, 4, 6. In this case, we compare number by number: (2,4,6) = (2,4,6), so either numbering is correct.
3-ethyl-2-methylpentane:3-ethyl-2-methylpentane:
If we number from the left, we have an ethyl at position 3 and a methyl at position 2. If we number from the right, we have an ethyl at position 3 and a methyl at position 4. The set (2,3) is lower than (3,4), so the first numbering is correct: 3-ethyl-2-methylpentane.