Hydrocarbons · Lesson 2 of 14
Naming Branched Alkanes
“Translate accurately between branched carbon skeletons, substituent names, and systematic names.”
• Choose and number the correct parent carbon chain. • Recognise alkyl groups and distinguish their attachment positions. • Apply substituent ordering and repeated-group prefixes. • Build a valid structure from a name and diagnose incorrect names.
Choose the chain before naming the branches
A complicated drawing is easier to name when treated as a parent chain carrying substituents. Trace the longest continuous carbon path first. This path may turn through what appears visually to be a side branch. Only after selecting it should you identify the groups left outside it; choosing the most horizontal line is an unreliable shortcut.
Number the parent from the end giving the lower sequence of substituent positions at the first point of difference. List substituent names alphabetically and use di-, tri-, or tetra- for repetitions. These multiplying prefixes do not control alphabetisation. Separate numbers by commas and numbers from words by hyphens. Alphabetical precedence helps resolve a genuine numbering tie, rather than replacing the lowest-locant comparison.
Problem
Correct the proposed names 2-ethylpentane and 5-ethyl-3-methylheptane.
- 1.For 2-ethylpentane, draw the five-carbon chain and attach the ethyl group. A path beginning at the outer end of that group and continuing to the far end of the original chain contains six carbons.
- 2.Numbering that six-carbon path appropriately leaves a methyl substituent at carbon 3. The correct name is 3-methylhexane.
- 3.For the second structure, both directions give the locant set 3,5. Give the lower locant to ethyl, which comes before methyl alphabetically. The name is 3-ethyl-5-methylheptane.
Alkyl groups retain an attachment point
Removing one hydrogen from an alkane gives an alkyl substituent, generally . The free valence specifies where this group attaches to another atom. Removing hydrogens from equivalent positions gives the same group; removing them from inequivalent positions can produce different groups. A carbon skeleton and an attachment point must both be specified.
| Five-carbon skeleton | Distinct attachment positions | Alcohol obtained after attachment to hydroxyl |
|---|---|---|
| Pentane | Terminal, next-to-terminal, central | Pentan-1-ol; pentan-2-ol; pentan-3-ol |
| 2-Methylbutane | Two equivalent methyl ends at the branched centre; the other end; the secondary centre; the tertiary centre | 2-Methylbutan-1-ol; 3-methylbutan-1-ol; 3-methylbutan-2-ol; 2-methylbutan-2-ol |
| 2,2-Dimethylpropane | All four methyl groups equivalent | 2,2-Dimethylpropan-1-ol |
Thus the three five-carbon alkane skeletons produce eight distinct pentyl attachment patterns. The alcohol names in this comparison are a way to make the attachment position visible: the hydroxyl group occupies the position from which hydrogen was removed. They do not change the rule for choosing the hydrocarbon skeleton.
More elaborate names still use the same logic
A substituent can itself be branched. Number that substituent from its point of attachment and enclose its full name in parentheses when needed. Familiar names such as isopropyl, sec-butyl, and tert-butyl occur in common usage. In alphabetising these familiar forms, sec- and tert- are ignored, whereas iso is treated as part of the name.
| Naming situation | Representative name | What to check |
|---|---|---|
| Different simple branches | 4-Ethyl-2-methylhexane | Locate ethyl and methyl correctly, then alphabetise |
| Repeated and familiar branches | 3,3-Diethyl-5-isopropyl-4-methyloctane | Ignore di- in alphabetisation |
| A sec-butyl branch | 5-sec-Butyl-4-isopropyldecane | Alphabetise butyl before isopropyl |
| A numbered complex branch | 5-(2,2-Dimethylpropyl)nonane | Start branch numbering at its attachment carbon |
| Equal locant sets | 3-Ethyl-5-methylheptane | Give ethyl the lower locant in the tie |
Problem
Name and .
- 1.In the first molecule, trace from a methyl at one end through the first central carbon, the CH2 group, the other central carbon, and a terminal methyl. This is a five-carbon chain.
- 2.Two methyl branches remain at carbon 2 and two at carbon 4, giving 2,2,4,4-tetramethylpentane.
- 3.In the second molecule, trace through both ethyl groups and their common central carbon. The parent is pentane and the remaining two methyl groups both attach at carbon 3: 3,3-dimethylpentane.
Problem
Find a systematic name for tetra-tert-butylmethane.
- 1.The central carbon is bonded to four tert-butyl groups. The longest path runs from a methyl in one group, through its substituted carbon and the central carbon, to a methyl in a second group: five carbons.
- 2.The parent is pentane. Two methyl groups remain at carbon 2 and two at carbon 4, while the other two tert-butyl groups attach at carbon 3.
- 3.The name is 3,3-di-tert-butyl-2,2,4,4-tetramethylpentane. The crowded appearance does not alter the longest-path method.
Reverse the process to draw a structure
To build a structure from a name, draw and number the parent chain, attach every branch at its stated position, then complete carbon valence with hydrogens. Check the longest chain again at the end. A name that directs a formally drawable structure can still be incorrect if it has chosen a shorter parent than that structure actually contains.
Problem
Draw 3-ethyl-2,2-dimethylpentane and 3,4,4,5-tetramethylheptane.
- 1.For the first, draw five parent carbons, put two methyl groups on carbon 2, and an ethyl group on carbon 3. Complete the formula as .
- 2.For the second, draw seven carbons and add methyl groups at 3, 4, 4, and 5. The result is .
- 3.In each formula the carbon carrying four carbon–carbon single bonds has no hydrogen. Check each CH, CH2, or CH3 label against the number of bonds already drawn.
Do not name by adding locants and choosing the smallest total. Compare the ordered locant sequences at their first difference. Do not count di-, tri-, or tetra- when alphabetising simple substituents.
Quiz
Which chain is normally selected as the parent of a branched alkane?
Which name is correct for the structure misleadingly called 2-ethylpentane?
Which prefix is ignored when alphabetising repeated methyl substituents?
How many distinct attachment patterns arise for groups?
In a branched substituent, where does its own numbering begin?
Practice Problems
- Name .
- Draw and name all eight distinct attachment patterns.
- Draw 2,5-dimethylhexane and check the hydrogen count at every carbon.
- Explain why 5-ethyl-3-methylheptane needs renumbering.
- Draw 5-(2,2-dimethylpropyl)nonane, showing the branch numbering separately.
- Explain the different roles of parent-chain length, locant comparison, and alphabetical order.
Key Takeaways
• Trace the longest carbon path before assigning branches. • Number using the lowest locant sequence at its first difference. • An alkyl group includes both a skeleton and a point of attachment. • Drawing from a name requires a final valence and parent-chain check.