Chemistry · General chemistry II · Worked example
Name a branched alkane
Name the alkane (CH₃)₂CHCH₂CH(C₂H₅)CH₂CH₂CH(CH₃)₂.
Find the longest chain
The longest continuous chain runs from one end methyl, through the carbon with the ethyl group, to the other end: eight carbons, so the parent is octane.
Name the branches
Off the eight-carbon chain hang one ethyl group and two methyl groups.
Number the chain
From one end the branches are at 2, 4 and 7; from the other end, at 2, 5 and 7. The first point of difference is 4 against 5, so number from the end that gives 2, 4, 7.
| Direction | Methyl | Ethyl | Branch numbers |
|---|---|---|---|
| From the left | 2, 7 | 4 | 2, 4, 7 |
| From the right | 2, 7 | 5 | 2, 5, 7 |
Write the name
Ethyl comes before methyl in alphabetical order, and the two methyl groups become 2,7-dimethyl.
Result
4-ethyl-2,7-dimethyloctane.
Your turn
Name CH₃CH₂CH(CH₃)CH₂CH₂CH₃.
Show the answer and explanation
3-methylhexane.
The longest chain has six carbons (hexane), with a methyl group on the third carbon from the nearer end. Calling it 2-ethylpentane uses a five-carbon chain that isn’t the longest, and 4-methylhexane numbers from the wrong end.
Keep exploring
Draw it in the Skeletal tool with the name set as its target, turn Check on, then change the target to 5-ethyl-2,7-dimethyloctane: it still describes the structure, but it isn’t the systematic name, because the chain is numbered from the wrong end.
Return to the concept →Sources and scope
Authored study material. Tool results depend on the stated inputs and model assumptions.
Try in the workspace
Open the example inputs, change a value and keep a useful result on your board.
Draw it and check the name Check the name 2-ethylpentane on 3-methylhexane See 3-methylhexane in every representation Open worked example on a board Organic nomenclature in Chemistry ReferenceYour existing work stays on this device. Examples open as editable copies.