Convert names to structures with ethyl branches and longer parent chains.
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Method and worked examples
To convert a hydrocarbon name to a condensed structure, draw the named parent chain first, number it, place double bonds and then add each branch at its locant. Complete four bonds around every carbon using hydrogens. A methyl branch is CH3 and an ethyl branch is CH2CH3. Bracketed groups in a condensed formula attach to the immediately preceding carbon.
For structure-to-name questions, check the longest permitted parent chain rather than counting only the unbracketed symbols. Include the required C=C bonds in an alkene parent and give those bonds low locants. Count branch carbons as part of the molecular formula even though they do not all belong to the chosen parent chain.
Example 1
Write a condensed structure for 3−ethylhexane.
Use a six-carbon parent with CH2CH3 at carbon 3:CH3CH2CH(CH2CH3)CH2CH2CH3.
Example 2
Name CH3CH2CH2CH(CH2CH3)CH2CH2CH2CH3.
The longest chain has eight carbons. Numbering from the nearer end gives an ethyl branch at 4, so the name is 4−ethyloctane.
Watch out:Ethyl is CH2CH3 and contributes two carbon atoms.
Hydrocarbon names and condensed structures practice | Year 11 Chemistry | Ascent Arcade