Classify straight-chain, branched and cyclic structures
Describe carbon skeletons as straight-chain, branched or cyclic from their connectivity.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Use the published functional-group scope and nomenclature limits; draw formula and mechanism evidence with correct connectivity, electron movement and stereochemistry. Written practice is self-checked
Before you begin
- Use covalent structures, valency, molecular formulas, lone pairs, and single, double and triple bonds.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What feature makes a carbon skeleton cyclic?
What you will learn
- Recognise an unbranched carbon chain
- Identify a branch in a carbon skeleton
- Recognise a closed carbon ring
Shape labels describe connectivity
A straight-chain molecule has a continuous, unbranched carbon sequence; its drawn chain may zigzag. A branched molecule has at least one carbon attached as a side branch from the selected parent chain.
A cyclic molecule has a closed path of atoms. A cyclic carbon skeleton can also have branches; the labels describe structural features rather than mutually exclusive physical shapes.
Interpret skeletal diagrams carefully
Each line end and vertex in a skeletal formula represents carbon unless another atom is written. Trace bonds to determine whether the structure is open, branched or closed.
Do not confuse a bent zigzag drawing with a branched chain. A straight-chain molecule is not necessarily drawn as a straight line on the page.
Pause and explain
Does 2-methylbutane have a branched carbon skeleton?
Worked example
Classify propane, 2-methylpropane and cyclohexane by carbon-skeleton connectivity.
Show the worked solution
- Propane has an open unbranched chain.
- 2-methylpropane has a side branch.
- Cyclohexane has a closed six-carbon ring.
Answer Propane is straight-chain, 2-methylpropane is branched, and cyclohexane is cyclic.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Classify a six-carbon unbranched open chain drawn as a zigzag.
Give me a hint
Count connectivity, not visual line direction.
Compare my reasoning
- The chain is open, not closed.
- No carbon is a side branch.
- It is straight-chain.
Straight-chain.
Look for these in your work
- Open chain recognized
- Branching checked
- Term applied
A skeletal formula has a four-carbon main path and a one-carbon side bond. Classify the skeleton.
Give me a hint
Look for a carbon attached outside the selected parent path.
Compare my reasoning
- The side bond ends at an extra carbon.
- That carbon is a branch.
- The molecule is branched.
Branched.
Look for these in your work
- Carbon endpoint counted
- Side branch recognized
- Class stated
A six-carbon skeletal formula joins its last carbon back to its first. What feature does that establish?
Give me a hint
Trace whether the carbon path closes.
Compare my reasoning
- The chain forms a closed loop.
- A closed carbon path is a ring.
- The skeleton is cyclic.
A cyclic carbon skeleton.
Look for these in your work
- Closure detected
- Ring identified
- Correct label
Common mistakes
- Calling every zigzag chain branched
- Treating a ring as an open chain
- Assuming cyclic and branched cannot both describe one molecule
What can you explain now?
What feature makes a carbon skeleton cyclic?
Compare with the explanation
A closed path of atoms
A cyclic structure contains a ring.
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Make it stick
Why is a zigzag drawing not necessarily a branched molecule?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.