Structural Isomers
High-Yield Summary
- Structural (constitutional) isomers share the same molecular formula but differ in atom connectivity — a real difference, not cosmetic, producing different boiling points/reactivity.
- Systematic method: start with the longest chain, shorten by one carbon at a time placing leftover carbons as substituents, try every unique position, stop once shortening only recreates isomers already found.
- Two rules avoid double-counting: a substituent at either chain-end just recreates the longest chain; always number to give the substituent the lowest position (higher-number placements are duplicates).
- C₆H₁₄ has exactly 5 structural isomers: n-hexane, 2-methylpentane, 3-methylpentane, 2,3-dimethylbutane, 2,2-dimethylbutane.
- Isomer family flowchart: same connectivity? No → structural isomers. Yes → stereoisomers → (bond-breaking to interconvert?) configurational or conformational → (mirror images?) enantiomers or diastereomers → (immovable bond/ring?) cis-trans (geometric) isomers.
Finding All Structural Isomers of a Formula
- 1Start with the longest possible carbon chain (the straight-chain arrangement).
- 2Shorten the chain by one carbon; place the leftover carbon as a methyl substituent at each unique position.
- 3Continue shortening, redistributing the growing number of leftover carbons as substituents at each unique position.
- 4Stop once further shortening only recreates structures already found — the set is complete.
- 5For molecules with functional groups: same method, but also systematically place the functional group along the longest chain containing it, keeping every carbon at full valency (4 bonds).
Key Terms
- Structural (constitutional) isomers
- Same molecular formula, different atom connectivity — the isomer type covered in this article.
- Stereoisomers
- Same connectivity, different spatial arrangement — the next branch of the isomer family tree, covered in the following subtopics.
Isomer Family Decision Points
| Question | Answer → Classification |
|---|---|
| Same connectivity? | No → structural (constitutional) isomers |
| Same connectivity? | Yes → stereoisomers (next question applies) |
| Bond-breaking needed to interconvert? | Yes → configurational isomers; No → conformational isomers |
| Non-superimposable mirror images? | Yes → enantiomers; No → diastereomers |
| Immovable bond (double bond) or ring involved? | Yes → cis-trans (geometric) isomers |
Common MCAT Trap
- Placing a substituent at the very end of a shortened chain doesn't create a new isomer — rotate the molecule and it's just the longest straight chain again.
- A substituent placement that gives a higher locant than necessary from one end is a duplicate — always renumber from the other end and check the lowest-position rule before counting it as new.
Quick Recall
How many structural isomers does C₆H₁₄ have, and what are they?
What's the first branch point in the isomer family flowchart?
Why doesn't placing a substituent at the end of a chain create a new structural isomer?
What distinguishes configurational isomers from conformational isomers?