Structural Isomers
Structural isomers share a molecular formula but differ in how their atoms are connected.
Structural isomers — also called constitutional isomers — are compounds that share the same molecular formula but differ in how their atoms are connected. That difference in connectivity, not just atom count, is what drives differences in physical and chemical properties between isomers. This article covers the systematic method for finding every structural isomer of a given formula, works it through in full for C₆H₁₄, and places structural isomers within the larger isomer family you'll build on in the rest of this chapter.
Key Takeaways
Structural (constitutional) isomers share the same molecular formula but differ in how their atoms are connected — a connectivity difference that produces real differences in physical and chemical properties.
Find all structural isomers of a formula systematically: start with the longest chain, shorten it one carbon at a time, and try every unique substituent placement — stop once further shortening only recreates isomers already found.
Two rules avoid double-counting: a substituent at either chain-end just recreates the longest chain, and the chain is always numbered to give substituents the lowest possible position.
C₆H₁₄ has exactly five structural isomers: n-hexane, 2-methylpentane, 3-methylpentane, 2,3-dimethylbutane, and 2,2-dimethylbutane.
The same method extends to molecules with functional groups — place the functional group systematically along the longest chain containing it, keeping every carbon's valency satisfied.
In the bigger isomer picture: 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.
What Are Structural (Constitutional) Isomers?
Isomers, by definition, are molecules with the same molecular formula but different chemical structures. Structural (constitutional) isomers are the isomer type where the atoms are bonded together differently — same number and types of atoms, different connectivity.
This connectivity difference is significant precisely because it isn't cosmetic: two structural isomers can have noticeably different boiling points, reactivity, and other physical or chemical behavior, even though they're built from identical atomic parts.
The Systematic Method for Finding All Structural Isomers
To find every structural isomer of a molecular formula, work through the carbon skeleton systematically rather than guessing:
Start with the longest possible carbon chain. This straight-chain arrangement is the simplest structure for the formula.
Shorten the chain by one carbon and place the leftover carbon as a substituent (a methyl group). Try each unique position along the shortened chain.
Continue shortening the chain, redistributing the growing number of leftover carbons as substituents at each step, again trying every unique position.
Stop once shortening the chain further only recreates structures you've already found. That's the signal the set of structural isomers is complete.
Two placement rules keep you from double-counting the same structure:
Placing a substituent at either end of the chain doesn't create anything new — it's equivalent to the longest straight chain once the molecule is rotated.
Always number the chain to give the substituent the lowest possible position. If a placement would get a higher number than necessary, renumber from the other end — that placement is a duplicate of one you've already counted, not a new isomer.
If the molecule carries one or more functional groups instead of being a simple hydrocarbon, the same process applies: identify the longest carbon chain, then systematically work through every unique placement of the functional group(s), making sure each carbon keeps its full valency of four bonds with all attached hydrogens accounted for.
Worked Example: The Five Structural Isomers of C₆H₁₄
Applying the method to the molecular formula C₆H₁₄:
Six-carbon chain: all six carbons in a row is n-hexane — the simplest structure, and the starting point.
Five-carbon chain (one leftover carbon): shortening to a five-carbon chain leaves one carbon to place as a methyl substituent.
Methyl group on carbon 2 → 2-methylpentane.
Methyl group on carbon 3 → 3-methylpentane.
Methyl on carbon 1 or carbon 5 isn't a new structure — it's just n-hexane again once rotated. Methyl on carbon 4 isn't new either — renumbering from the other end makes it equivalent to carbon 2, since the chain is always numbered to give the substituent the lowest possible position.
Four-carbon chain (two leftover carbons): shortening to a four-carbon (butane) backbone leaves two carbons to place.
Both methyl groups on carbons 2 and 3 → 2,3-dimethylbutane.
Both methyl groups on carbon 2 → 2,2-dimethylbutane.
Three-carbon chain (three leftover carbons): shortening to a three-carbon (propane) backbone and distributing three leftover carbons only recreates structures already identified above — no new isomer results.
That accounts for every way to arrange the carbon skeleton of C₆H₁₄. C₆H₁₄ has exactly five structural isomers: n-hexane, 2-methylpentane, 3-methylpentane, 2,3-dimethylbutane, and 2,2-dimethylbutane.
Structural Isomers with Functional Groups
The same longest-chain-first method extends directly to molecules with functional groups — the only difference is that instead of placing plain carbon substituents, you're also systematically placing the functional group itself along the chain. Start with the longest chain containing the functional group, then work through each unique position, keeping every carbon's valency of four bonds satisfied and accounting for all attached hydrogens at every step.
Where Structural Isomers Fit in the Isomer Family
Structural isomers are one branch of a larger isomer classification. Working through the questions in order:
Do the isomers have the same connectivity of atoms? No → they're structural (constitutional) isomers — the type this article covers. Yes → they're stereoisomers, and a second question applies.
(For stereoisomers) Do they require breaking bonds to interconvert? Yes → configurational isomers. No → conformational isomers — different arrangements reached by simply rotating around single bonds.
(For configurational isomers) Are they non-superimposable mirror images of each other? Yes → enantiomers — chiral molecules related the way a left hand is related to a right hand. No → diastereomers — stereoisomers that differ in spatial arrangement without being mirror images.
(For diastereomers) Does the difference involve an immovable bond, such as a double bond, or a ring? Yes → cis-trans isomers, also called geometric isomers — substituents fall on the same side (cis) or opposite sides (trans) because rotation around that bond or ring is restricted.
This flowchart is worth keeping in mind as a map for the rest of the chapter: conformational isomers, configurational isomers, enantiomers, diastereomers, and cis-trans isomers are each developed in depth in the subtopics that follow.
Common MCAT Mistakes
Assuming two molecules with the same molecular formula are the same compound. Molecular formula alone doesn't determine structure — same formula with different connectivity means different structural isomers, often with different physical and chemical properties.
Counting a substituent placed at either end of the chain as a new isomer. It isn't — a substituent at the chain's terminal position just recreates the longest straight chain once the molecule is rotated.
Forgetting to renumber from the other end before counting a placement as new. A placement that looks like a higher-numbered position may be identical to a lower-numbered one already counted — the chain is always numbered to give the substituent the lowest possible position first.
Calling any two molecules with the same molecular formula "stereoisomers." Stereoisomers specifically share the same connectivity; if the atoms are bonded together differently, the molecules are structural (constitutional) isomers instead.
MCAT-Style Concept Check
Question: Two molecules both have the molecular formula C₄H₁₀. One is a straight four-carbon chain; the other has a one-carbon branch at the second carbon. What is the relationship between these two molecules, and why?
A) Stereoisomers, because they share the same molecular formula
B) Structural (constitutional) isomers, because their atoms are connected differently despite sharing the same molecular formula
C) Conformational isomers, because one can be converted into the other by rotating around a single bond
D) The same compound, since both contain four carbons and ten hydrogens
Answer: B
Explanation: Sharing a molecular formula is necessary for two molecules to be isomers of any kind, but it isn't sufficient to determine which kind. The deciding question is connectivity: do the atoms bond together the same way? Here they don't — one structure is unbranched and the other has a branch — so these are structural (constitutional) isomers, not the same compound. Option A is wrong because stereoisomers require identical connectivity, which these molecules don't have. Option C is wrong because conformational isomers are reached by rotating around single bonds within the same connectivity, not by rearranging which atoms bond to which. Option D is wrong because identical atom counts don't make two structures the same compound — connectivity, not formula, defines structure.
FAQ
What's the difference between structural isomers and stereoisomers?
Structural (constitutional) isomers have different connectivity — their atoms are bonded together in different arrangements. Stereoisomers have the same connectivity as each other; they differ only in the spatial arrangement of atoms. That single question — same connectivity or not — is the first branch point in classifying any pair of isomers.
How many structural isomers does C₆H₁₄ have?
Exactly five: n-hexane, 2-methylpentane, 3-methylpentane, 2,3-dimethylbutane, and 2,2-dimethylbutane. They're found systematically by starting with the six-carbon straight chain, then shortening the chain one carbon at a time and trying every unique position for the leftover carbon(s) as substituents.
Why doesn't placing a substituent at the end of the chain count as a new isomer?
Because a substituent at either terminal carbon just extends the chain by one carbon once the molecule is rotated — it recreates the longest straight-chain structure that's already been counted, rather than producing a genuinely different connectivity.
Do structural isomers actually behave differently, or is the connectivity difference just a technicality?
They behave differently. Even though structural isomers are built from identical atoms in identical quantities, their different connectivity produces real, measurable differences in physical properties (like boiling point) and chemical reactivity — connectivity, not atom count, is what determines a molecule's behavior.
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