Difficulty: Introductory | Prerequisites: Lewis structures, basic bonding
Constitutional isomers are molecules that share the same molecular formula but differ in how their atoms are connected. IUPAC nomenclature gives every organic molecule a unique, systematic name by identifying the longest carbon chain, numbering it to give substituents the lowest possible locants, and listing substituents in alphabetical order. Getting comfortable with both concepts is essential before moving into stereochemistry.
Constitutional isomers (structural isomers)
Molecules with the same molecular formula but different connectivity of atoms. The atoms are bonded together in a different order or arrangement.
Think of it as: same Lego pieces, built into different shapes.
Molecular formula
The formula that states the exact number and type of each atom in a molecule (e.g. C₆H₁₄), without saying anything about how they are arranged.
In simple terms, it is the ingredient list, not the recipe.
Parent chain (longest continuous carbon chain)
The longest unbroken chain of carbon atoms in a molecule. This chain determines the base name of the compound (methane, ethane, propane, butane, pentane, hexane, heptane, etc.).
Think of it as: the backbone or spine of the molecule. Everything else hangs off it.
Substituent
An atom or group of atoms attached to the parent chain, replacing a hydrogen. Common examples: methyl (–CH₃), ethyl (–C₂H₅), propyl (–C₃H₇), isopropyl (–CH(CH₃)₂).
In simple terms, the branches sticking off the main chain.
Locant
The number assigned to a carbon on the parent chain that tells you where a substituent or functional group is attached.
Think of it as: the address number on the backbone that says where each branch lives.
Degree of unsaturation (index of hydrogen deficiency)
A calculation that tells you how many rings or pi bonds a molecule contains, based on its molecular formula. For C_nH_m with no other elements: (2n + 2 – m) / 2.
In simple terms, it tells you how far below the "fully saturated" hydrogen count a molecule sits, which hints at double bonds, triple bonds, or rings.
Two molecules are constitutional isomers if and only if they have the same molecular formula but different atom-to-atom connectivity. To check:
Write out the molecular formula of each compound. If the formulas differ, they cannot be isomers of any kind.
If the formulas match, examine the connectivity. Are the atoms bonded in the same sequence? If the bonding sequence differs, they are constitutional isomers.
Molecules that look different on paper but have identical connectivity (just drawn or rotated differently) are the same compound, not isomers.
From PSO Worksheet 2, Problem 3:
Set (a): The two structures have the same molecular formula but different hydrogen counts when examined closely, so they are not constitutional isomers.
Set (b): The two structures share the same molecular formula and have different connectivity, so they are constitutional isomers.
The worksheet asks for at least three isomers each for C₆H₁₄ and C₅H₁₂O.
C₆H₁₄ (saturated alkane, no rings or double bonds):
Hexane (straight chain of six carbons)
2-Methylpentane (five-carbon chain with a methyl branch at C2)
3-Methylpentane (five-carbon chain with a methyl branch at C3)
2,3-Dimethylbutane (four-carbon chain with methyl groups at C2 and C3)
2,2-Dimethylbutane (four-carbon chain with two methyl groups at C2)
C₅H₁₂O (one degree of unsaturation short of needing one, so this is a saturated alcohol or ether):
1-Pentanol (OH at the end of a five-carbon chain)
2-Pentanol (OH at C2 of a five-carbon chain)
3-Methyl-1-butanol (four-carbon chain, methyl at C3, OH at C1)
2-Methyl-2-butanol (four-carbon chain, methyl and OH both at C2)
Diethyl ether (CH₃CH₂–O–CH₂CH₃, an ether rather than an alcohol)
The strategy is systematic: start with the longest possible chain, then shorten the chain by one carbon and place that carbon as a branch, then shorten again. For formulas containing oxygen, remember that the oxygen can appear as an alcohol (–OH) or as an ether (C–O–C).
Step 1: Find the parent chain.
Identify the longest continuous chain of carbon atoms. This gives the base name (methane = 1C, ethane = 2C, propane = 3C, butane = 4C, pentane = 5C, hexane = 6C, heptane = 7C, octane = 8C).
Step 2: Number the chain.
Number from the end that gives the substituents the lowest set of locants. Compare locant sets digit by digit at the first point of difference.
Step 3: Name and number each substituent.
Identify each branch. Common alkyl groups: methyl (1C), ethyl (2C), propyl (3C), isopropyl (branched 3C), butyl (4C).
Step 4: Alphabetise substituents.
List substituents in alphabetical order in the name. Multiplying prefixes (di-, tri-, tetra-) do not count for alphabetical ordering, but prefixes that are part of the substituent name (e.g. "iso" in isopropyl) do count.
Step 5: Assemble the name.
Locants separated by commas, substituent names joined by hyphens to the parent name. Example: 2,3-dimethylpentane.
"2-Ethyl-3-propylheptane" is incorrect.
When you draw out this structure, the actual longest chain is longer than seven carbons. The correct approach: redraw the structure, find the true longest chain, and rename. The worksheet answer indicates this should be 2-ethyl-3-propylheptane-1,4-diol (the –OH groups were omitted from the original name).
"1-Methyl-5-ethylhexane" is incorrect.
Two errors here. First, substituents must be listed in alphabetical order, and "ethyl" comes before "methyl." Second, a carbon bearing a substituent at position 1 usually means the chain should be extended to include that branch as part of the parent chain. The corrected name is 1-ethyl-5-methylhexane (alphabetical order, E before M).
Students often assume that molecules drawn differently on paper must be different compounds. Two line drawings that look different may represent the same molecule, just rotated or flipped. Always check the connectivity, not the visual layout.
A frequent naming error is forgetting to alphabetise substituents. "Dimethyl" and "ethyl" are alphabetised by the substituent name (ethyl before methyl), not by the prefix. The "di-" does not count.
Students sometimes pick a chain that looks horizontal on the page rather than the actual longest chain. The longest chain can zigzag across the structure. Always count carefully.
When numbering the parent chain, students sometimes number from whichever end is on the left. The correct rule is to number from the end that gives the lowest set of locants to the substituents, regardless of left or right.
True or false: Two molecules with different molecular formulas can be constitutional isomers. (False, they must have the same molecular formula.)
Fill in the blank: C₆H₁₄ has ___ constitutional isomers that are alkanes. (Five)
True or false: In IUPAC naming, "di-" counts when alphabetising substituents. (False, multiplying prefixes are ignored for alphabetical order.)
Fill in the blank: The parent chain is the ___ continuous chain of carbon atoms. (Longest)
True or false: Butanol and diethyl ether are constitutional isomers. (False, butanol is C₄H₁₀O and diethyl ether is C₄H₁₀O, so they are constitutional isomers. True.)
Q: Draw three constitutional isomers of C₄H₁₀O.
A: 1-Butanol (CH₃CH₂CH₂CH₂OH), 2-butanol (CH₃CH(OH)CH₂CH₃), and diethyl ether (CH₃CH₂OCH₂CH₃). All three have the formula C₄H₁₀O but different connectivity.
Q: What is wrong with the name "3-ethyl-2-methylbutane"? Give the correct IUPAC name.
A: If you draw this structure out, the longest chain is five carbons, not four. The ethyl group at C3 extends the chain. The correct name is 3-methylpentane.
Q: Are pentane and cyclopentane constitutional isomers?
A: No. Pentane is C₅H₁₂ and cyclopentane is C₅H₁₀. They have different molecular formulas, so they cannot be constitutional isomers.
Q: Name the compound with a six-carbon parent chain, a methyl group at C2, and an ethyl group at C4.
A: 4-Ethyl-2-methylhexane. Ethyl comes before methyl alphabetically.
Q: A student names a compound "1-ethyl-3-methylcyclopentane." Is this correct? If not, why?
A: In a cyclic compound, you number to give the lowest set of locants. If ethyl is at position 1, methyl at position 3, check whether numbering the other direction gives lower locants (e.g. 1-ethyl-3-methyl vs 1-methyl-3-ethyl). Since E comes before M alphabetically, ethyl should get the lower number when possible. 1-Ethyl-3-methylcyclopentane is correct if that numbering direction gives the lowest locants.
Constitutional isomers reappear throughout organic chemistry whenever you need to predict products of a reaction (different isomers can form from the same starting material). IUPAC nomenclature is the foundation for naming everything that follows: alcohols, alkenes, alkynes, and functional group chemistry all build on these same rules. Stereoisomers (cis/trans, R/S) are the next logical step after constitutional isomers and require you to be fluent in naming before you can layer on stereochemical descriptors.
constitutional isomers, structural isomers, IUPAC nomenclature, IUPAC naming, organic chemistry nomenclature, parent chain, longest chain, substituent, alkyl group, methyl, ethyl, propyl, isopropyl, locant, numbering, alphabetical order, constitutional isomers examples, C6H14 isomers, drawing isomers, naming organic compounds, CHM 255, Purdue organic chemistry, PSO worksheet 2, degree of unsaturation, index of hydrogen deficiency