Difficulty: Introductory to Intermediate | Prerequisites: Lewis structures, basic knowledge of alkanes and functional groups.
Constitutional isomers and IUPAC naming are the language layer of organic chemistry. Before you can discuss reactions, mechanisms or properties, you need to be able to identify when two structures are the same compound drawn differently, when they are genuinely different compounds sharing a molecular formula, and how to name any structure unambiguously. This material appears on every exam from here through the end of the course, and naming errors are among the most common reasons students lose marks on otherwise correct answers.
Constitutional isomers are molecules with the same molecular formula but different atom-to-atom connectivity. To name an organic compound, find the longest continuous carbon chain (the parent), number it to give substituents the lowest possible locants, then list substituents alphabetically. The most common naming mistakes come from not finding the true longest chain or from numbering in the wrong direction.
Constitutional isomers (structural isomers)
Compounds that share the same molecular formula but differ in the connectivity of their atoms. Think of it as: same collection of atoms, wired together differently.
Molecular formula
The formula that tells you exactly how many of each element are in one molecule (e.g., C₆H₁₄). Two compounds must have the same molecular formula to be constitutional isomers. If the formulas differ, they are simply different compounds.
Parent chain
The longest continuous chain of carbon atoms in the molecule. This determines the base name (methane, ethane, propane, butane, pentane, hexane, heptane, octane, etc.). In simple terms, you trace every possible path through the carbon skeleton and pick the one with the most carbons.
Substituent
An atom or group attached to the parent chain. Common examples: methyl (–CH₃), ethyl (–C₂H₅), propyl (–C₃H₇), isopropyl (–CH(CH₃)₂). Think of it as: the branches hanging off the main chain.
Locant
The number assigned to a carbon on the parent chain to specify where a substituent is attached. You number the chain so that substituents receive the lowest possible set of locants.
IUPAC nomenclature
The systematic naming system maintained by the International Union of Pure and Applied Chemistry. It gives every organic compound one unambiguous name based on its structure. In simple terms, this is the "official" naming system that replaces common names with a set of rules everyone agrees on.
Degree of unsaturation (index of hydrogen deficiency)
A formula that tells you how many rings or pi bonds a molecule contains: DoU = (2C + 2 + N – H – X) / 2, where C, N, H, X are the counts of carbon, nitrogen, hydrogen and halogen atoms. Useful for quickly checking whether two molecules with the same formula could be constitutional isomers, or whether one must have a ring or double bond the other does not.
Two compounds are constitutional isomers if and only if:
They have the same molecular formula, and
Their atoms are connected differently (different connectivity).
If the molecular formulas differ, they cannot be isomers of any kind. If the formulas match but the connectivity is identical (just drawn or rotated differently), they are the same compound.
Worked example from the worksheet (Question 3):
(a) Cyclopentane (C₅H₁₀) vs. a straight-chain structure with formula C₅H₁₂. Different molecular formulas → not constitutional isomers.
(b) Cyclopentane (C₅H₁₀) vs. a straight-chain structure that also has formula C₅H₁₀. Same molecular formula, different connectivity (one is a ring, one is an open chain with a double bond or different ring) → yes, constitutional isomers.
The quick check: write out the molecular formula for each compound. If they match, look at whether the bonding pattern differs.
When asked to draw constitutional isomers for a given molecular formula, systematically vary the carbon skeleton:
Start with the straight chain.
Shorten the chain by one carbon and attach that carbon as a branch. Move the branch to every unique position.
Shorten again and try two branches.
For formulas containing oxygen (e.g., C₅H₁₂O), remember the oxygen can be placed in different positions: as an ether (C–O–C) or as an alcohol (C–OH), and each of those can have different carbon skeletons.
Worked example (Question 4a): Three isomers of C₆H₁₄
Hexane (straight chain of 6 carbons)
2-methylpentane (5-carbon chain with a methyl branch at C2)
3-methylpentane (5-carbon chain with a methyl branch at C3)
2,3-dimethylbutane (4-carbon chain with methyl branches at C2 and C3)
2,2-dimethylbutane (4-carbon chain with two methyl branches at C2)
Any three of the above would be a valid answer.
Worked example (Question 4b): Three isomers of C₅H₁₂O
Here the oxygen opens up two functional group categories:
Ethers: e.g., methyl tert-butyl ether (CH₃–O–C(CH₃)₃), diethyl methyl configurations
Alcohols: e.g., 1-pentanol, 2-pentanol, 3-pentanol, 2-methyl-1-butanol
Any three distinct structures with formula C₅H₁₂O and different connectivity count.
Find the longest continuous carbon chain. This is the parent chain. Its length gives the base name.
Number the chain from the end that gives the substituents the lowest set of locants. Compare locant sets at the first point of difference.
Name each substituent and assign it the locant of the carbon it is attached to.
List substituents alphabetically in the final name. Prefixes like di-, tri-, tetra- are used for repeated substituents but are ignored when alphabetising. ("Dimethyl" is alphabetised under M, not D.)
Use commas between numbers and hyphens between numbers and words.
Worked example (Question 6a): "2-Ethyl-3-methylpentane" is wrong.
When you draw the structure this name describes and trace the longest chain, you find a chain of 6 carbons (not 5). The pentane chain claimed in the name misses carbons that are part of the "ethyl" branch. If you start at the terminal carbon of the ethyl group, you get a 6-carbon chain. The correct name is 3,4-dimethylhexane.
Lesson: always check that no substituent's carbons, when included, create a longer parent chain than the one you chose.
Worked example (Question 6b): "1-Methyl-5-ethylhexane" is wrong.
Two errors here. First, a "methyl" at C1 is not a branch; it is just the terminal carbon of a longer chain. Second, by incorporating the C from the methyl and the 2 carbons from the ethyl into the main chain, you find an 8-carbon chain. The correct name is 3-methyloctane.
Lesson: a substituent at C1 means you have not found the true longest chain. The parent chain must always be extended through a C1 branch.
Students often assume two molecules drawn differently must be different compounds. Always check: can you trace the same connectivity? If so, they are the same compound, just drawn from a different angle or orientation.
Students forget to look for a longer parent chain that runs through a substituent. If your "substituent" at C1 or at the end of the chain can be absorbed into a longer chain, it should be.
Students alphabetise prefixes like "di" and "tri." These multiplying prefixes are ignored when determining alphabetical order. Dimethyl is listed under M, triethyl under E.
Students sometimes list substituents by locant order rather than alphabetical order. The rule is alphabetical: ethyl before methyl, regardless of which has the lower locant number.
IUPAC naming is how pharmaceutical companies, chemical suppliers and regulatory bodies communicate about specific compounds without ambiguity. When a chemist orders "2-methylpentane" from a supplier, both parties know exactly which molecule is meant. Constitutional isomers matter because molecules with the same formula but different connectivity can have wildly different properties: n-butane is a gas used as lighter fuel, while its constitutional isomer isobutane is used as a refrigerant and aerosol propellant.
⚠️ "Is this name correct? If not, give the correct IUPAC name" is a staple exam question. The two errors from this worksheet (failing to find the longest chain, and naming a C1 substituent) are the most frequently tested.
⚠️ Drawing multiple constitutional isomers for a given formula tests your ability to systematically vary the carbon skeleton. Exams typically ask for 3 to 5. Be methodical: start long and progressively shorten the chain.
⚠️ Constitutional isomers vs. identical compounds vs. stereoisomers: you will need to distinguish all three. For now, focus on connectivity. Stereoisomers come later.
True or False: Two compounds with the formula C₄H₁₀ but different structures are constitutional isomers. (True.)
True or False: Cyclopentane and pentane are constitutional isomers. (False. Cyclopentane is C₅H₁₀; pentane is C₅H₁₂. Different molecular formulas.)
Fill in the blank: The longest continuous carbon chain in a molecule determines the ___. (Parent chain / base name.)
True or False: "Di" and "tri" count when alphabetising substituents. (False. They are ignored.)
Fill in the blank: A substituent at carbon 1 means you have not found the ___. (Longest chain.)
Q: Draw three constitutional isomers of C₅H₁₂.
A: Pentane (straight chain), 2-methylbutane (isopentane) and 2,2-dimethylpropane (neopentane). These are the only three.
Q: The name "3-ethyl-2-methylbutane" is incorrect. What is the correct IUPAC name?
A: The longest chain runs through the ethyl branch, giving a 5-carbon parent chain. The correct name is 2,3-dimethylpentane (or 3-methylpentane, depending on how the structure is drawn; the key point is that the ethyl at C3 of a butane signals a longer chain has been missed).
Q: Are butane and isobutane constitutional isomers? Explain.
A: Yes. Both have the molecular formula C₄H₁₀. Butane is a straight 4-carbon chain; isobutane (2-methylpropane) has a 3-carbon chain with a methyl branch. Same formula, different connectivity.
Q: Name the compound with a 5-carbon chain, a methyl group on C2 and a methyl group on C4.
A: 2,4-dimethylpentane.
Q: Why is "1-methylhexane" not a valid IUPAC name?
A: A methyl group on C1 simply extends the chain by one carbon. Including that carbon gives a 7-carbon straight chain, so the compound is just heptane.
Constitutional isomers are the first type of isomerism you encounter; stereoisomers (cis/trans, enantiomers, diastereomers) come next and build directly on this foundation. IUPAC naming extends to alkenes, alkynes, alcohols, ethers, aldehydes, ketones, carboxylic acids and amines, each adding a suffix or prefix to the same parent-chain framework. The concept of connectivity also matters for reaction mechanisms: constitutional isomers can have very different reactivity, so being able to draw and name them correctly is essential for predicting products.
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