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Two compounds can share a formula such as C4H10O and still boil at different temperatures, react differently and carry different names. Isomer questions test whether you can see these differences quickly and count them without missing or repeating a structure. This post defines the four types of structural isomerism NCERT uses, works through every structural isomer of C4H10O, and shows the drawing slips that lead to wrong counts.
Where is this in the syllabus?
The JEE Main 2026 syllabus lists "Isomerism - structural and stereoisomerism" in Some Basic Principles of Organic Chemistry. The CBSE Chemistry curriculum for 2026-27 includes isomerism in the Class 11 organic chemistry unit. The textbook section is 8.6 of the NCERT Class 11 chapter Organic Chemistry: Some Basic Principles and Techniques.
What makes two compounds structural isomers?
NCERT defines isomerism as two or more compounds having the same molecular formula but different properties. Structural isomers have the same formula but different structures, meaning their atoms are linked in different ways. Stereoisomers, by contrast, have the same links but a different arrangement in space. That second family is covered in our post on stereoisomerism, chirality, R/S and E/Z.
What are the four types?
NCERT's examples:
Chain isomerism: different carbon skeletons. C5H12 is pentane, isopentane (2-methylbutane) and neopentane (2,2-dimethylpropane).
Position isomerism: the same skeleton, with a substituent atom or functional group in a different position. C3H8O gives propan-1-ol and propan-2-ol.
Functional group isomerism: different functional groups. C3H6O gives propanone (a ketone) and propanal (an aldehyde).
Metamerism: different alkyl chains on either side of the functional group. C4H10O gives methoxypropane (CH3OC3H7) and ethoxyethane (C2H5OC2H5).
Worked example: all structural isomers of C4H10O
Step 1: check the formula. C4H10O has the same H count as an alkane, C4H10, so there are no rings and no double bonds. The oxygen must be in an alcohol (C–OH) or an ether (C–O–C).
Step 2: alcohols. Use each four-carbon skeleton and put the OH on every distinct carbon.
Straight chain C–C–C–C: butan-1-ol and butan-2-ol. Putting OH on carbon 3 or 4 just gives these again, numbered from the other end.
Branched chain, 2-methylpropane: 2-methylpropan-1-ol (OH on an end CH3) and 2-methylpropan-2-ol (OH on the central carbon).
That makes four alcohols.
Step 3: ethers. Split the four carbons between the two sides of the O: 1 + 3 or 2 + 2.
1 + 3 with a straight propyl group: 1-methoxypropane.
1 + 3 with an isopropyl group: 2-methoxypropane.
2 + 2: ethoxyethane.
That makes three ethers, and 4 + 3 = 7 structural isomers in all.
Step 4: name the relationships.
Butan-1-ol and butan-2-ol: position isomers.
Butan-1-ol and 2-methylpropan-1-ol: chain isomers.
Any alcohol and any ether: functional group isomers.
Ethoxyethane and 1-methoxypropane: metamers.
1-methoxypropane and 2-methoxypropane: the oxygen is attached at a different position on the three-carbon chain, so they are usually treated as position isomers.
Why do students get the count wrong?
The usual cause is drawing the same molecule in a different shape. A straight chain drawn with a bend is still a straight chain. CH3CH2CH2CH2OH drawn as an L is still butan-1-ol. Name every structure by IUPAC rules before you count it, and if two names match, you have a repeat. Our guide to IUPAC naming of organic compounds gives the numbering rules.
The second cause is mixing in stereoisomers. Butan-2-ol is chiral, so it exists as two mirror-image forms. A question asking for structural isomers counts it once. A question asking for all isomers, including stereoisomers, counts it twice, giving 8 for C4H10O. Read the question carefully.
Common mistakes and how to avoid them
Missing a skeleton. List every chain first: for four carbons, straight and one branched.
Counting OH on carbon 3 of butane as a new compound. Renumber from the other end and you get butan-2-ol.
Forgetting the ethers. Any formula CnH2n+2O with n of 2 or more allows ethers.
Calling every pair of ethers metamers. Metamers differ in the alkyl groups on either side. Check whether that is the difference.
Labelling propanal and propanone as position isomers. The C=O changes from an aldehyde to a ketone, so they are functional group isomers.
A practice routine
Daily, 15 minutes: pick one formula, such as C5H12, C4H9Cl, C3H6O or C4H8O2, and list its structural isomers with IUPAC names.
Twice a week: take five pairs from your list and name the type of isomerism for each.
Weekly: redo the C4H10O count from memory, then add stereoisomers for each chiral structure.
When a formula like C4H8O2 gives you a different count each time, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer.
Today, list the structural isomers of C4H9Cl and check that you have four.
