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Counting the isomers of C4H10O is a standard question, but exams go one step further: "Compound X, C4H10O, reacts with sodium and gives a ketone on oxidation. Identify X." To answer that you need more than the count. You need to know which class each isomer belongs to and which test picks it out. This post sorts all seven isomers into classes, matches each class to its NCERT test, and works through three identification puzzles.
Where is this in the syllabus?
The JEE (Main) 2026 syllabus lists "Isomerism - structural and stereoisomerism" in the basic principles of organic chemistry, and under alcohols, "Identification of primary, secondary and tertiary alcohols". The NEET (UG) 2026 syllabus has the same two lines. In the 2026-27 NCERT books, structural isomerism is section 8.6.1 of the Class 11 unit on organic chemistry, whose metamerism example is C4H10O itself (methoxypropane and ethoxyethane). The tests are in the Class 12 unit Alcohols, Phenols and Ethers. Check your own exam year's syllabus.
What are the seven isomers, by class?
If you have not yet done the full count, our post on the four types of structural isomerism builds it step by step. Here is the same list arranged by what the tests can see:
Primary alcohols (OH on a carbon bonded to one other carbon): butan-1-ol and 2-methylpropan-1-ol.
Secondary alcohol: butan-2-ol. It is also the only chiral isomer, because C-2 carries H, OH, CH3 and C2H5.
Tertiary alcohol: 2-methylpropan-2-ol.
Ethers: 1-methoxypropane, 2-methoxypropane and ethoxyethane.
That is four alcohols and three ethers. Hold on to the split into primary, secondary and tertiary, because every test below responds to it.
Which tests tell the classes apart?
NCERT gives four pieces of evidence you can use.
Sodium metal. Alcohols "react with active metals such as sodium, potassium and aluminium to yield corresponding alkoxides" and hydrogen. Ethers have no O–H bond, so they do not. This splits alcohols from ethers.
Lucas test (concentrated HCl with ZnCl2). The alcohol dissolves, but the alkyl chloride it forms does not, so the mixture turns cloudy. NCERT says tertiary alcohols give turbidity immediately and primary alcohols give none at room temperature. Secondary alcohols are in between.
Hot copper at 573 K. NCERT states that vapours of a primary or secondary alcohol passed over heated copper are dehydrogenated to an aldehyde or a ketone, "while tertiary alcohols undergo dehydration" to an alkene.
Boiling point. NCERT's table puts butan-1-ol at 390 K and ethoxyethane at 307.6 K, close to n-pentane at 309.1 K. Same formula, a gap of more than 80 K, because alcohol molecules hydrogen-bond to each other and ether molecules cannot.
Worked puzzle 1: an optically active alcohol
X is C4H10O. It releases hydrogen with sodium. Its vapour over copper at 573 K gives a ketone. X is optically active.
Reacts with sodium: X is one of the four alcohols.
Gives a ketone: X is secondary. Only one C4H10O alcohol is secondary.
Optically active: this confirms it, since butan-2-ol is the only chiral isomer.
X is butan-2-ol, and the ketone is butan-2-one. The chirality clue was a check, not a need, but use every clue given: if they disagree, you have misread one.
Worked puzzle 2: no reaction with sodium
Y is C4H10O. It does not react with sodium. It is a metamer of 1-methoxypropane.
No reaction with sodium: Y is an ether.
Metamers have different alkyl groups on either side of the same functional group. 1-Methoxypropane has methyl and propyl (1 + 3 carbons). A metamer must split the carbons differently, which leaves only 2 + 2.
Y is ethoxyethane. 2-Methoxypropane is not the answer: it has the same 1 + 3 split (methyl and isopropyl) and differs from 1-methoxypropane only in where the oxygen sits on the three-carbon chain, so it is usually treated as a position isomer.
Worked puzzle 3: instant turbidity
Z is C4H10O. With Lucas reagent it turns cloudy at once. Over hot copper it gives an alkene, not a carbonyl compound.
Instant turbidity: tertiary alcohol.
Alkene over copper: tertiary alcohols dehydrate rather than dehydrogenate, which agrees.
Z is 2-methylpropan-2-ol, and the alkene is 2-methylpropene.
Common mistakes and how to avoid them
Calling 2-methylpropan-1-ol secondary because the chain is branched. Classify by the carbon that carries the OH, not by the branching elsewhere. Here the OH carbon is bonded to one carbon, so it is primary.
Forgetting the ethers. A question that says "isomers of C4H10O" includes them. One that says "isomeric alcohols" does not.
Expecting tertiary alcohols to give a ketone. The carbon bearing the OH has no hydrogen left to lose, so NCERT notes they do not undergo oxidation.
Counting butan-2-ol twice. It has two mirror-image forms. For structural isomers count it once; for all isomers, including stereoisomers, the total is eight. Our post on chirality and R/S naming explains why.
A practice routine
Day 1: Draw the seven isomers from memory and label each as 1°, 2°, 3° or ether.
Day 2: For each, write what sodium, the Lucas test and hot copper would show. That is 21 predictions; check them against the rules above.
Day 3: Repeat for C5H12O. You should find 14 structural isomers: eight alcohols (four primary, three secondary, one tertiary) and six ethers.
Day 4: Write two puzzles of your own and swap them with a friend.
If puzzles like these keep sending you to the wrong isomer, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer.
Tonight, write the hot-copper product for each of the four C4H10O alcohols and check that exactly one gives an alkene.
