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NCERT's chapter on carbon compounds asks "How many structural isomers can you draw for pentane?" Most Class 10 students draw four or five, because the same molecule drawn twice looks different on paper. This post shows a simple way to draw every isomer of butane and pentane exactly once, explains why they really are different substances, and works through NCERT's harder question about bromopentane.
Is this in the Class 10 syllabus?
Yes. The CBSE Class X Science curriculum for 2026-27 lists "Carbon and its Compounds", including the "versatile nature of carbon" and hydrocarbons. Isomers appear in section 4.2.2, Chains, Branches and Rings, of the 2026-27 NCERT Class 10 chapter Carbon and its Compounds. The textbook is still the same rationalised edition, reprinted for 2026-27. Check your school's syllabus for the current year.
What are structural isomers?
NCERT defines them as "compounds with identical molecular formula but different structures." The formula tells you which atoms and how many. The structure tells you how they are joined.
NCERT's own example is butane, C4H10. Four carbon atoms can be joined in two ways:
A straight chain: CH3–CH2–CH2–CH3. This is butane.
A branched chain: a chain of three carbons with the fourth attached to the middle one, CH3–CH(CH3)–CH3. Its systematic name, which you will meet in Class 11, is 2-methylpropane; it is also called isobutane.
Fill every carbon's remaining bonds with hydrogens and both have 4 carbons and 10 hydrogens. Same formula, different structures.
Are isomers really different substances?
Yes, and you can see it in their properties. PubChem, the chemistry database of the US National Library of Medicine, lists boiling points of −0.5 °C for butane and −11.7 °C for isobutane. Same atoms, but a difference of about 11 °C. Branched molecules are more compact, so they touch their neighbours less and are easier to separate into a gas.
How do you draw every isomer of pentane exactly once?
Use the longest chain as your guide, and work from long to short.
Step 1. Longest chain of 5. All five carbons in a row: CH3–CH2–CH2–CH2–CH3. This is pentane.
Step 2. Longest chain of 4. Put four carbons in a row and attach the fifth as a branch. On carbon 1 or 4, the branch just makes the chain longer, so you are back to step 1. On carbon 2 or 3, you get the same molecule, because carbon 3 is carbon 2 counted from the other end. So there is one: CH3–CH(CH3)–CH2–CH3, called 2-methylbutane or isopentane.
Step 3. Longest chain of 3. Put three carbons in a row and attach two branches. Both must go on the middle carbon, or the chain gets longer again. That gives C(CH3)4, a central carbon bonded to four CH3 groups: 2,2-dimethylpropane, or neopentane.
Step 4. Longest chain of 2. Impossible: five carbons cannot be arranged so that no chain is longer than two.
So pentane, C5H12, has three structural isomers. PubChem lists their boiling points as about 36.1 °C for pentane, 28 °C for isopentane and 9.5 °C for neopentane. The more branched, the lower the boiling point.
How fast does the number of isomers grow?
Very fast. The Online Encyclopedia of Integer Sequences, entry A000602, lists the number of alkanes with n carbons, ignoring stereoisomers: 1, 1, 1 for one to three carbons, then 2 for butane, 3 for pentane, 5 for hexane, 9 for heptane, and 75 for decane. This is the "versatile nature of carbon" that NCERT describes: a few atoms can be put together in many ways.
Worked example: NCERT's bromopentane question
NCERT asks you to draw bromopentane and adds, "Are structural isomers possible for bromopentane?" Bromopentane is C5H11Br: a pentane skeleton with one hydrogen replaced by bromine. Go skeleton by skeleton and put the Br on each different kind of carbon.
Straight chain: Br on carbon 1, 2 or 3. Carbons 4 and 5 repeat 2 and 1. That is 3.
2-methylbutane skeleton: Br on the end CH3 next to the branch, on the branch carbon itself, on the CH2, or on the far CH3. That is 4.
Neopentane skeleton: all four CH3 groups are equivalent, so Br on any one gives the same molecule. That is 1.
Total: 3 + 4 + 1 = 8 structural isomers. So yes, they are possible, and there are eight.
Which mistakes cost the most marks?
Drawing a bent chain and calling it new. A chain drawn with a corner is still a straight chain. Bonds can rotate.
Branching from the end carbon. That just makes the main chain longer.
Counting the same position twice. Carbon 2 and carbon 4 of a five-carbon chain are the same position read from opposite ends.
Changing the formula. Count carbons and hydrogens in every drawing: all butane isomers are C4H10, all pentane isomers C5H12.
Thinking isomers are the same substance. Different structures give different boiling points.
For the rest of the chapter, see our guide to carbon and its compounds for Class 10. When you reach Class 11, our post on the four types of structural isomerism takes this further.
A practice routine
Day 1: Draw both butane isomers and all three pentane isomers from memory, with every hydrogen shown, and count the atoms.
Day 2: Draw all five isomers of hexane, C6H14, using the longest-chain method. Check your count against the sequence above.
Day 3: Redo the bromopentane count without looking, then try chlorobutane, C4H9Cl, which has four.
If you are unsure why carbon makes four bonds in the first place, the Fermi Concept Library covers every concept in chemistry, each with its own lesson.
Tonight, draw the three isomers of pentane and write each one's boiling point beside it.
