Q.Which among the following is an isomeric pair
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Chain Isomerism: The Skeleton of a Molecule
Imagine you have a set of 5 identical Lego bricks. You can snap them together in a straight line — one after the other — to make a long, thin rod. Or, you could take the same 5 bricks and make a "T" shape: a straight chain of 4 bricks, with the 5th brick attached to the middle one. Both structures use exactly the same number and type of bricks, but their shapes are different.
That difference in shape — specifically, in the carbon skeleton — is the heart of chain isomerism.
The Intuition
Carbon atoms love to bond with each other to form chains. But they don't always have to form a single, straight line. They can branch off. When two molecules have the same molecular formula (same number of each atom) but differ in how their carbon atoms are connected — one is a straight chain, another is branched — they are chain isomers.
Think of it like a family tree. The same set of people can be arranged in a direct line (grandparent → parent → child) or with branches (grandparent has two children, each with their own children). The "people" (carbon atoms) are the same, but the "connections" (bonds) form different patterns.
The Precise Statement
Chain isomerism is a type of structural isomerism where two or more compounds have the same molecular formula but differ in the arrangement of the carbon skeleton (the carbon chain). This difference arises because the carbon atoms can be linked in a straight chain or in one or more branched chains.
Chain isomers always have the same molecular formula but different structural formulas due to different carbon skeletons. They are distinct compounds with different physical and chemical properties.
A Concrete Example: Pentane (C5H12)
The molecular formula C5H12 can represent three different chain isomers. Let's draw them:
- n-Pentane (normal pentane): A straight chain of 5 carbons.
CH3−CH2−CH2−CH2−CH3
- Isopentane (2-methylbutane): A chain of 4 carbons with a 1-carbon branch (a methyl group, −CH3) attached to the second carbon.
CH3−CH(CH3)−CH2−CH3
- Neopentane (2,2-dimethylpropane): A central carbon bonded to four other carbons — a highly branched structure.
C(CH3)4
| Isomer Name | Common Name | Structure (Condensed) | Boiling Point (°C) |
|---|---|---|---|
| Pentane | n-Pentane | CH3(CH2)3CH3 | 36.1 |
| 2-Methylbutane | Isopentane | (CH3)2CHCH2CH3 | 27.7 |
| 2,2-Dimethylpropane | Neopentane | C(CH3)4 | 9.5 |
Notice how the boiling point drops as the molecule becomes more branched. This is because branching reduces the surface area available for intermolecular forces (London dispersion forces), making the molecule easier to vaporize.
A common mistake is to think that any difference in structure is chain isomerism. It is only about the carbon skeleton. If two molecules have the same skeleton but different positions of a double bond or a functional group, that is a different type of isomerism (position isomerism), not chain isomerism.
When Does Chain Isomerism Occur?
Chain isomerism is possible only when the carbon chain is long enough to allow branching. For alkanes: …
Isomers have the same molecular formula but different structures. Butane and 2-methylpropane are both C4H10 (chain isomers), so they form an isomeric pair. …
Butane and 2-methylpropane are chain isomers (both C4H10).
Isomers are compounds with the same molecular formula but different arrangements of atoms. Checking the pairs:
- Butane: CH3-CH2-CH2-CH3, formula C4H10
- 2-methylpropane (isobutane): (CH3)3CH, formula C4H10 …
- CBSE 2026Set ANNUAL1 markQ.Write the isomers of pentane.
›Reveal solutionSolution
Pentane (C5H12) has exactly three chain (structural) isomers: n-pentane, isopentane, and neopentane.
All three isomers share the same molecular formula, C5H12, but differ in how the 5 carbons are connected:
- n-Pentane: CH3-CH2-CH2-CH2-CH3 — a straight, unbranched chain of 5 carbons.
- Isopentane (2-methylbutane): (CH3)2CH-CH2-CH3 — a 4-carbon main chain (butane) with one methyl branch on C2.
- Neopentane (2,2-dimethylpropane): C(CH3)4 — a central carbon bonded to four methyl groups, i.e., a 3-carbon main chain (propane) with two methyl branches, both on C2. …
- CBSE 2024Set ANNUAL1 markMCQQ.CH3-CH(CH3)-CH2-CH2-CH3 is(a) Isopentane(b) Neopentane(c) Isohexane(d) n-pentane
›Reveal solutionSolution
Counting all carbons in CH3-CH(CH3)-CH2-CH2-CH3 gives 6 total, so this is a hexane isomer (2-methylpentane), commonly called isohexane, not isopentane (which only has 5 carbons).
Structure: CH3-CH(CH3)-CH2-CH2-CH3
Count the carbons: the main chain CH3-CH-CH2-CH2-CH3 has 5 carbons, plus the branch -CH3 on the second carbon adds 1 more, for a total of 6 carbons -- this is a hexane (C6H14) isomer.
IUPAC name: numbering the main chain to give the substituent the lowest locant, the methyl group is on C2, giving 2-methylpentane.
Common names for the pentane/hexane isomers: …
- CBSE 2023Set ANNUAL1 markMCQQ.Which among the following is an isomeric pair(a) Propane and Propene(b) Butane and 2-methyl propane(c) Pentane and propane(d) Butane and Isopropane
›Reveal solutionSolution
Butane and 2-methylpropane are chain isomers (both C4H10).
Isomers are compounds with the same molecular formula but different arrangements of atoms. Checking the pairs:
- Butane: CH3-CH2-CH2-CH3, formula C4H10
- 2-methylpropane (isobutane): (CH3)3CH, formula C4H10 …
- CBSE 2018Set annual21 markMCQQ.The structure of Isobutane is:(a) CH3-CH2-CH3(b) CH3-CH(CH3)-CH3(c) CH3-CH3(d) CH4
›Reveal solutionSolution
Isobutane is 2-methylpropane — a central CH carbon flanked by three methyl groups — distinguishing it from the straight-chain propane, ethane, and methane shown in the other options.
Isobutane (also called 2-methylpropane, molecular formula C4H10) is the branched-chain structural isomer of n-butane. Its structure has a central carbon atom bonded to three -CH3 (methyl) groups and one hydrogen atom:
CH3-CH(CH3)-CH3, i.e., a central CH carbon with three methyl substituents (equivalent to writing (CH3)3CH).
Checking the other options:
- (a) CH3-CH2-CH3 is propane (C3H8), a straight-chain 3-carbon alkane — not isobutane.
- (c) CH3-CH3 is ethane (C2H6), a 2-carbon alkane. …
- CBSE 2018Set annual21 markMCQQ.The number of chain isomers in Hexane is:(a) 4(b) 5(c) 3(d) 6
›Reveal solutionSolution
Counting every distinct way to arrange six carbons into a chain, with or without branches, gives exactly five different hexane skeletons.
Chain (structural) isomers of an alkane are compounds with the same molecular formula but different arrangements (connectivity) of the carbon skeleton. For hexane, C6H14, the five possible chain isomers are:
- n-Hexane: CH3-CH2-CH2-CH2-CH2-CH3 (straight chain)
- 2-Methylpentane: (CH3)2CH-CH2-CH2-CH3
- 3-Methylpentane: CH3-CH2-CH(CH3)-CH2-CH3
- 2,2-Dimethylbutane: (CH3)3C-CH2-CH3
- 2,3-Dimethylbutane: (CH3)2CH-CH(CH3)2 …
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