Consider the compounds I to VII: I. CH3—CH2—CH2—CH2—OH II. CH3—CH2—CH(OH)—CH3 III. (CH3)3C—OH IV. CH3—CH(CH3)—

NCERT Exemplar · Q26

Q. Consider the compounds I to VII: I. CH3—CH2—CH2—CH2—OH II. CH3—CH2—CH(OH)—CH3 III. (CH3)3C—OH IV. CH3—CH(CH3)—CH2—OH V. CH3—CH2—O—CH2—CH3 VI. CH3—O—CH2—CH2—CH3 VII. CH3—O—CH(CH3)2 Identify the pairs of compounds that represents chain isomerism.

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Concept understanding — IUPAC Nomenclature

IUPAC Nomenclature (Organic Compounds)

IUPAC nomenclature is a systematic way to name a compound so that its name alone tells you its exact structure, with no ambiguity. Every organic name follows the same underlying recipe, whatever the functional group.

The Recipe

Identify the principal characteristic group. If the molecule has a functional group senior enough to be named as a suffix (carboxylic acid > ester > amide > nitrile > aldehyde > ketone > alcohol > amine, and so on down the seniority order), that group decides the suffix and must be included in the parent chain. A halogen is never senior enough to be a suffix — it is always named as a prefix ("halo-"), whatever else is present.
Choose the parent chain. The parent is the longest continuous carbon chain that contains the principal characteristic group (if there is one). Among chains of the same length, the one with the most substituents wins.
Number the chain. Number from whichever end gives the LOWEST LOCANT to the principal characteristic group first. If there is no principal group (e.g. a simple haloalkane, or an alkene with a halogen substituent), lowest locant goes to the site of unsaturation (double/triple bond) first, then to substituents as a set.

Watch out

When two numbering directions give the SAME locant for the principal group/unsaturation (a genuine tie), the tie-break is the lowest locant SET for the substituents as a group — compare the two sets at their first point of difference. Only if the sets are themselves tied does the alphabetically-first substituent get the lower number.
Name and cite the substituents as prefixes, in alphabetical order (ignoring multiplying prefixes like di-/tri- but not ignoring structural prefixes like iso-/cyclo-), each with its own locant.
Assemble the name: locants + substituent prefixes (alphabetical) + parent chain name + suffix (if any).

Worked Example

$CH_3-CH(Cl)-CH(CH_3)-CH_2-CH_3$ : the longest chain is 5 carbons (pentane), no principal characteristic group (just a halogen substituent), so number for the lowest locant set. From the left: Cl at C2, methyl at C3 → set {2,3}. From the right: methyl at C3, Cl at C4 → set {3,4}. {2,3} is lower, so numbering from the left wins: 2-chloro-3-methylpentane. …

Why this formula?

IUPAC Nomenclature: Why the Rules Work the Way They Do

IUPAC nomenclature is not a single formula, but a system of rules designed to give every organic compound a unique, unambiguous name. The "why" behind these rules lies in clarity, consistency, and communication — ensuring that a chemist in Tokyo and one in Toronto draw the same structure from the same name.

1. The Core Principle: The Longest Carbon Chain

Rule: Identify the longest continuous chain of carbon atoms. This becomes the parent chain (e.g., pentane, hexane).

Why?

The longest chain represents the backbone of the molecule.
It gives the most stable, fundamental name — shorter chains would be branches, not the main structure.
Example: In a molecule with 5 carbons in a row and a 2-carbon branch, calling it "pentane" (not "ethane") tells you the core skeleton is 5 carbons long.

Key idea: The parent chain is the maximum continuous path — not necessarily the one that looks "straight" on paper.

2. Numbering: Lowest Locants (The "First Point of Difference" Rule)

Rule: Number the parent chain so that substituents get the smallest possible numbers. When there's a tie, compare the first point of difference.

Why?

This ensures reproducibility — two chemists will always number the same way.
It avoids ambiguity: "2-methylpentane" is unambiguous; "3-methylpentane" would be a different compound.
The first point of difference rule: If you have substituents at positions 2,4 and 3,5, choose 2,4 because 2 < 3 (the first number is smaller).

Example:

For a methyl group on carbon 2 vs. carbon 4 of a 5-carbon chain:
- 2-methylpentane (correct)
- 4-methylpentane (wrong — higher number)

3. Alphabetical Order of Substituents

Rule: List substituents in alphabetical order (ignoring prefixes like di-, tri-, sec-, tert- but not iso-).

Why?

Alphabetical order is a universal sorting convention — no need to remember priority based on size or complexity.
It makes names searchable and predictable.
Example: "3-ethyl-2-methylpentane" (e before m) — not "2-methyl-3-ethylpentane".

Exception: Prefixes like iso- and neo- are considered part of the name (e.g., isopropyl comes before methyl because "i" < "m").

4. Multiple Bonds: The "Lowest Locant" Rule for Alkenes/Alkynes

Rule: Number the chain so that the double or triple bond gets the lowest possible number, even if it means giving a substituent a higher number.

Why?

The functional group (alkene/alkyne) is more important than alkyl substituents.
The bond position defines the compound's reactivity and geometry.
Example: In pent-2-ene (not pent-3-ene), the double bond is between carbons 2 and 3 — the lower number (2) is used.

Priority order:

Principal functional group (e.g., -OH, -COOH, C=C)
Multiple bonds
Substituents (alkyl, halo, etc.)

5. The "Suffix" and "Prefix" System …

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Chain isomerism occurs between compounds with the same molecular formula and functional group but different carbon skeletons. The pairs of compounds exhibiting chain isomerism are (I, III), (I, IV), (II, III), (II, IV), (V, VI), (V, VII), and (VI, VII).

In organic chemistry, isomers are compounds that have the same molecular formula but different arrangements of atoms. Chain isomerism is a specific type of structural isomerism where compounds share the same molecular formula and the same functional group, but differ in the arrangement of their carbon atoms, forming different carbon skeletons (e.g., straight chain vs. branched chain).

To identify chain isomers, we follow these steps:

Determine the molecular formula for each compound.
Identify the functional group present in each compound.
Determine the carbon skeleton (straight or branched) for each compound.
Group compounds with the same molecular formula and the same functional group.
Within these groups, compare the carbon skeletons to find pairs that differ in their carbon chain arrangement.

Let's analyze each compound:

Analyze each compound's molecular formula, functional group, and carbon skeleton.

We will first determine the molecular formula, functional group, and the nature of the carbon chain for each given compound.

Compound	Structural Formula	Molecular Formula	Functional Group	IUPAC Name	Carbon Skeleton
I	CH3—CH2—CH2—CH2—OH	C4H10O	Alcohol	Butan-1-ol	Straight 4-carbon chain
II	CH3—CH2—CH(OH)—CH3	C4H10O	Alcohol	Butan-2-ol	Straight 4-carbon chain
III	(CH3)3C—OH	C4H10O	Alcohol	2-Methylpropan-2-ol	Branched 3-carbon chain (with a methyl branch)
IV	CH3—CH(CH3)—CH2—OH	C4H10O	Alcohol	2-Methylpropan-1-ol	Branched 3-carbon chain (with a methyl branch)
V	CH3—CH2—O—CH2—CH3	C4H10O	Ether	Ethoxyethane	Two ethyl groups (2C-O-2C)
VI	CH3—O—CH2—CH2—CH3	C4H10O	Ether	1-Methoxypropane	Methyl and n-propyl groups (1C-O-3C straight)
VII	CH3—O—CH(CH3)2	C4H10O	Ether	2-Methoxypropane	Methyl and isopropyl groups (1C-O-3C branched)

Group compounds by molecular formula and functional group.

All seven compounds have the same molecular formula, C4H10O. However, they belong to two different functional groups:
- Alcohols: I, II, III, IV
- Ethers: V, VI, VII
For compounds to be chain isomers, they must have the same functional group. Therefore, we will look for chain isomerism within the group of alcohols and within the group of ethers separately. Alcohols and ethers with the same molecular formula are functional group isomers, not chain isomers.
Identify chain isomers within the alcohol group (I, II, III, IV).
- I (Butan-1-ol) and II (Butan-2-ol) both have a straight 4-carbon chain. They differ only in the position of the hydroxyl group, making them position isomers, not chain isomers.
- III (2-Methylpropan-2-ol) and IV (2-Methylpropan-1-ol) both have a branched 3-carbon chain (a propane skeleton with a methyl group at the second carbon). They differ in the position of the hydroxyl group, making them position isomers, not chain isomers.
Now, let's compare the straight-chain alcohols with the branched-chain alcohols:
- I (Butan-1-ol) has a straight 4-carbon chain.
- III (2-Methylpropan-2-ol) has a branched 3-carbon chain. Since they have the same molecular formula (C4H10O) and functional group (alcohol) but different carbon skeletons, (I, III) are chain isomers.
- IV (2-Methylpropan-1-ol) also has a branched 3-carbon chain. Similarly, (I, IV) are chain isomers. …

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Q.Consider the compounds I to VII: I. CH3—CH2—CH2—CH2—OH II. CH3—CH2—CH(OH)—CH3 III. (CH3)3C—OH IV. CH3—CH(CH3)—CH2—OH V. CH3—CH2—O—CH2—CH3 VI. CH3—O—CH2—CH2—CH3 VII. CH3—O—CH(CH3)2 Identify the pairs of compounds that represents chain isomerism.

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Q.Consider the compounds I to VII:
I. CH3—CH2—CH2—CH2—OH
II. CH3—CH2—CH(OH)—CH3
III. (CH3)3C—OH
IV. CH3—CH(CH3)—CH2—OH
V. CH3—CH2—O—CH2—CH3
VI. CH3—O—CH2—CH2—CH3
VII. CH3—O—CH(CH3)2
Identify the pairs of compounds that represents chain isomerism.

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