Consider the following four compounds: I. CH3—CH2—CH2—CH2—CHO II. CH3—CH2—CH2—CO—CH3 III. CH3—CH2—CO—CH2—CH3 I

NCERT Exemplar · Q20

Q. Consider the following four compounds: I. CH3—CH2—CH2—CH2—CHO II. CH3—CH2—CH2—CO—CH3 III. CH3—CH2—CO—CH2—CH3 IV. CH3—CH(CH3)—CH2—CHO Which of the following pairs are not functional group isomers? (Note: more than one of the given options may be correct.)

(i) II and III

(ii) II and IV

(iii) I and IV

(iv) I and II

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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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Functional group isomers are compounds with the same molecular formula but different functional groups. We will determine the molecular formula and functional group for each compound and then check which pairs have the same functional group, as those will not be functional group isomers. The pairs that are not functional group isomers are (i) II and III and (iii) I and IV.

In organic chemistry, isomers are compounds that have the same molecular formula but different structural arrangements of atoms. Functional group isomers are a specific type of isomerism where compounds share the same molecular formula but possess different functional groups. This difference in functional groups leads to distinct chemical and physical properties.

To identify functional group isomers, we follow a two-step process:

Determine the molecular formula: Count the number of carbon, hydrogen, and oxygen atoms in each compound. For compounds to be isomers, their molecular formulas must be identical.
Identify the functional group: Determine the characteristic functional group present in each compound. If the molecular formulas are the same but the functional groups are different, then the compounds are functional group isomers. If both the molecular formula and the functional group are the same, they are not functional group isomers (they might be other types of isomers like chain or position isomers).

Let's apply this to the given compounds:

Analyze each compound to determine its molecular formula and functional group:
- Compound I: CH3—CH2—CH2—CH2—CHO
  - This is an aldehyde, characterised by the terminal —CHO group.
  - Counting atoms:
    - Carbon (iii): $1 (\text{CH}_3) + 1 (\text{CH}_2) + 1 (\text{CH}_2) + 1 (\text{CH}_2) + 1 (\text{CHO}) = 5$
    - Hydrogen (H): $3 (\text{CH}_3) + 2 (\text{CH}_2) + 2 (\text{CH}_2) + 2 (\text{CH}_2) + 1 (\text{CHO}) = 10$
    - Oxygen (O): $1 (\text{CHO})$
  - Molecular Formula: $\text{C}_5\text{H}_{10}\text{O}$
  - Functional Group: Aldehyde
- Compound II: CH3—CH2—CH2—CO—CH3
  - This is a ketone, characterised by the carbonyl group (—CO—) bonded to two alkyl groups.
  - Counting atoms:
    - Carbon (iii): $1 (\text{CH}_3) + 1 (\text{CH}_2) + 1 (\text{CH}_2) + 1 (\text{CO}) + 1 (\text{CH}_3) = 5$
    - Hydrogen (H): $3 (\text{CH}_3) + 2 (\text{CH}_2) + 2 (\text{CH}_2) + 3 (\text{CH}_3) = 10$
    - Oxygen (O): $1 (\text{CO})$
  - Molecular Formula: $\text{C}_5\text{H}_{10}\text{O}$
  - Functional Group: Ketone
- Compound III: CH3—CH2—CO—CH2—CH3
  - This is also a ketone, with the carbonyl group (—CO—) bonded to two ethyl groups.
  - Counting atoms:
    - Carbon (iii): $1 (\text{CH}_3) + 1 (\text{CH}_2) + 1 (\text{CO}) + 1 (\text{CH}_2) + 1 (\text{CH}_3) = 5$
    - Hydrogen (H): $3 (\text{CH}_3) + 2 (\text{CH}_2) + 2 (\text{CH}_2) + 3 (\text{CH}_3) = 10$
    - Oxygen (O): $1 (\text{CO})$
  - Molecular Formula: $\text{C}_5\text{H}_{10}\text{O}$
  - Functional Group: Ketone
- Compound IV: CH3—CH(CH3)—CH2—CHO
  - This is an aldehyde, with a methyl branch on the second carbon from the main chain.
  - Counting atoms:
    - Carbon (iii): $1 (\text{CH}_3) + 1 (\text{CH}) + 1 (\text{CH}_3 \text{ branch}) + 1 (\text{CH}_2) + 1 (\text{CHO}) = 5$
    - Hydrogen (H): $3 (\text{CH}_3) + 1 (\text{CH}) + 3 (\text{CH}_3 \text{ branch}) + 2 (\text{CH}_2) + 1 (\text{CHO}) = 10$
    - Oxygen (O): $1 (\text{CHO})$
  - Molecular Formula: $\text{C}_5\text{H}_{10}\text{O}$
  - Functional Group: Aldehyde …

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Q.Consider the following four compounds: I. CH3—CH2—CH2—CH2—CHO II. CH3—CH2—CH2—CO—CH3 III. CH3—CH2—CO—CH2—CH3 IV. CH3—CH(CH3)—CH2—CHO Which of the following pairs are not functional group isomers? (Note: more than one of the given options may be correct.)

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Q.Consider the following four compounds:
I. CH3—CH2—CH2—CH2—CHO
II. CH3—CH2—CH2—CO—CH3
III. CH3—CH2—CO—CH2—CH3
IV. CH3—CH(CH3)—CH2—CHO
Which of the following pairs are not functional group isomers? (Note: more than one of the given options may be correct.)

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