Assertion (A): Pent-1-ene and pent-2-ene are position isomers. Reason (R): Position isomers differ in the posi

NCERT Exemplar · Q57

Q. Assertion (A): Pent-1-ene and pent-2-ene are position isomers. Reason (R): Position isomers differ in the position of functional group or a substituent.

(i) Both A and R are correct and R is the correct explanation of A.

(ii) Both A and R are correct but R is not the correct explanation of A.

(iii) Both A and R are not correct.

(iv) A is not correct but R is correct.

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Concept understanding — Structural Isomerism

Structural Isomerism: The First Meeting

Imagine you have a box of identical Lego bricks — four red, ten blue, and six yellow. You build two different models: a car and a house. Both use exactly the same number of each colour brick, but the structures are completely different. That is the core idea of isomerism: same atoms, different arrangement.

In chemistry, molecules are not just a list of atoms. How those atoms are connected matters enormously. Two molecules can have the exact same molecular formula (same number of each atom) but be connected in different ways. Those are structural isomers (also called constitutional isomers).

The Precise Statement

Structural isomers are compounds that have the same molecular formula but different connectivity of atoms — that is, different structural formulas.

The key word is connectivity. Which atom is bonded to which? If you change that, you get a different substance with different physical and chemical properties.

A Concrete Example: C₄H₁₀

Take butane, C₄H₁₀. There are exactly two ways to connect four carbon atoms and ten hydrogen atoms:

n-Butane — a straight chain: C–C–C–C
Isobutane (2-methylpropane) — a branched chain: a central carbon bonded to three methyl groups

Both have formula C₄H₁₀. But n-butane boils at –0.5 °C, while isobutane boils at –11.7 °C. Same atoms, different connectivity → different substance.

Note

Structural isomers are not the same molecule. They are distinct compounds that happen to share a molecular formula. You cannot rotate or flip one to get the other — you must break and reform bonds.

The Three Main Types

Structural isomerism comes in three flavours:

Type	What changes	Example (C₃H₆O)
Chain isomerism	The carbon skeleton (straight vs. branched)	Butane vs. isobutane
Position isomerism	The location of a functional group or substituent	Propan-1-ol vs. propan-2-ol (OH on carbon 1 vs. carbon 2)
Functional group isomerism	The atoms are rearranged into a different functional group	Propanal (aldehyde) vs. propanone (ketone) — both C₃H₆O

Watch out

Do not confuse structural isomers with stereoisomers. Stereoisomers have the same connectivity but differ in spatial arrangement (like left and right hands). That is a completely different chapter. For now: structural isomers = different bond connections.

Why This Matters …

Why this formula?

Structural Isomerism: Why the Key Ideas Hold

Structural isomerism arises when molecules share the same molecular formula but differ in the connectivity of atoms. There is no single "formula" for structural isomerism — instead, the key is understanding why different arrangements are possible.

The Core Principle: Connectivity ≠ Composition

A molecular formula tells you how many of each atom are present, but not how they are joined. Structural isomers exist because atoms can form bonds in multiple distinct sequences while satisfying valency rules.

Why This Happens: The Valency Constraint

Each atom has a fixed bonding capacity (valency):

Carbon: 4 bonds
Hydrogen: 1 bond
Oxygen: 2 bonds
Nitrogen: 3 bonds

Example: For $C_4H_{10}$ , the formula satisfies $4(4) + 10(1) = 26$ valence electrons. But the carbon atoms can be arranged as:

A straight chain: $CH_3-CH_2-CH_2-CH_3$ (n-butane)
A branched chain: $CH_3-CH(CH_3)-CH_3$ (isobutane)

Both satisfy valency, but the connectivity differs.

The "Formula" for Counting Isomers: Why It's Not Simple

There is no closed-form formula to count structural isomers for a given molecular formula. The number grows rapidly and depends on:

Carbon skeleton branching possibilities
Functional group positions
Ring formation possibilities

Why No Simple Formula Exists

The problem is combinatorial — the number of possible trees (acyclic graphs) with $n$ carbon atoms grows exponentially. For example:

$C_4H_{10}$ : 2 structural isomers
$C_5H_{12}$ : 3 structural isomers
$C_6H_{14}$ : 5 structural isomers
$C_{10}H_{22}$ : 75 structural isomers

The pattern follows Cayley's formula for trees, but even that counts only carbon skeletons — not functional group positions.

Key Reasoning: The Branching Principle

The fundamental reason structural isomers exist is that carbon chains can branch. Consider $C_5H_{12}$ :

Straight chain: $C-C-C-C-C$ (n-pentane)
One branch: $C-C-C(C)-C$ (isopentane) — the branch can be at position 2 or 3, but these are identical due to symmetry
Two branches: $C-C(C)(C)-C$ (neopentane) — a quaternary carbon

Why position matters: The branch location changes the carbon's environment, altering physical and chemical properties.

The Functional Group Position Rule

For compounds with functional groups (e.g., alcohols $C_nH_{2n+2}O$ ), the position of the -OH group creates isomers:

$CH_3CH_2CH_2OH$ (propan-1-ol) — OH at end
$CH_3CH(OH)CH_3$ (propan-2-ol) — OH in middle …

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Position isomers differ only in where a functional group or substituent sits on the same carbon skeleton. Pent-1-ene and pent-2-ene have their double bond at different positions along the same five-carbon chain, making them position isomers. Both statements are correct, and R explains A perfectly.

Understanding Position Isomerism

Structural isomers share the same molecular formula but differ in how their atoms are connected. Within structural isomerism, position isomers form a specific subclass: they have the same carbon skeleton and the same functional group, but that functional group occupies a different position on the chain.

Think of it as rearranging furniture in the same room. The room (carbon skeleton) stays identical, the furniture piece (functional group) is the same, but you've moved it to a different spot.

Analyzing the Assertion and Reason

Let me work through each statement systematically.

1. Examining the Reason (R) first

The reason states that position isomers differ in the position of a functional group or substituent. This is the textbook definition of position isomerism. For alkenes, the functional group is the $\ce{C=C}$ double bond. For alcohols, it's the $\ce{-OH}$ group. For haloalkanes, it's the halogen atom. The definition is accurate and complete.

2. Checking the Assertion (A)

Pent-1-ene has the structure:

\ce{CH2=CH-CH2-CH2-CH3}

The double bond sits between carbon-1 and carbon-2.

Pent-2-ene has the structure:

\ce{CH3-CH=CH-CH2-CH3}

The double bond sits between carbon-2 and carbon-3.

Both molecules have:

The same molecular formula: $\ce{C5H10}$
The same carbon skeleton: an unbranched five-carbon chain
The same functional group: one $\ce{C=C}$ double bond
Different positions for that double bond

This matches the definition of position isomers exactly.

3. Does R explain A? …

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Q.Assertion (A): Pent-1-ene and pent-2-ene are position isomers. Reason (R): Position isomers differ in the position of functional group or a substituent.

Understanding Position Isomerism

Analyzing the Assertion and Reason

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Q.Assertion (A): Pent-1-ene and pent-2-ene are position isomers.
Reason (R): Position isomers differ in the position of functional group or a substituent.

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