Assertion: It is difficult to replace chlorine by –OH in chlorobenzene in comparison to that in chloroethane.

NCERT Exemplar · Q92

Q. Assertion: It is difficult to replace chlorine by -OH in chlorobenzene in comparison to that in chloroethane. Reason: Chlorine-carbon (C-Cl) bond in chlorobenzene has a partial double bond character due to resonance.

(i) Assertion and reason both are correct and reason is correct explanation of assertion.

(ii) Assertion and reason both are wrong statements.

(iii) Assertion is correct but reason is wrong statement.

(iv) Assertion is wrong but reason is correct statement.

(v) Assertion and reason both are correct statements but reason is not correct explanation of assertion.

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Concept understanding — Inductive Effect on Acidity

Inductive Effect on Acidity – From Intuition to Precision

Imagine you are holding a rope tied to a heavy box. If you pull the rope, the box moves toward you. Now imagine the rope is made of rubber bands — the pull still reaches the box, but it gets weaker the farther away you are. That is exactly how the inductive effect works inside a molecule.

The Core Intuition

An acid donates a proton ( $H^+$ ). After it does, the remaining part (the conjugate base) carries a negative charge. The stability of that negative charge determines how willing the molecule is to give up the proton. More stable conjugate base → stronger acid.

Now, some atoms or groups are electron-withdrawing — they pull electron density toward themselves through the sigma bonds. If such a group is attached near the acidic proton, it pulls some electron density away from the negative charge on the conjugate base. That spreads out (delocalises) the negative charge, making the conjugate base more stable. The acid becomes stronger.

Conversely, electron-donating groups push electron density toward the negative charge, concentrating it and making the conjugate base less stable. The acid becomes weaker.

Note

The inductive effect operates through sigma bonds only. It does not involve pi bonds or resonance. It is a permanent, through-bond polarisation.

The Precise Statement

Inductive effect on acidity: The acidity of a compound increases with the presence of electron-withdrawing groups (EWGs) near the acidic site, and decreases with electron-donating groups (EDGs). The effect is strongest when the group is closest to the acidic proton, and diminishes rapidly with distance.

Mathematically, for a series of substituted carboxylic acids:

\text{R-COOH} \quad \text{where R = substituent}

The acid dissociation constant $K_a$ changes as:

If R is electron-withdrawing (e.g., $-\text{Cl}$ , $-\text{NO}_2$ , $-\text{CF}_3$ ): $K_a$ increases → stronger acid.
If R is electron-donating (e.g., $-\text{CH}_3$ , $-\text{C}_2\text{H}_5$ ): $K_a$ decreases → weaker acid.

Why Distance Matters

The inductive effect falls off with distance because sigma bonds are localised. Each bond attenuates the effect by roughly a factor of 2–3. For example, compare:

Compound	$pK_a$	Explanation
$\text{CH}_3\text{COOH}$	4.76	Reference (no EWG)
$\text{ClCH}_2\text{COOH}$	2.86	Cl withdraws through one bond
$\text{Cl}_2\text{CHCOOH}$	1.29	Two Cl atoms, stronger withdrawal
$\text{Cl}_3\text{CCOOH}$	0.65	Three Cl atoms, strongest withdrawal
$\text{CH}_3\text{CH}_2\text{COOH}$	4.87	Ethyl group is electron-donating (slightly weaker acid)

Notice: $\text{ClCH}_2\text{COOH}$ is about 100 times stronger than acetic acid ( $\Delta pK_a \approx 1.9$ ). But if the Cl is moved further away:

Compound	$pK_a$
$\text{ClCH}_2\text{CH}_2\text{COOH}$	4.08
$\text{ClCH}_2\text{CH}_2\text{CH}_2\text{COOH}$	4.52

The effect fades as the chlorine moves farther from the carboxyl group. …

Why this formula?

Inductive Effect on Acidity: Why It Works

The inductive effect is a through-bond electron displacement caused by differences in electronegativity. When we ask why it affects acidity, we must first understand what acidity means at the molecular level.

The Core Idea: Stabilising the Conjugate Base

Acidity is governed by the equilibrium:

\text{HA} \rightleftharpoons \text{H}^+ + \text{A}^-

The stronger the acid, the more it favours the right side. This happens when the conjugate base $\text{A}^-$ is more stable. The inductive effect directly influences this stability.

Why Electron-Withdrawing Groups (EWG) Increase Acidity

Consider a carboxylic acid with an electronegative atom (like Cl) attached to the carbon chain:

\text{Cl} - \text{CH}_2 - \text{COOH}

The inductive pull: The Cl atom is more electronegative than carbon. It pulls electron density toward itself through the sigma bonds.
Effect on the O–H bond: This electron withdrawal travels along the carbon chain, reducing electron density around the O–H bond. The bond becomes more polarised, making the H⁺ easier to remove.
Stabilising the conjugate base: After losing H⁺, the negative charge on the carboxylate ion ( $\text{RCOO}^-$ ) is delocalised by resonance. But the inductive effect further stabilises this negative charge by pulling electron density away from the oxygen atoms. This makes the conjugate base less reactive (more stable), shifting equilibrium toward dissociation.

Key insight: The inductive effect doesn't just weaken the O–H bond — it stabilises the anion that forms after deprotonation.

The Quantitative Relationship: Hammett Equation

For substituted benzoic acids, the effect is quantified by the Hammett equation:

\log \left( \frac{K_a}{K_a^0} \right) = \sigma \rho

Where:

$K_a$ = acid dissociation constant of substituted acid
$K_a^0$ = acid dissociation constant of unsubstituted benzoic acid
$\sigma$ = substituent constant (measures inductive + resonance effect)
$\rho$ = reaction constant (sensitivity of the reaction to substituent effects)

Why This Formula Holds

The derivation comes from linear free-energy relationships:

Free energy change: For any acid dissociation:

\Delta G^\circ = -RT \ln K_a

Effect of substituent: A substituent changes $\Delta G^\circ$ by an amount proportional to its electronic effect:

\Delta (\Delta G^\circ) = -RT \ln \left( \frac{K_a}{K_a^0} \right)

Separability assumption: The total effect of a substituent on any reaction can be factored into:
- A substituent-specific term ( $\sigma$ ) — how strongly it pulls/pushes electrons
- A reaction-specific term ( $\rho$ ) — how sensitive the reaction is to electronic effects
Empirical validation: Hammett found that for meta and para substituted benzoic acids, plotting $\log(K_a/K_a^0)$ against $\sigma$ gives a straight line. This confirms the additive nature of inductive effects.

The Inductive Effect Constant ( $\sigma_I$ ) …

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The key idea is that resonance in chlorobenzene gives the C–Cl bond partial double-bond character, making it stronger and harder to break than the single C–Cl bond in chloroethane. Both the assertion and the reason are correct, and the reason correctly explains the assertion.

The question tests your understanding of how resonance affects bond strength and reactivity. In chlorobenzene, the chlorine atom donates its lone pairs into the benzene ring, creating resonance structures where the C–Cl bond has double-bond character. This makes the bond stronger and shorter, so replacing chlorine with a nucleophile like –OH becomes difficult. In chloroethane, there is no such resonance — the C–Cl bond is a pure single bond, weaker and more easily broken.

Let’s break it down step by step.

Resonance in chlorobenzene

The lone pairs on chlorine can delocalise into the aromatic ring. This gives resonance structures where the carbon attached to chlorine has a partial positive charge and the C–Cl bond is a double bond.
$The resonance hybrid shows a C-Cl bond order between 1 and 2, i.e., partial double-bond character.$
Effect on bond strength

A double bond is stronger and shorter than a single bond. The partial double-bond character in chlorobenzene means the C–Cl bond is harder to break. In chloroethane, the C–Cl bond is a pure single bond — weaker and more reactive toward nucleophilic substitution.
Nucleophilic substitution comparison …

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Here are the common mistakes students make on this exact question, along with how to avoid each.

Mistake 1: Thinking the Reason is Wrong

What students do: They see "partial double bond character" and think, "No, that's not right — it's just inductive effect."
Why it’s wrong: The reason is correct. In chlorobenzene, the lone pairs on chlorine resonate with the benzene ring, giving the C–Cl bond partial double bond character. This is a real, exam-tested fact.
How to avoid: Memorise the resonance structures of chlorobenzene. Draw them: the lone pair from Cl moves into the ring, creating a double bond between C and Cl in one canonical form. That’s the partial double bond character.

Mistake 2: Confusing “Difficult to replace” with “Reaction doesn’t happen”

What students do: They think “Chlorobenzene does undergo nucleophilic substitution under extreme conditions, so the assertion is false.”
Why it’s wrong: The assertion says “difficult to replace” — not impossible. It is indeed much harder than in chloroethane. The assertion is correct.
How to avoid: Read the exact wording. “Difficult” means slower, harsher conditions needed — not “never happens.” Compare rates: chlorobenzene requires high temperature/pressure; chloroethane reacts easily.

Mistake 3: Mixing up Inductive Effect and Resonance

What students do: They think the C–Cl bond in chlorobenzene is stronger only because of the –I effect of chlorine.
Why it’s wrong: The –I effect is present, but the main reason for the bond’s strength and low reactivity is resonance (partial double bond character). The –I effect alone would weaken the bond (by pulling electron density), not strengthen it.
How to avoid: Remember:
- Inductive effect is through sigma bonds — it withdraws electrons, making C slightly positive.
- Resonance involves pi bonds — it donates electrons from Cl into the ring, creating double bond character. …

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Q.Assertion: It is difficult to replace chlorine by –OH in chlorobenzene in comparison to that in chloroethane.
Reason: Chlorine-carbon (C–Cl) bond in chlorobenzene has a partial double bond character due to resonance.

Concept understanding — Inductive Effect on Acidity

Inductive Effect on Acidity – From Intuition to Precision

The Core Intuition

The Precise Statement

Why Distance Matters

Why this formula?

Inductive Effect on Acidity: Why It Works

The Core Idea: Stabilising the Conjugate Base

Why Electron-Withdrawing Groups (EWG) Increase Acidity

The Quantitative Relationship: Hammett Equation

Why This Formula Holds

The Inductive Effect Constant ( $\sigma_I$ ) …

Method: Resonance Effect Analysis for Nucleophilic Substitution Reactivity

Step 1: Identify the structural difference

Step 2: Analyse resonance in chlorobenzene

Step 3: Compare with chloroethane

Step 4: Relate to nucleophilic substitution

Mistake 1: Thinking the Reason is Wrong

Mistake 2: Confusing “Difficult to replace” with “Reaction doesn’t happen”

Mistake 3: Mixing up Inductive Effect and Resonance

Q.Assertion: It is difficult to replace chlorine by –OH in chlorobenzene in comparison to that in chloroethane. Reason: Chlorine-carbon (C–Cl) bond in chlorobenzene has a partial double bond character due to resonance.

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Q.Assertion: It is difficult to replace chlorine by –OH in chlorobenzene in comparison to that in chloroethane.
Reason: Chlorine-carbon (C–Cl) bond in chlorobenzene has a partial double bond character due to resonance.

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