Assertion: Acetanilide is less basic than aniline. Reason: Acetylation of aniline results in decrease of elect

NCERT Exemplar · Q77

Q. Assertion: Acetanilide is less basic than aniline. Reason: Acetylation of aniline results in decrease of electron density on nitrogen.

(i) Both assertion and reason are wrong.

(ii) Both assertion and reason are correct statements but reason is not correct explanation of assertion.

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

(iv) Both assertion and reason are correct statements and reason is 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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Acetylation converts the lone pair on aniline’s nitrogen into an amide, where resonance with the carbonyl group delocalises the electrons. This drastically reduces electron density on nitrogen, making acetanilide far less basic than aniline. The reason correctly explains the assertion.

The question tests your understanding of how resonance (not just inductive effect) governs basicity in nitrogen compounds. Basicity depends on how readily the nitrogen atom can donate its lone pair to a proton. Anything that delocalises or withdraws that lone pair makes the compound less basic.

Aniline’s nitrogen lone pair is already partially delocalised into the benzene ring — that’s why aniline is a weaker base than aliphatic amines like methylamine. But acetylation takes this delocalisation much further.

What acetylation does structurally

Acetylation of aniline replaces one hydrogen on the –NH₂ group with a –COCH₃ group, forming acetanilide (C₆H₅–NH–COCH₃). The nitrogen is now part of an amide functional group.
The key resonance effect

In an amide, the nitrogen lone pair is in conjugation with the carbonyl (C=O) group. The major resonance contributor places a positive charge on nitrogen and a negative charge on oxygen:

\text{C}_6\text{H}_5\text{–NH–}\overset{\text{O}}{\overset{||}{\text{C}}}\text{–CH}_3 \longleftrightarrow \text{C}_6\text{H}_5\text{–}\overset{+}{\text{N}}\text{H}=\overset{-}{\text{C}}\text{(O}^-\text{)–CH}_3

This resonance is so strong that the C–N bond in amides has partial double-bond character. The lone pair is no longer available for protonation — it’s tied up in the π-system.

Comparing electron density on nitrogen

In aniline, the lone pair is only partially delocalised into the benzene ring (the ring is less electron-withdrawing than a carbonyl). In acetanilide, the carbonyl group is a much stronger electron-withdrawing group via resonance, so the electron density on nitrogen is drastically reduced.
$Basicity order: aniline (pKₐ of conjugate acid \approx 4.6) >> acetanilide (pKₐ \approx -0.5) The difference is about 5 orders of magnitude — acetanilide is essentially non-basic in water.$
Why the reason is correct and is the explanation …

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Here’s a breakdown of the common mistakes students make on this exact question, along with how to avoid each.

🧠 The Correct Answer First

Option (D) is correct.

Assertion: Acetanilide is less basic than aniline. ✓
Reason: Acetylation of aniline results in decrease of electron density on nitrogen. ✓
Reason is the correct explanation of the assertion.

✗ Mistake 1: Confusing “Basic” with “Acidic”

What students do wrong:

They see “inductive effect” and immediately think about acidity trends (e.g., electron-withdrawing groups increase acidity). Here, the question is about basicity — the ability of nitrogen to donate its lone pair.

How to avoid:

Basicity depends on electron density on nitrogen. More electron density → more basic.
Acidity depends on stability of conjugate base.
Always check: Is the question about donating or accepting a proton?

✗ Mistake 2: Forgetting the Role of Resonance in Acetanilide

What students do wrong:

They only consider the inductive effect of the acetyl group ( $-COCH_3$ ) and think it’s a weak electron-withdrawing group, so the effect on basicity is small.

The truth:

The acetyl group causes resonance withdrawal — the lone pair on nitrogen is delocalized into the carbonyl group:

\ce{Ph-NH-COCH3 <-> Ph-NH+=C(O-)CH3}

This dramatically reduces electron density on nitrogen, making acetanilide much less basic than aniline.

How to avoid:

Always check if resonance (mesomeric effect) is possible.
For amides (like acetanilide), resonance dominates over inductive effects.

✗ Mistake 3: Misreading the Reason Statement

What students do wrong:

They think the reason says “acetylation increases electron density” or they misinterpret “decrease in electron density” as a wrong statement.

How to avoid:

Read the reason carefully: “Acetylation of aniline results in decrease of electron density on nitrogen.”
This is factually correct — the acetyl group pulls electron density away via resonance and induction.
Don’t overthink: if both assertion and reason are true and linked, pick (D).

✗ Mistake 4: Thinking the Reason is Not the Correct Explanation

What students do wrong: …

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Q.Assertion: Acetanilide is less basic than aniline.
Reason: Acetylation of aniline results in decrease of electron density on nitrogen.

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: Inductive Effect & Resonance Analysis for Basicity

🧠 The Correct Answer First

✗ Mistake 1: Confusing “Basic” with “Acidic”

✗ Mistake 2: Forgetting the Role of Resonance in Acetanilide

✗ Mistake 3: Misreading the Reason Statement

✗ Mistake 4: Thinking the Reason is Not the Correct Explanation

Q.Assertion: Acetanilide is less basic than aniline. Reason: Acetylation of aniline results in decrease of electron density on nitrogen.

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Q.Assertion: Acetanilide is less basic than aniline.
Reason: Acetylation of aniline results in decrease of electron density on nitrogen.

Unlock everything free for 14 days