Assertion (A): The $\alpha$-hydrogen atom in carbonyl compounds is less acidic. Reason (R): The anion formed after the loss of the $\alpha$-hydrogen atom in carbonyl compounds is resonance stabilised. (i) Assertion and Reason both are correct and Reason is the correct explanation of Assertion. (ii) Assertion and Reason both are wrong. (iii) Assertion is correct but Reason is wrong. (iv) Assertion is wrong but Reason is correct.

Assertion (A): The α-hydrogen atom in carbonyl compounds is less acidic. Reason (R): The anion formed after th

NCERT Exemplar · Q48

\alpha

(i) Assertion and Reason both are correct and Reason is the correct explanation of Assertion.

(ii) Assertion and Reason both are wrong.

(iii) Assertion is correct but Reason is wrong.

(iv) Assertion is wrong but Reason is correct.

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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 assertion is wrong because alpha-hydrogens in carbonyl compounds are actually more acidic (not less), and the reason is correct because the enolate anion is resonance-stabilised. The correct option is (iv).

The carbonyl group is strongly electron-withdrawing. When an alpha-hydrogen is removed, the negative charge left on the alpha-carbon is delocalised onto the oxygen via resonance (R-C(=O)-CH2- <-> R-C(O-)=CH2). This stabilises the enolate anion enormously, making the alpha-hydrogen MORE acidic than a typical alkane C-H (pKa ~20 for acetone vs …

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Here’s a breakdown of the common mistakes students make on this specific Assertion-Reason question about the Inductive Effect on Acidity in carbonyl compounds, and how to avoid each.

Common Mistake #1: Misreading the Assertion

The mistake:

Students think the Assertion says “α-hydrogen is more acidic” (which is actually true) and therefore mark the Assertion as correct.

Why it happens:

They confuse the actual fact (α-hydrogens are acidic due to enolate stabilisation) with the wording of the statement.

How to avoid:

Read the Assertion literally — it says “less acidic”. That is wrong.

Key fact: α-hydrogens in carbonyl compounds are more acidic than typical alkanes because the conjugate base (enolate) is resonance-stabilised.

Common Mistake #2: Assuming the Reason is Wrong

The mistake:

Students think the Reason is false because they associate “resonance stabilisation” only with aromatic systems or carboxylate ions.

Why it happens:

They forget that the enolate ion formed after losing α-hydrogen is also resonance-stabilised.

How to avoid:

Remember the enolate resonance:

\ce{R-C(=O)-CH2- -> R-C(O-)=CH2}

The negative charge is delocalised between carbon and oxygen. This stabilises the anion, making the α-hydrogen more acidic.

Common Mistake #3: Confusing “Inductive Effect” with “Resonance Effect”

The mistake:

Students try to explain the acidity using only inductive effects (e.g., electronegativity of oxygen) and ignore resonance.

Why it happens:

The question mentions “Inductive Effect on Acidity” in the title, so students focus only on induction.

How to avoid:

Understand that resonance stabilisation is the dominant factor here. The inductive effect of the carbonyl group does pull electron density, but the real reason for enhanced acidity is the resonance in the enolate.

Common Mistake #4: Choosing Option (A) — Both correct, Reason explains Assertion

The mistake:

Students know the Reason is correct and assume the Assertion must also be correct. …

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Q.Assertion (A): The $\alpha$ -hydrogen atom in carbonyl compounds is less acidic.
Reason (R): The anion formed after the loss of the $\alpha$ -hydrogen atom in carbonyl compounds is resonance stabilised.

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 Stabilisation Analysis of Conjugate Base

Steps

Common Mistake #1: Misreading the Assertion

Common Mistake #2: Assuming the Reason is Wrong

Common Mistake #3: Confusing “Inductive Effect” with “Resonance Effect”

Common Mistake #4: Choosing Option (A) — Both correct, Reason explains Assertion

Q.Assertion (A): The α\alphaα-hydrogen atom in carbonyl compounds is less acidic. Reason (R): The anion formed after the loss of the α\alphaα-hydrogen atom in carbonyl compounds is resonance stabilised.

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Q.Assertion (A): The $\alpha$ -hydrogen atom in carbonyl compounds is less acidic.
Reason (R): The anion formed after the loss of the $\alpha$ -hydrogen atom in carbonyl compounds is resonance stabilised.

Unlock everything free for 14 days