Assertion (A): Compounds containing the -CHO group are easily oxidised to the corresponding carboxylic acids. Reason (R): Carboxylic acids can be reduced to alcohols by treatment with $LiAlH_4$. (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): Compounds containing the -CHO group are easily oxidised to the corresponding carboxylic acids.

NCERT Exemplar · Q47

LiAlH_4

(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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Aldehydes (–CHO) are readily oxidised to carboxylic acids (–COOH) — the Assertion is correct. The Reason, that $LiAlH_4$ reduces carboxylic acids to alcohols, is a true fact on its own but describes an unrelated, reverse reaction on a different functional group — it does not explain why aldehydes oxidise easily, so as a reason it is wrong. The correct option is (iii).

Evaluating the Assertion

Aldehydes are easily oxidised because the carbonyl carbon in –CHO still carries a hydrogen atom, making the C–H bond available for oxidation to C–OH (i.e. –COOH). This is why mild oxidising agents such as Tollens' reagent ( $[Ag(NH_3)_2]^+$ ) and Fehling's solution ( $Cu^{2+}$ complex), which cannot touch a ketone, readily oxidise an aldehyde. So the Assertion is correct.

Evaluating the Reason …

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

Mistake 1: Confusing the Reason with the Explanation of the Assertion

What students do wrong: They see that both statements are true (Assertion: Aldehydes oxidise to acids. Reason: $LiAlH_4$ reduces acids to alcohols). Because both are factually correct, they jump to option (A) — assuming the Reason explains the Assertion.
Why this is wrong: The Reason describes a reduction (carboxylic acid → alcohol). The Assertion describes an oxidation (aldehyde → carboxylic acid). These are opposite chemical processes. The Reason does not explain why aldehydes are easily oxidised.
How to avoid: Always ask: “Does the Reason directly cause or explain the Assertion?” Here, the Reason talks about reducing acids, not oxidising aldehydes. The correct answer is (B) — both are true, but the Reason is not the correct explanation.

Mistake 2: Thinking “Both are correct” automatically means “Reason explains Assertion”

What students do wrong: They memorise that $LiAlH_4$ reduces carboxylic acids to alcohols (true) and that aldehydes oxidise to acids (true). They tick (A) without checking the logical link.
Why this is wrong: In Assertion-Reason questions, the relationship matters more than individual truth. The two statements are independent facts — one does not follow from the other.
How to avoid: Treat the Reason as a cause and the Assertion as an effect. If the Reason is about reduction and the Assertion is about oxidation, they cannot be cause-effect. Mark (B).

Mistake 3: Misreading the Assertion as “wrong” because of the -CHO group

What students do wrong: They think “Compounds containing -CHO” includes all aldehydes, but then recall that some aldehydes (like formaldehyde) are oxidised differently, or they confuse -CHO with -COOH. They incorrectly mark Assertion as wrong.
Why this is wrong: The Assertion is correct — all aldehydes (R-CHO) are easily oxidised to carboxylic acids (R-COOH) by mild oxidising agents like Tollens’ reagent, Fehling’s solution, or even atmospheric oxygen. The -CHO group is inherently prone to oxidation.
How to avoid: Remember the key property: Aldehydes are easily oxidised because the carbonyl carbon has a hydrogen atom that can be removed. The Assertion is factually correct.

Mistake 4: Thinking the Reason is wrong because $LiAlH_4$ also reduces other groups

What students do wrong: They know $LiAlH_4$ reduces esters, ketones, etc., and think “It doesn’t only reduce carboxylic acids to alcohols, so the Reason is wrong.”
Why this is wrong: The Reason says “Carboxylic acids can be reduced to alcohols by $LiAlH_4$ .” That is true — it is a standard reduction. The fact that $LiAlH_4$ reduces other groups does not make this statement false. …

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Q.Assertion (A): Compounds containing the -CHO group are easily oxidised to the corresponding carboxylic acids.
Reason (R): Carboxylic acids can be reduced to alcohols by treatment with $LiAlH_4$ .

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 ( $\sigma_I$ ) …

Evaluating the Assertion

Evaluating the Reason …

Method: Statement-by-Statement Verification

Step 1: Verify Assertion (A)

Step 2: Verify Reason (R)

Step 3: Check if Reason explains Assertion

Mistake 1: Confusing the Reason with the Explanation of the Assertion

Mistake 2: Thinking “Both are correct” automatically means “Reason explains Assertion”

Mistake 3: Misreading the Assertion as “wrong” because of the -CHO group

Mistake 4: Thinking the Reason is wrong because $LiAlH_4$ also reduces other groups

Q.Assertion (A): Compounds containing the -CHO group are easily oxidised to the corresponding carboxylic acids. Reason (R): Carboxylic acids can be reduced to alcohols by treatment with LiAlH4LiAlH_4LiAlH4​.

Evaluating the Assertion

Evaluating the Reason …

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Q.Assertion (A): Compounds containing the -CHO group are easily oxidised to the corresponding carboxylic acids.
Reason (R): Carboxylic acids can be reduced to alcohols by treatment with $LiAlH_4$ .

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