Phenol is less acidic than ______. (i) ethanol (ii) o-nitrophenol (iii) o-methylphenol (iv) o-methoxyphenol

NCERT Exemplar · Q12

Q. Phenol is less acidic than ____________.

(i) ethanol

(ii) o-nitrophenol

(iii) o-methylphenol

(iv) o-methoxyphenol

Uttar Pradesh UpmspMCQ· 1mImportance★★★★★

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Concept understanding — Acidity Of Phenol

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?

Acidity of Phenol: Why It's More Acidic Than Alcohols

Let's build this from first principles — understanding why phenol is acidic is the key to mastering organic chemistry.

1. The Core Observation

Phenol ( $\ce{C6H5OH}$ ) has a pKa ≈ 10, while ethanol ( $\ce{CH3CH2OH}$ ) has a pKa ≈ 16.

This means phenol is about 1 million times more acidic than a typical alcohol.

The question: Why does the O–H bond in phenol break so much more easily?

2. The Key: Stability of the Conjugate Base

Acidity is determined by the stability of the conjugate base after losing $\ce{H+}$ .

Alcohol conjugate base: $\ce{CH3CH2O-}$ (alkoxide ion) — negative charge is localized on oxygen.
Phenol conjugate base: $\ce{C6H5O-}$ (phenoxide ion) — negative charge is delocalized into the benzene ring.

The Resonance Explanation

The phenoxide ion has multiple resonance structures:

\ce{C6H5O-} \leftrightarrow \text{(several resonance forms where negative charge moves to ortho/para carbons)}

Draw the structures mentally:

One structure has the negative charge on oxygen.
Other structures show the negative charge on carbon atoms at the ortho and para positions of the ring.

This delocalization spreads the negative charge over more atoms, making the ion more stable.

Key principle: The more stable the conjugate base, the stronger the acid.

3. Why Alcohols Can't Do This

In an alkoxide ion ( $\ce{RO-}$ ), the negative charge is stuck on oxygen.

There are no empty p-orbitals or conjugated $\pi$ systems nearby to accept the charge.

Result: The alkoxide is less stable, so the alcohol is less acidic.

4. The Inductive Effect Also Helps (But Resonance Dominates)

The benzene ring is slightly electron-withdrawing (due to its $sp^2$ carbons being more electronegative than $sp^3$ ).

This inductive effect pulls electron density away from the O–H bond, making the proton slightly more positive and easier to remove.

However, resonance stabilization of the conjugate base is the dominant factor — inductive effects alone cannot explain the million-fold difference.

5. The Quantitative Picture (pKa Values)

Compound	pKa	Conjugate base stability
Ethanol	~16	Localized charge on O
Phenol	~10	Delocalized charge via resonance
Acetic acid	~4.76	Even more resonance (two O atoms)

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Phenol is less acidic than o-nitrophenol because the nitro group is strongly electron-withdrawing, stabilising the phenoxide ion via resonance and induction and lowering its pKa. The correct option is (ii).

Concept

Acidity depends on the stability of the conjugate base (the phenoxide ion). Electron-withdrawing groups (EWGs) stabilise the phenoxide and raise acidity; electron-donating groups (EDGs) destabilise it and lower acidity.

Checking each option

Option (i), ethanol: the ethoxide ion has no resonance stabilisation, so ethanol (pKa about 16) is far less acidic than phenol (pKa about 10) - phenol is not less acidic than ethanol, so this is wrong. …

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Here are the common mistakes students make on this question and how to avoid each.

Mistake 1: Forgetting the effect of substituents on acidity

Many students pick (A) ethanol because they remember that phenol is more acidic than alcohols. However, the question asks for a compound that is more acidic than phenol (i.e., phenol is less acidic than it).

Why it’s wrong: Ethanol is an alcohol. Phenol is more acidic than ethanol because the phenoxide ion is stabilized by resonance with the benzene ring. So phenol is more acidic than ethanol, not less.
How to avoid: Always compare the stability of the conjugate base. For phenol vs. alcohol, the phenoxide ion has resonance stabilization; the alkoxide ion does not. Therefore, phenol is the stronger acid.

Mistake 2: Ignoring the direction of the comparison

Students sometimes misread the phrase “Phenol is less acidic than” and think it means “Phenol is more acidic than.”

Why it’s wrong: The sentence structure is: Phenol < X in acidity. So X must be a stronger acid than phenol.
How to avoid: Rewrite the statement in your mind: “Which of these is a stronger acid than phenol?” Then check each option.

Mistake 3: Not knowing the effect of electron-donating groups (EDG)

Options (C) o-methylphenol and (D) o-methoxyphenol contain electron-donating groups (methyl and methoxy). These groups decrease acidity by destabilizing the negative charge on the conjugate base.

Why they are wrong:
- Methyl ( $-CH_3$ ) is an EDG via hyperconjugation and inductive effect.
- Methoxy ( $-OCH_3$ ) is a strong EDG via resonance (though it also has a weak -I effect, the resonance donation dominates). Both make the phenoxide ion less stable, so these compounds are weaker acids than phenol.
How to avoid: Memorize the common EDGs: alkyl groups ( $-R$ ), $-OH$ , $-OR$ , $-NH_2$ . Any substituent that donates electrons will reduce acidity.

Mistake 4: Not recognizing the effect of electron-withdrawing groups (EWG)

Option (B) o-nitrophenol has a nitro group ( $-NO_2$ ), which is a strong electron-withdrawing group (EWG) via both inductive and resonance effects.

Why it is correct: The nitro group stabilizes the phenoxide ion by delocalizing the negative charge, making o-nitrophenol a stronger acid than phenol. …

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Q.Phenol is less acidic than ____________.

Concept understanding — Acidity Of Phenol

Inductive Effect on Acidity – From Intuition to Precision

The Core Intuition

The Precise Statement

Why Distance Matters

Why this formula?

Acidity of Phenol: Why It's More Acidic Than Alcohols

1. The Core Observation

2. The Key: Stability of the Conjugate Base

The Resonance Explanation

3. Why Alcohols Can't Do This

4. The Inductive Effect Also Helps (But Resonance Dominates)

5. The Quantitative Picture (pKa Values)

Concept

Checking each option

Method: Substituent Effect on Acidity of Phenols

Step-by-step reasoning

Mistake 1: Forgetting the effect of substituents on acidity

Mistake 2: Ignoring the direction of the comparison

Mistake 3: Not knowing the effect of electron-donating groups (EDG)

Mistake 4: Not recognizing the effect of electron-withdrawing groups (EWG)

Q.Phenol is less acidic than ____________.

Concept

Checking each option

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