Out of o-nitrophenol and o-cresol, which is more acidic?

NCERT Exemplar · Q27

Q. Out of o-nitrophenol and o-cresol, which is more acidic?

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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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The key idea is that electron-withdrawing groups (like –NO₂) increase phenol acidity by stabilising the phenoxide ion, while electron-donating groups (like –CH₃) decrease it. o-Nitrophenol is more acidic than o-cresol.

Understand the core concept: Acidity of phenols

Phenol ( $\ce{C6H5OH}$ ) is weakly acidic because the phenoxide ion ( $\ce{C6H5O-}$ ) is stabilised by resonance delocalisation of the negative charge into the benzene ring. Any substituent that further stabilises the phenoxide ion increases acidity; any substituent that destabilises it decreases acidity.
Identify the substituents
- o-Nitrophenol: has a nitro group ( $-\ce{NO2}$ ) at the ortho position.
- o-Cresol: has a methyl group ( $-\ce{CH3}$ ) at the ortho position.
Effect of the nitro group

The nitro group is a strong electron-withdrawing group (EWG) by both inductive effect (through sigma bonds) and resonance effect (through pi bonds). It pulls electron density away from the oxygen of the phenoxide ion, thereby stabilising the negative charge. This makes o-nitrophenol more acidic than phenol itself.
Effect of the methyl group

The methyl group is an electron-donating group (EDG) via hyperconjugation and the inductive effect. It pushes electron density toward the oxygen, destabilising the phenoxide ion (since the negative charge becomes more concentrated). This makes o-cresol less acidic than phenol.
Compare the two …

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Here are the common mistakes students make when comparing the acidity of o-nitrophenol and o-cresol, along with clear ways to avoid each.

Mistake 1: Forgetting the effect of substituents on the phenoxide ion

What students do wrong:

They only look at the substituent’s effect on the phenol molecule itself, not on the phenoxide ion (the conjugate base). Acidity is about how stable the conjugate base is.

How to avoid:

Always ask: “Which substituent makes the negative charge on the oxygen more stable?”

o-nitrophenol has a strong electron-withdrawing group ( $-NO_2$ ) that stabilizes the phenoxide ion by delocalizing the negative charge.
o-cresol has an electron-donating group ( $-CH_3$ ) that destabilizes the phenoxide ion by pushing more electron density onto the oxygen.

Key rule: Electron-withdrawing groups (EWG) increase acidity; electron-donating groups (EDG) decrease acidity.

Mistake 2: Ignoring the ortho effect or overcomplicating it

What students do wrong:

They think the ortho position always creates steric hindrance or hydrogen bonding that changes the trend. For these two compounds, the ortho effect is not the deciding factor.

How to avoid:

In o-nitrophenol, the $-NO_2$ group can form an intramolecular hydrogen bond with the $-OH$ group. This slightly reduces acidity compared to para-nitrophenol, but o-nitrophenol is still far more acidic than o-cresol.
In o-cresol, the $-CH_3$ group does not form any such bond — it only donates electrons.

Bottom line: Compare the electronic effect first. The ortho effect is a secondary correction, not the main reason.

Mistake 3: Confusing “cresol” with “phenol” or “carboxylic acid”

What students do wrong:

They treat o-cresol as if it were a simple phenol, forgetting that $-CH_3$ is electron-donating. Or they mistakenly think “cresol” is more acidic because it sounds like “carboxylic acid.”

How to avoid:

Cresol = methylphenol. The $-CH_3$ group is an electron-donating group (EDG) via hyperconjugation and inductive effect.
Phenol itself has $pK_a \approx 10$ . o-Cresol has $pK_a \approx 10.3$ (less acidic). o-Nitrophenol has $pK_a \approx 7.2$ (more acidic).

Memory aid: “Methyl makes it mild (less acidic). Nitro makes it nasty (more acidic).”

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Q.Out of o-nitrophenol and o-cresol, which is more acidic?

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)

Method: Inductive & Resonance Effect Analysis

Step 1 – Recall the acidity principle

Step 2 – Identify the substituents

Step 3 – Analyze each substituent’s effect

Mistake 1: Forgetting the effect of substituents on the phenoxide ion

Mistake 2: Ignoring the ortho effect or overcomplicating it

Mistake 3: Confusing “cresol” with “phenol” or “carboxylic acid”

Q.Out of o-nitrophenol and o-cresol, which is more acidic?

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