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

The key idea is that electron-withdrawing groups increase acidity by stabilising the conjugate base, while electron-donating groups decrease acidity by destabilising it.

1. In o-nitrophenol, the nitro group ($-\text{NO}_2$) is strongly electron-withdrawing via both inductive and resonance effects. It pulls electron density away from the O–H bond, making the phenoxide ion more stable. …

1. 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.

2. 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.

3. 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.

4. 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.

5. Compare the two …

Method: Inductive & Resonance Effect Analysis

This method compares acidity by analyzing how substituents stabilize the phenoxide ion (the conjugate base).

Step 1 – Recall the acidity principle

Phenol is acidic because the phenoxide ion is stabilized by resonance (negative charge delocalized into the ring). Any substituent that further stabilizes the phenoxide ion increases acidity; any substituent that destabilizes it decreases acidity.

Step 2 – Identify the substituents

• o-Nitrophenol: –NO₂ group at ortho position
• o-Cresol: –CH₃ group at ortho position

Step 3 – Analyze each substituent’s effect

For o-nitrophenol:

• The –NO₂ group is electron-withdrawing (by both inductive effect and resonance effect).
• It pulls electron density away from the phenoxide ion, stabilizing the negative charge.
• Result: Increases acidity.

For o-cresol:

• The –CH₃ group is electron-donating (by hyperconjugation and inductive effect).
• It pushes electron density toward the phenoxide ion, destabilizing the negative charge. …

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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