Question
Arrange the following benzoic acids in increasing order of acidity and explain your reasoning:
- Benzoic acid (C₆H₅COOH)
- 4-Nitrobenzoic acid (p-NO₂C₆H₄COOH)
- 4-Methoxybenzoic acid (p-CH₃OC₆H₄COOH)
- 4-Methylbenzoic acid (p-CH₃C₆H₄COOH)
Solution — Step by Step
Acidity of a substituted benzoic acid depends on how stable the carboxylate anion (–COO⁻) is after losing H⁺. More stable the anion, more acidic the compound. Any substituent that withdraws electron density from the ring stabilises the anion → increases acidity.
- –NO₂ at para: Strong electron withdrawing group (EWG). Both –I effect and –M effect operate in the same direction. Pulls electron density away from the ring and the –COO⁻.
- –OCH₃ at para: This one trips up most students. It has –I effect (withdrawing) but a dominant +M effect (donating via lone pair into ring). Net effect: electron donating group (EDG).
- –CH₃ at para: Pure +I effect (hyperconjugation too). Always electron donating. Destabilises the anion.
- –H: Benzoic acid is our reference.
The carboxylate carbon is directly attached to the ring, so anything that drains the ring’s electron density also drains the –COO⁻, stabilising it.
Flip the arrow for EDG.
EDGs destabilise anion → weakest acids. EWGs stabilise anion → strongest acid.
| Compound | Net substituent effect | Relative acidity |
|---|---|---|
| 4-Methoxybenzoic acid | EDG (strong +M wins) | Weakest |
| 4-Methylbenzoic acid | EDG (weak +I) | 2nd weakest |
| Benzoic acid | No substituent | Reference |
| 4-Nitrobenzoic acid | EWG (–I + –M) | Strongest |
Increasing order of acidity:
The actual pKₐ values confirm this beautifully:
- p-Methoxybenzoic acid: ~4.47
- p-Methylbenzoic acid: ~4.37
- Benzoic acid: ~4.20
- p-Nitrobenzoic acid: ~3.44
Lower pKₐ = stronger acid. The –NO₂ group drops pKₐ by almost 0.8 units — that’s a massive effect.
Why This Works
The –COOH group conjugates with the benzene ring. This means electron density changes anywhere on the ring do transmit to the carboxylate. This is why substituent effects on benzoic acid are so significant and why this system is the textbook example for teaching electronic effects.
The –OCH₃ group is the conceptually tricky one. Students see “oxygen” and think “electronegative = electron withdrawing = stronger acid.” That’s only half the story. Oxygen has lone pairs that donate into the ring by resonance — and at the para position, that +M effect wins decisively over the –I effect. This is why para-methoxybenzoic acid is actually weaker than benzoic acid.
The –NO₂ group is the gold standard EWG because both its inductive and resonance effects pull in the same direction. At para, it directly stabilises the negative charge on –COO⁻ through the resonance chain.
Alternative Method — Using Resonance Structures
Draw the resonance structures of the carboxylate anion for each compound.
For p-nitrobenzoic acid, you can draw a structure where negative charge moves all the way to the –NO₂ group’s oxygen atoms. This extra delocalisation is possible only because –NO₂ is a resonance acceptor. More resonance structures = more stabilisation = more acidic.
For p-methoxybenzoic acid, the –OCH₃ pushes electron density into the ring, which increases electron density at the carboxylate. The anion is destabilised by this electron buildup.
Quick rule for JEE: At para position, the +M effect of –OCH₃ always beats its –I effect. So para-methoxy always makes benzoic acid weaker. This comes up directly in JEE Main — if you see –OH or –OCH₃ at para or ortho, treat it as net electron donating.
Common Mistake
Calling –OCH₃ an electron withdrawing group because oxygen is electronegative. This is a classic inorganic-thinking error applied to organic chemistry. In resonance analysis, the lone pairs on oxygen override the electronegativity argument. –OCH₃ is electron donating by resonance (+M), and at para position this effect dominates. As a result, p-methoxybenzoic acid ends up less acidic than benzoic acid — the opposite of what inductive-only thinking would predict. This exact reasoning was tested in JEE Main 2023 as a conceptual MCQ.