Question
Arrange the following acids in increasing order of acid strength and explain the role of inductive effect:
Also explain why trichloroacetic acid is a much stronger acid than acetic acid, even though both are carboxylic acids.
Solution — Step by Step
An acid is strong when it donates easily — meaning after losing , the resulting anion (conjugate base) must be stable.
For carboxylic acids, the conjugate base is the carboxylate anion . The more we can spread (delocalize) this negative charge, the more stable it is, and the stronger the acid.
Inductive effect is the transmission of electron density through sigma bonds due to electronegativity differences. No pi bonds involved — purely through the chain.
An electron-withdrawing group (−I effect) like Cl, F, NO₂ pulls electron density toward itself through sigma bonds. This reduces electron density on the carboxylate oxygen, helping it bear the negative charge more comfortably.
When Cl is attached to the alpha carbon of , it pulls electron density away from the group.
This disperses the negative charge over a larger region — the carboxylate anion becomes more stable, so the acid releases more readily.
Each additional Cl atom adds more electron-withdrawing pull. Three Cl atoms in create a cumulative −I effect far stronger than a single Cl.
This is why pKa values drop sharply:
| Acid | pKa |
|---|---|
| 4.76 | |
| 2.86 | |
| 1.48 | |
| 0.70 |
Lower pKa = stronger acid. The trend is stark.
Increasing acid strength (weakest → strongest):
Trichloroacetic acid () is roughly 10,000 times stronger than acetic acid ().
Why This Works
The key insight is that acid strength is not about the acid itself — it’s about how stable the conjugate base is. We always ask: after losing , how happy is the anion?
The carboxylate anion already has resonance stabilization (negative charge shared between two oxygens). But when electron-withdrawing groups are nearby, they add an extra layer of stabilization by pulling even more of that negative charge away from the oxygen atoms.
The inductive effect weakens with distance — a Cl on the beta carbon has less effect than one on the alpha carbon. This is why we specify where the substituent is, not just what it is.
Alternative Method — Using pKa and Hammett Equation
For JEE Advanced, you might encounter this framing: substituents are classified by their sigma constants (σ). Electron-withdrawing groups have positive σ values; electron-donating groups have negative σ values.
For our purposes, the ranking logic is the same — just compare the number and type of −I groups directly attached to or near the carboxyl group. More −I substituents, closer to , means stronger acid.
Shortcut for board exams: Whenever you see halogens attached to the carbon next to , the acid is stronger than plain acetic acid. More halogens = even stronger. This single rule handles most NCERT and CBSE questions on this topic.
Common Mistake
Students confuse +I and −I effects on acid strength.
The methyl group () has a +I effect — it donates electrons toward the carboxylate, destabilising the anion. This makes acetic acid () weaker than formic acid ().
Many students assume “more substituents = stronger acid” without checking whether the group is electron-withdrawing or electron-donating. An alkyl group makes the acid weaker; a halogen makes it stronger. Always check the direction of the inductive effect first.