Question
Arrange the following in increasing order of acidity and justify: (i) acetic acid, (ii) chloroacetic acid, (iii) trichloroacetic acid, (iv) formic acid, (v) benzoic acid.
Solution — Step by Step
Acidity depends on stability of the carboxylate anion (the conjugate base). Electron-withdrawing groups (EWG) stabilize negative charge, increasing acidity. Electron-donating groups (EDG) destabilize it, decreasing acidity.
(i) CHCOOH: methyl is a mild EDG, slightly destabilizes carboxylate. , .
(ii) ClCHCOOH: one Cl pulls electrons (-I effect), stabilizes carboxylate. .
(iii) ClCCOOH: three Cl’s pull strongly. — very strong acid.
(iv) HCOOH: just H, no group to donate or withdraw. .
(v) CHCOOH: phenyl is mildly EWG via inductive but EDG via resonance — overall slightly EWG. .
Acidity (lowest first): (iii) ClCCOOH (0.65) > (ii) ClCHCOOH (2.86) > (iv) HCOOH (3.75) > (v) CHCOOH (4.20) > (i) CHCOOH (4.76).
(i) < (v) < (iv) < (ii) < (iii).
Final answer: CHCOOH < CHCOOH < HCOOH < ClCHCOOH < ClCCOOH.
Why This Works
The effect of halogens delocalizes negative charge from the carboxylate oxygen to the rest of the molecule, stabilizing the anion. Each additional Cl adds more , so trichloroacetate is most stabilized → trichloroacetic acid is most acidic.
Methyl, in contrast, donates electrons (+I), making CHCOO less stable than HCOO (no +I), so formic acid is more acidic than acetic acid.
Benzoic acid: phenyl’s net effect is slightly electron-withdrawing in the carboxylic-acid context, making it slightly more acidic than acetic acid.
Alternative Method
Compare values directly if memorized. The pattern for the alpha substituent is a standard JEE-NEET ranking.
Common Mistake
Students often place benzoic acid above formic acid in acidity, reasoning that phenyl is “aromatic and electron-withdrawing”. But phenyl’s resonance donation slightly counteracts its inductive withdrawal, leaving it weaker overall than HCOOH. Memorize: .