3 free questions from this chapter (the full set has 30). Marking: Multi-correct: +4 all correct, +1 per correct option if none wrong, −2 if any wrong · Numerical: +4 / 0 · Matching: +3 / −1.
Question 1Level 2 · ExamSingle correct
Consider the acids (I) HCOOH, (II) CH3COOH, (III) C6H5COOH and (IV) (CH3)3CCOOH in water at 25 °C. The correct decreasing order of acid strength is ______. Exactly one option is correct.
AI > II > III > IV
BI > III > II > IV
CIII > I > II > IV
DI > III > IV > II
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Answer: B
Acid strength depends on the stability of the carboxylate ion.
(I) HCOOH: no electron-releasing alkyl group on the carboxyl carbon, so HCOO− is the most stable anion here (pKa ≈ 3.75).
(III) C6H5COOH: the sp2 ring carbon attached to COOH is more electronegative than an sp3 alkyl carbon, so benzoic acid (pKa ≈ 4.20) is stronger than acetic acid, but weaker than formic acid.
(II) CH3COOH: the +I effect of CH3 destabilises the anion (pKa ≈ 4.76).
(IV) (CH3)3CCOOH: three methyl groups give a larger +I effect, so it is the weakest (pKa ≈ 5.03).
Order: I > III > II > IV.
Answer: (B).
Difficulty: HardQuestion 2Level 3 · AdvancedSingle correct
Consider (I) benzoic acid, (II) 2-methylbenzoic acid, (III) 4-methylbenzoic acid and (IV) 4-methoxybenzoic acid in water at 25 °C. The correct decreasing order of acid strength is ______. Exactly one option is correct.
AI > II > III > IV
BI > III > II > IV
CII > I > IV > III
DII > I > III > IV
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Answer: D
(II) 2-Methylbenzoic acid: the ortho CH3 pushes the COOH group out of the plane of the ring. The ring can then no longer donate electron density to the carboxyl group by resonance (this donation stabilises the neutral acid more than the anion). The result is the ortho effect: almost every ortho-substituted benzoic acid is stronger than benzoic acid, irrespective of the electronic nature of the substituent. pKa ≈ 3.91.
(I) Benzoic acid: pKa ≈ 4.20.
(III) 4-Methylbenzoic acid: +I and hyperconjugation of the para CH3 destabilise the carboxylate: pKa ≈ 4.37.
(IV) 4-Methoxybenzoic acid: from the para position OCH3 donates electrons by resonance (+R), which outweighs its –I effect; this destabilises the anion more than para CH3 does: pKa ≈ 4.47.
Order: II > I > III > IV.
Answer: (D).
Difficulty: Very hardQuestion 3Level 3 · AdvancedSingle correct
Consider (I) Cl2CHCOOH, (II) FCH2COOH, (III) CH3CHClCOOH and (IV) ClCH2CH2COOH in water at 25 °C. The correct decreasing order of acid strength is ______. Exactly one option is correct.
AI > II > III > IV
BII > I > III > IV
CI > III > II > IV
DII > III > I > IV
Show answer and solution
Answer: A
Electron-withdrawing (–I) groups stabilise the carboxylate ion; the effect grows with the number and electronegativity of the halogens and falls off quickly with distance.
(I) Cl2CHCOOH: two α-Cl atoms; their combined –I effect is the largest here (pKa ≈ 1.3).
(II) FCH2COOH: one α-F; F is more electronegative than Cl, but one F is weaker than two Cl (pKa ≈ 2.6). This matches the NCERT order Cl2CHCOOH > FCH2COOH > ClCH2COOH.
(III) CH3CHClCOOH: one α-Cl (weaker than α-F), and the extra CH3 (+I) opposes it slightly (pKa ≈ 2.8).
(IV) ClCH2CH2COOH: Cl is at the β-carbon, so its –I effect at the carboxylate is much weaker (pKa ≈ 4.0).
Order: I > II > III > IV.
Answer: (A).
Difficulty: Very hard