Q.26 The following carboxylic acids have a general formula, R-COOH.
(i) HCOOH
(ii) CH3 – COOH
(iii) ClCH2 – COOH
(iv) CF3 – COOH
Which one of the following represents the decreasing order of their respective 𝑝𝐾𝑎 values?
(A) i > ii > iii > iv (B) ii > i > iii > iv (C) iv > iii > i > ii (D) iv > iii > ii > i
The decreasing order of pKa values for these carboxylic acids is (iv) > (iii) > (ii) > (i), corresponding to option (D).
Lower pKa indicates stronger acidity due to better stabilization of the conjugate base by electron-withdrawing groups.
pKa Values
Actual pKa values confirm the order:
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(iv) CF₃COOH: ~0.5
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(iii) ClCH₂COOH: ~2.86
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(ii) CH₃COOH: ~4.76
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(i) HCOOH: ~3.75
Thus: CF₃COOH (0.5) < ClCH₂COOH (2.86) < CH₃COOH (4.76) < HCOOH (3.75), so decreasing pKa: iv > iii > ii > i.
Acid Strength Factors
Acidity depends on conjugate base stability. Electron-withdrawing groups (-I effect) delocalize the carboxylate anion’s negative charge, lowering pKa.
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CF₃- exerts strongest -I effect (three F atoms), making (iv) strongest acid.
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ClCH₂- has moderate -I effect from one Cl.
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CH₃- is +I (electron-donating), weakening (ii).
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H in (i) lacks +I donation, so HCOOH > CH₃COOH.
Option Analysis
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(A) i > ii > iii > iv: Wrong; ignores strong -I effects of Cl and CF₃.
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(B) ii > i > iii > iv: Wrong; places CH₃COOH > HCOOH despite +I effect.
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(C) iv > iii > i > ii: Wrong; HCOOH pKa (3.75) < CH₃COOH (4.76).
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(D) iv > iii > ii > i: Correct, matches experimental pKa trend.
The decreasing order of pKa values for carboxylic acids like HCOOH, CH3COOH, ClCH2COOH, and CF3COOH reveals key principles of acidity influenced by inductive effects, essential for CSIR NET Life Sciences and organic chemistry exams.
Inductive Effects on pKa
Electron-withdrawing groups (EWGs) stabilize the conjugate base (RCOO⁻), reducing pKa. CF₃COOH has the lowest pKa (~0.5) due to three fluorine atoms’ strong -I effect. ClCH₂COOH follows (~2.86) with chlorine’s -I pull. CH₃COOH (~4.76) is weakened by methyl’s +I donation, while HCOOH (~3.75) lacks this.
Exam-Relevant Comparison
| Acid | Formula | pKa | Key Factor |
|---|---|---|---|
| Trifluoroacetic | CF₃COOH (iv) | 0.5 | Strongest -I (3F) |
| Chloroacetic | ClCH₂COOH (iii) | 2.86 | Moderate -I (Cl) |
| Acetic | CH₃COOH (ii) | 4.76 | +I (CH₃) |
| Formic | HCOOH (i) | 3.75 | No R-group donation |
This decreasing pKa order (iv > iii > ii > i) matches option (D). For CSIR NET aspirants, master -I vs +I effects for similar questions on acidity trends.
