92. Which one of the following represents the decreasing order of their respective pKa values?
(i) HCOOH
(ii) CH₃COOH
(iii) ClCH₂COOH
(iv) CF₃COOH
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
Decreasing Order of pKa Values of Carboxylic Acids – Complete Explanation Based on the Inductive Effect
In this question, four carboxylic acids are given, each containing a different substituent attached to the carboxyl group. The objective is to arrange them in the decreasing order of their pKa values. Since pKa is inversely related to acid strength, this problem can be solved by first comparing the relative acidities of the compounds.
Understanding pKa and Acid Strength
The pKa of an acid is a measure of its tendency to donate a proton. A lower pKa value indicates a stronger acid because the acid ionizes more readily in solution. Conversely, a higher pKa value corresponds to a weaker acid.
Therefore, while solving this question, it is important to remember the following relationship:
Higher pKa → Weaker Acid
Lower pKa → Stronger Acid
Thus, determining the decreasing order of pKa is equivalent to determining the increasing order of acid strength and then reversing the sequence.
Role of the Inductive Effect
The acidity of carboxylic acids depends primarily on the stability of the conjugate base formed after the loss of a proton.
If a substituent attached to the carbon chain withdraws electron density through the negative inductive effect (−I effect), it stabilizes the negatively charged carboxylate ion. A more stable conjugate base makes proton loss easier, increasing the acidity and lowering the pKa.
On the other hand, electron-donating groups exhibit a positive inductive effect (+I effect). These groups push electron density toward the carboxylate ion, destabilizing the negative charge. As a result, the acid becomes weaker and its pKa increases.
Analysis of Each Compound
(i) Formic Acid (HCOOH)
Formic acid contains only a hydrogen atom attached to the carbonyl carbon. Hydrogen neither donates nor withdraws electrons significantly through the inductive effect.
Therefore, formic acid serves as a useful reference compound and is stronger than acetic acid because it lacks the electron-donating methyl group.
Approximate pKa ≈ 3.75
(ii) Acetic Acid (CH3COOH)
The methyl group exhibits a +I (electron-donating) effect. It pushes electron density toward the carboxyl group, destabilizing the acetate ion formed after deprotonation.
Since the conjugate base becomes less stable, acetic acid is weaker than formic acid.
Approximate pKa ≈ 4.76
This is the highest pKa among the given compounds.
(iii) Chloroacetic Acid (ClCH2COOH)
Chlorine is highly electronegative and exerts a strong −I effect. It withdraws electron density from the carboxylate ion, stabilizing the negative charge.
Consequently, chloroacetic acid is much stronger than both formic acid and acetic acid.
Approximate pKa ≈ 2.86
(iv) Trifluoroacetic Acid (CF3COOH)
The CF3 group contains three highly electronegative fluorine atoms, each exerting an exceptionally powerful −I effect.
Together, these fluorine atoms withdraw a large amount of electron density from the carboxylate ion, providing extraordinary stabilization to the conjugate base.
As a result, trifluoroacetic acid is one of the strongest simple carboxylic acids.
Approximate pKa ≈ 0.23
This is the lowest pKa among the given compounds.
Comparison of pKa Values
| Compound | Approximate pKa | Relative Acidity |
|---|---|---|
| CH3COOH | 4.76 | Weakest |
| HCOOH | 3.75 | Stronger than Acetic Acid |
| ClCH2COOH | 2.86 | Strong |
| CF3COOH | 0.23 | Strongest |
Therefore, the decreasing order of pKa values is:
CH3COOH > HCOOH > ClCH2COOH > CF3COOH
or
ii > i > iii > iv
Explanation of Every Option
Option (A): i > ii > iii > iv
This option is incorrect because it places formic acid above acetic acid. In reality, acetic acid has a higher pKa due to the electron-donating methyl group, making it the weaker acid.
Option (B): ii > i > iii > iv
This is the correct answer. Acetic acid has the highest pKa because of the +I effect of the methyl group. Formic acid follows, while chloroacetic acid has a much lower pKa due to the −I effect of chlorine. Trifluoroacetic acid possesses the lowest pKa because three fluorine atoms strongly stabilize the conjugate base.
Option (C): iv > iii > i > ii
This option is incorrect because it completely reverses the acidity trend. Trifluoroacetic acid is actually the strongest acid and therefore has the smallest pKa, not the largest.
Option (D): iv > iii > ii > i
This option is also incorrect because it places trifluoroacetic acid at the highest pKa value. The strong −I effect of the CF3 group makes trifluoroacetic acid the strongest acid among the given compounds.
Why Electron-Withdrawing Groups Increase Acidity
Whenever an acid loses a proton, a negatively charged conjugate base is formed. If electron-withdrawing substituents are present, they help disperse and stabilize this negative charge through the inductive effect. A more stable conjugate base is less likely to regain the proton, making the acid stronger.
Conversely, electron-donating groups increase electron density on the conjugate base, making it less stable. As a result, proton loss becomes less favorable, reducing acidity and increasing the pKa value.
Final Answer
Correct Option: (B)
Correct decreasing order of pKa values:
CH3COOH > HCOOH > ClCH2COOH > CF3COOH
or
ii > i > iii > iv
