Quick Summary
Concept Tested: Acid Strength & Inductive Effect
Chapter: Aldehydes Ketones and Carboxylic Acids
Difficulty: ⭐⭐
Time Required: 1 min
Key Formula: $$pK_a = -\log_{10} K_a$$
Answer: (C)
Reasoning: Formic acid (HCOOH) lacks electron-donating alkyl groups, making its conjugate base (HCOO⁻) the most stable, resulting in the lowest pKₐ value.
The Question
Which of the following has the lowest pKₐ value?
(A) CH3CH2COOH
(B) (CH3)2CH-COOH
(C) HCOOH
(D) CH3COOH
Quick Answer
The correct answer is (C) HCOOH (Formic Acid).
Formic acid is the strongest acid in this list because its conjugate base is not destabilized by any electron-donating alkyl groups. In contrast, acetic, propanoic, and isobutanoic acids have alkyl groups that donate electron density to the carboxylate ion, destabilizing it and increasing the pKₐ value.
Why Other Options Are Incorrect
(A) CH3CH2COOH (Propanoic Acid): The ethyl group (CH3CH2) is electron-donating via the inductive effect. This increases electron density on the conjugate base (CH3CH2COO⁻), destabilizing it. Consequently, this acid has a higher pKₐ than formic acid.
(B) (CH3)2CH-COOH (2-Methylpropanoic Acid): The isopropyl group is a branched alkyl group and a strong electron-donor via the inductive effect, destabilizing the conjugate base and giving this acid a pKₐ (≈4.86) close to propanoic acid’s — both noticeably higher than formic acid’s.
(D) CH3COOH (Acetic Acid): The methyl group (CH3) is electron-donating, which destabilizes the conjugate base (CH3COO⁻). However, the inductive effect of a methyl group is weaker than that of ethyl or isopropyl groups. Thus, it has a higher pKₐ than formic acid but a lower pKₐ than propanoic acid.
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Understanding the Concept
To determine acidity, we look at the stability of the conjugate base. The stronger the acid, the more stable its conjugate base. For carboxylic acids, the presence of alkyl groups attached to the carboxyl carbon influences this stability.
The Inductive Effect: Alkyl groups are electron-releasing (electron-donating). When attached to the carboxyl group, they push electron density towards the carboxylate ion (the conjugate base). This destabilizes the negative charge, making the acid weaker (higher pKₐ).
Conversely, an electron-withdrawing group (like a halogen) would stabilize the conjugate base and increase acidity. Since formic acid has no alkyl group, it lacks this destabilizing effect.
Detailed Step-by-Step Solution
Step 1: Define the Relationship between pKₐ and Acid Strength
The acidity of an acid is quantified by the acid dissociation constant, Ka. The pKa is defined as:
$$pK_a = -\log_{10} K_a$$
From this relationship, we can deduce that a lower pKₐ corresponds to a higher Ka and thus a stronger acid. Our goal is to find the acid with the lowest pKₐ value.
Step 2: Analyze the Structure of the Conjugate Bases
Let’s examine the conjugate base for each option to assess its stability:
- (A) CH3CH2COO⁻: The negative charge is on the oxygen, but there is an electron-donating ethyl group attached to the carbon.
- (B) (CH3)2CHCOO⁻: The negative charge is on the oxygen, and it is attached to a carbon bearing two electron-donating methyl groups.
- (C) HCOO⁻: The negative charge is on the oxygen, but the carbon is attached only to a hydrogen atom. There are no electron-donating alkyl groups to destabilize the charge.
- (D) CH3COO⁻: The negative charge is on the oxygen, and it is attached to a carbon bearing one electron-donating methyl group.
Step 3: Compare Stability and Calculate pKₐ Values
Since alkyl groups are electron-releasing, they destabilize the carboxylate ion by pushing electron density towards the site of the negative charge.
Comparing the destabilizing effects:
1. HCOO⁻ (Formate): No destabilization. Most stable conjugate base.
2. CH3COO⁻ (Acetate): Destabilized by one methyl group.
3. CH3CH2COO⁻ (Propionate): Destabilized by one ethyl group (more electron-donating than methyl).
4. (CH3)2CHCOO⁻ (Isobutyrate): Destabilized by two methyl groups (branched, strong donation).
Therefore, the stability order of conjugate bases is: HCOO⁻ > CH3COO⁻ > CH3CH2COO⁻ > (CH3)2CHCOO⁻.
Correspondingly, the pKₐ values (from experimental data) are:
- HCOOH: 3.75
- CH3COOH: 4.76
- CH3CH2COOH: 4.87
- (CH3)2CHCOOH: 4.86
The lowest value is 3.75, belonging to Formic Acid.
Final Answer
(C) HCOOH (Formic Acid) is the correct answer.
Essential Formulas for This Topic
1. pKₐ Definition:
$$pK_a = -\log_{10} K_a$$
2. Relationship between pKₐ and Ka:
$$K_a = 10^{-pK_a}$$
3. Acid Strength Trend:
$$\text{Lower pK}_a \rightarrow \text{Higher K}_a \rightarrow \text{Stronger Acid}$$
Common Mistakes to Avoid
Mistake 1: Confusing pKₐ and Ka
Wrong Thinking: Thinking that a lower pKₐ means a weaker acid because the number is smaller.
Correct Approach: Always remember the negative sign in the definition: lower pKₐ = higher Ka = stronger acid.
Mistake 2: Assuming Size of Alkyl Group Determines Acidity
Wrong Thinking: Thinking that larger alkyl groups (like tert-butyl) are more acidic than smaller ones (like methyl) just because they are bigger.
Correct Approach: Focus on the inductive effect (electron donation). Larger, branched alkyl groups are actually better electron donors, which decreases acidity.
Mistake 3: Ignoring the Role of the Conjugate Base
Wrong Thinking: Focusing only on the acid itself without looking at what happens after it loses a proton.
Correct Approach: Acid strength is determined by how easily the acid donates a proton to form its conjugate base. The stability of the conjugate base is the key factor.
Key Concept Summary
- $$pK_a = -\log_{10} K_a$$ defines the acidity scale.
- Alkyl groups are electron-donating via the inductive effect.
- Electron-donating groups destabilize the carboxylate ion, making the acid weaker.
- The absence of alkyl groups (as in HCOOH) results in the strongest acidity.
Golden Rule: Acid strength increases as the electron-withdrawing nature of the substituents attached to the carboxyl group increases.
Frequently Asked Questions
Q: Why is Formic acid stronger than Acetic acid?
A: Formic acid has a hydrogen atom directly attached to the carboxyl group. Hydrogen is the zero-reference point for the inductive effect (I = 0) — it neither donates nor withdraws electron density. A methyl group, by contrast, is electron-donating (+I), which destabilizes the conjugate base. Lacking that destabilizing +I effect is what makes formic acid’s conjugate base more stable and the acid stronger.
Q: Does resonance play a role in the acidity of these simple carboxylic acids?
A: No. In simple aliphatic carboxylic acids like these, resonance stabilization of the conjugate base is constant (the negative charge is delocalized over two oxygens). The differences in acidity arise solely from inductive effects, not resonance.
Q: What is the order of acidity for the given options?
A: HCOOH > CH3COOH > CH3CH2COOH > (CH3)2CHCOOH.
Q: How does branching affect the pKₐ value?
A: Branching (like in 2-methylpropanoic acid) increases the electron-donating capacity of the alkyl group, which destabilizes the carboxylate ion, resulting in a slightly higher pKₐ compared to its straight-chain counterpart.
Prerequisites to Solve This Question
- Understanding of the definition of pKa.
- Knowledge of the Inductive Effect (electron donation/withdrawal).
- Understanding of Acid-Base conjugate pairs.
After Solving This, You Can:
- ✔ Analyze structural features to predict acidity.
- ✔ Compare pKₐ values of different carboxylic acids.
- ✔ Identify the strongest acid in a given set.
Study Tips for This Topic
For JEE Main, focus on memorizing the approximate pKₐ values of common acids (Formic, Acetic, Benzoic, etc.). Often, questions will ask for the order of acidity. Remember that electron-withdrawing groups (EWG) increase acidity, while electron-donating groups (EDG) decrease it.
Difficulty Rating & Exam Frequency
Difficulty: ⭐⭐ (Easy-Medium)
JEE Main Frequency: High
JEE Advanced Frequency: Medium
Importance: High. This concept is foundational for organic chemistry and appears frequently in physical organic chemistry sections.
Related Questions from Aldehydes Ketones And Carboxylic Acids
Written by Nishant Kumar Gupta
Founder of Padho Likho JEE & Senior Chemistry Educator — 12+ Years Experience, Ex-Faculty Allen/Aakash/Narayana.
Last Updated: July 2026
Question Source: JEE Main 2005 PYQ
Topic: Aldehydes Ketones And Carboxylic Acids