Problem 221 - A Complete Resource Book in Chemistry for JEE Main

Question

Among the following acids which has the lowest pKa value? (a) \(\mathrm{CH}_{3} \mathrm{COOH}\) (b) \(\mathrm{HCOOH}\) (c) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CH}-\mathrm{COOH}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COH}\)

Step-by-Step Solution

Answer

Formic acid (HCOOH) has the lowest pKa value.

Step 1: Understand pKa Values

The pKa value of an acid indicates its strength; the lower the pKa value, the stronger the acid. A small pKa value means the acid dissociates easily in solution, releasing protons more readily.

Step 2: Examine Acid Structures

Look at the structural formulas of the given acids: (a) Acetic acid ((CH3)COOH), (b) Formic acid (HCOOH), (c) Isobutyric acid (((CH3)2)CH-COOH), and (d) Propionaldehyde (CH3CH2COH). Recognize that propionaldehyde is not a carboxylic acid, which means it won't have an acidic pKa value like the others.

Step 3: Compare Acidic Strength of Carboxylic Acids

The acid strength is influenced by the rest of the molecule connected to the carboxyl group. Formic acid (HCOOH) is the simplest carboxylic acid and often has a lower pKa due to its lack of additional electron-donating groups compared to acetic acid and isobutyric acid.

Step 4: Identify the Acid with Lowest pKa

Given that formic acid (HCOOH) has no alkyl groups to donate electrons to the carboxylic group, which would decrease its acidity, it is usually stronger than acetic acid and isobutyric acid. Therefore, formic acid typically has the lowest pKa value among carboxylic acids.

Key Concepts

pKa Value

In the realm of chemistry, understanding the strength of an acid is crucial, and that's where the concept of pKa value shines. A pKa value is derived from the acid dissociation constant, providing insight into how readily an acid will donate its proton in a solution. The most important takeaway here is: the lower the pKa value, the stronger the acid.

Let's break it down further:

• Acids with lower pKa values are stronger because they dissociate more completely in solution.
• A smaller pKa indicates that the acid releases protons more readily, making it more reactive.
• The value is logarithmic, meaning each unit change represents a tenfold difference in acidity.

In summary, knowing an acid's pKa value can help you deduce its strength, making it an indispensable tool for chemists.

Carboxylic Acids

Carboxylic acids are a functional group characterized by the presence of a carboxyl group (-COOH). These compounds are widespread in chemistry due to their important roles and inherent properties that lend themselves to many reactions.

Key features include:

• The carboxyl group is made up of a carbon atom double-bonded to an oxygen and also bonded to a hydroxyl group.
• The presence of the carboxyl group makes these compounds acidic, as they can donate a proton to form a carboxylate ion.

Formic acid (HCOOH), acetic acid (CH₃COOH), and isobutyric acid ((CH₃)₂CH-COOH) are examples of carboxylic acids mentioned in the exercise. Among these, formic acid is the simplest, having the least steric hindrance and electronic influence, thus often presenting a lower pKa.

Chemical Structure Influence

The structure of an acid plays a pivotal role in determining its strength, especially in the context of pKa values. This is due to the electron-withdrawing and donating nature of various substituents attached to the carboxyl group.

Here are some insights:

• Electron-withdrawing groups stabilize the negative charge on the conjugate base, increasing the acidity and resulting in a lower pKa.
• Conversely, electron-donating groups decrease acidity by destabilizing the conjugate base, leading to a higher pKa.
• In the given acids, formic acid lacks additional alkyl groups, making it stronger due to minimal electron donation to the carboxyl group.

This simple carboxylic structure of formic acid leads to its typically lower pKa value when compared to more substituted carboxylic acids like acetic acid or isobutyric acid.