Problem 191 - 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 ( 6 ext{HCOOH} 6) has the lowest pKa value.

Step 1: Understanding pKa

pKa is a measure of the acidity of a substance. A lower pKa value indicates a stronger acid because it means the acid dissociates more in water, releasing more hydrogen ions.

Step 2: Analyzing Structure and Effects

The acidity of carboxylic acids like \( ext{R-COOH}\) depends on the stability of the carboxylate ion \( ext{R-COO}^-\). Structures with substituents that stabilize this negative charge will generally be more acidic and have lower pKa values. Electron-withdrawing groups or smaller branched structures can stabilize the ion much better.

Step 3: Comparing Molecular Formulas

Examine the structures given: \(a) \) Acetic acid, \(b) \) Formic acid, \(c) \) Isobutyric acid, and \(d) \) Propionic acid (note this seems a type error as structure is incorrect). Smaller acids tend to be stronger, and formic acid is the simplest/parent structure in the group with additional hydrogens compared to others.

Step 4: Conclusion Based on Acidity Trends

Formic acid ( ext{HCOOH}) is known to be the strongest among the simple carboxylic acids listed here because it lacks any additional substituents that can donate electron density, leading to a lower pKa.

Key Concepts

Understanding pKa value

When it comes to understanding how acidic a substance is, we often refer to its pKa value. This value provides insight into the substance's acid strength. A pKa value is derived from the disassociation constant of an acid in water. It essentially tells us how easily the acid can donate its hydrogen ion (H⁺) in a solution.
A crucial point to remember is that a lower pKa value indicates a stronger acid. This is because the acid is more willing to give up its hydrogen ion. Conversely, a higher pKa value suggests a weaker acid, as it holds onto its hydrogen more tightly. In practical terms, when comparing acids, the one with the lowest pKa value is typically the most potent and has the greatest capacity to release its hydrogen ion.

• Lower pKa = Stronger Acid
• Higher pKa = Weaker Acid

Understanding pKa can help predict the behavior of different acids in reactions and how they might interact in various contexts. This makes it an essential concept in chemistry, particularly when discussing carboxylic acids.

Carboxylic Acids Explained

Carboxylic acids are a group of organic compounds that contain a carboxyl group \( \text{-COOH} \). This group is key to their acidic properties. In the world of acids, they are known for being relatively strong due to their ability to release a proton from the carboxyl group.
The structure of carboxylic acids plays a pivotal role in their acidity. Generally, the more stable the resulting carboxylate ion (\( \text{R-COO}^- \)) becomes, the stronger the acid. This stability is often influenced by the surrounding atoms or groups attached to the acid molecule. For instance, in simpler carboxylic acids like formic acid (\( \text{HCOOH} \)), the lack of additional groups makes them more acidic compared to those with extra substituents.

• Carboxyl group: \( \text{-COOH} \)
• Releases a proton (H⁺)
• Stability of the carboxylate ion dictates acidity

Knowing about these aspects of carboxylic acids can aid in understanding how they behave in solutions and reactions, making them a fundamental topic in organic chemistry.

Role of Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) are incredibly important in determining the acidity of acids, including carboxylic acids. They increase the acidity by stabilizing the charge on the carboxylate ion once the acid has donated a proton.
These groups function by pulling electron density away from the acidic portion of the molecule. This withdrawal of electron density helps to stabilize the negative charge on the deprotonated acid ion (\( \text{R-COO}^- \)). Consequently, an acid with electron-withdrawing groups is often a stronger acid.
In our original problem, the formic acid does not have substituents that donate back electron density, hence allowing the acid to be stronger due to the lack of interference and the greater stability of its conjugate base.

• EWGs pull electron density from the molecule
• Helps stabilize the resulting carboxylate ion
• Increases overall acidity of the compound

Understanding the influence of electron-withdrawing groups can make predicting the properties of complex molecules much easier and clarify why certain acids are stronger than others.