Problem 31 - Chemistry and Chemical Reactivity

Question

Which is the stronger of the following two acids? (a) benzoic acid, \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CO}_{2} \mathrm{H}, \mathrm{p} K_{\mathrm{a}}=4.20\) (b) 2 -chlorobenzoic acid, \(\mathrm{ClC}_{6} \mathrm{H}_{4} \mathrm{CO}_{2} \mathrm{H}, \mathrm{p} K_{3}=2.90\)

Step-by-Step Solution

Answer

2-chlorobenzoic acid is the stronger acid with a pKa of 2.90.

Step 1: Identify the pKa Values

The question provides the pKa values for two acids. Benzoic acid has a pKa of 4.20, while 2-chlorobenzoic acid has a pKa of 2.90.

Step 2: Understand the Relationship Between pKa and Acidity

The acidity of an acid is inversely proportional to its pKa value. This means the lower the pKa, the stronger the acid.

Step 3: Compare the pKa Values

Since 2-chlorobenzoic acid has a pKa of 2.90, which is lower than the pKa of 4.20 for benzoic acid, it is the stronger acid.

Key Concepts

Understanding pKa Values

The term "pKa" stands for the negative base-10 logarithm of the acid dissociation constant (Ka) of a solution. It's a parameter that helps us understand how strong or weak an acid is.
- A low pKa value indicates a strong acid, implying it donates protons ( H^+ ) easily to the surrounding environment. - Conversely, a high pKa value suggests a weak acid, which means it holds onto its protons more tightly.
To determine acidity strength in practical applications, we use pKa values as a comparison tool. The smaller the pKa value, the more capable the acid is of releasing hydrogen ions into a solution, which increases the solution's acidity.

Properties of Benzoic Acid

Benzoic acid is a simple aromatic carboxylic acid with the chemical formula C_6H_5CO_2H . It serves as a fundamental material in organic chemistry, often used in food preservation and as an intermediate in various chemical syntheses.
- It has a pKa value of 4.20, indicating it is comparatively weaker when compared to other chlorinated derivatives like 2-chlorobenzoic acid. - In benzoic acid, the aromatic benzene ring ensures a stable resonance, making it less eager to dissociate hydrogen ions.
The structure and resonance stability in benzoic acid make it a moderate acid. It’s widely present in nature, particularly in plants, serving valuable roles both industrially and in biological settings.

Characteristics of 2-Chlorobenzoic Acid

2-Chlorobenzoic acid is derived from benzoic acid, but with a chlorine atom substitution at the second position of the aromatic ring. Its chemical formula is ClC_6H_4CO_2H . This subtle structural change imparts significant effects on its chemical properties.
- Its pKa value is 2.90, making it a much stronger acid compared to benzoic acid. - The presence of the electron-withdrawing chloride group enhances the acidity by increasing the acid's ability to release protons.
This withdrawing effect from the chlorine destabilizes the protonative form while stabilizing the deprotonated form. As a result, 2-chlorobenzoic acid can release hydrogen ions more readily, making it a notable example of how small modifications in chemical structure can noticeably influence acidity.