Problem 27 - 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, \(\mathrm{ClC}_{6} \mathrm{H}_{4} \mathrm{CO}_{2} \mathrm{H}, \mathrm{p} K_{\mathrm{a}}=2.90\)

Step-by-Step Solution

Answer

2-chlorobenzoic acid is the stronger acid because it has a lower pKa value of 2.90.

Step 1: Understanding pKa Values

The strength of an acid is determined by its ability to donate a proton, which is represented by its pKa value. The smaller the pKa value, the stronger the acid, because it dissociates more completely in water.

Step 2: Compare the pKa Values

We have been given two acids: benzoic acid with a pKa of 4.20 and 2-chlorobenzoic acid with a pKa of 2.90. Comparing the two, 2-chlorobenzoic acid has a smaller pKa value than benzoic acid.

Step 3: Determine the Stronger Acid

Since 2-chlorobenzoic acid has a lower pKa value than benzoic acid, it is the stronger acid. A lower pKa means it donates its proton more readily, making it more acidic.

Key Concepts

pKa value

The concept of pKa is pivotal in understanding acid strength. Chemically, pKa is the negative logarithm of an acid's dissociation constant ( K_a ). It provides a numerical value that indicates how easily an acid can lose a proton. The key takeaway is that a smaller pKa value implies a stronger acid. This happens because a lower pKa signifies a greater degree of dissociation in water, meaning the acid more readily gives up protons.

To quickly determine acid strength:

• Observe the pKa value: lower values mean stronger acids.
• Acids dissociate more fully with a smaller pKa.

This straightforward relationship makes pKa an invaluable tool for chemists when comparing different acids. Understanding pKa helps in selecting the right acid for a specific reaction or application.

Benzoic acid

Benzoic acid is a simple aromatic carboxylic acid, with the chemical formula C_6H_5CO_2H . It's commonly found in nature and used industrially in various applications, such as preserving food. Understanding the structure helps clarify its properties.

Key characteristics:

• Contains a benzene ring connected to a carboxyl group (-COOH) .
• In water, it can partially dissociate to form its conjugate base and a proton.
• Its pKa value is 4.20, suggesting moderate acidity in comparison with stronger acids.

Benzoic acid's structure accounts for its properties, where the electron-withdrawing nature of the carboxyl group affects its acidity. It demonstrates moderate acidic behavior, meaning it does not dissociate as fully as stronger acids. This factor makes benzoic acid less effective at donating protons than acids with lower pKa values.

Chlorobenzoic acid

Chlorobenzoic acid, specifically 2-chlorobenzoic acid, is an intriguing modification of benzoic acid. By adding a chlorine atom to the benzene ring, we alter the acid's properties significantly. This subtle change increases the acidity, as evidenced by its lower pKa value of 2.90.

Impact of chlorine:

• Chlorine, an electronegative atom, enhances the acidity by drawing electron density away from the benzene ring.
• This shift allows the carboxyl group to release protons more easily.
• Collates to a stronger acid as compared to benzoic acid.

This is an excellent illustration of how substituents on an aromatic ring can affect acid strength. Chlorine increases chlorobenzoic acid's ability to donate protons, showcasing the importance of electronegative components in enhancing acid dissociation.