Question

The following carboxylic acids have a general formula, R-COOH.
(i) HCOOH
(ii) CH\(_3\) - COOH
(iii) ClCH\(_2\) - COOH
(iv) CF\(_3\) - COOH
Which one of the following represents the decreasing order of their respective pK\(_a\) values?

• A. i > ii > iii > iv
• B. ii > i > iv > iii
• C. iv > iii > i > ii
• D. iv > iii > ii > i

Hint:

Electronegative groups, especially halogens, lower the pK\(_a\) of carboxylic acids by making the hydrogen easier to lose, thus increasing acidity.

Answer 1

The Correct Option is B

To arrange the given carboxylic acids in decreasing order of their pK_a values, we need to understand the relationship between the structure of carboxylic acids and their acidity.

The pK_a value is a measure of the strength of an acid. The lower the pK_a, the stronger the acid. This is because a strong acid dissociates more completely in solution, releasing more protons (H⁺ ions).

In carboxylic acids, the presence of electron-withdrawing groups increases acidity (thus lowering the pK_a) by stabilizing the negative charge on the conjugate base (carboxylate ion) through resonance or induction. Conversely, electron-donating groups decrease acidity (thus increasing the pK_a).

1. HCOOH (Formic acid): This acid has no additional groups attached besides the carboxyl group, so it exhibits the baseline acidity of carboxylic acids.
2. CH₃COOH (Acetic acid): The CH₃ group is slightly electron-donating, which makes acetic acid less acidic than formic acid. Hence, it has a higher pK_a than formic acid.
3. ClCH₂COOH (Chloroacetic acid): The Cl group is highly electronegative and acts as an electron-withdrawing group through induction, thus making chloroacetic acid more acidic than both formic and acetic acids.
4. CF₃COOH (Trifluoroacetic acid): The CF₃ group is even more electron-withdrawing than the Cl group because F is more electronegative and there are three F atoms, making trifluoroacetic acid the most acidic among these compounds. Therefore, it has the lowest pK_a value.

Thus, the order of decreasing pK_a values (i.e., increasing acidity) is as follows:

• CH₃COOH (ii) > HCOOH (i) > CF₃COOH (iv) > ClCH₂COOH (iii)

Hence, the correct order is indeed ii > i > iv > iii.