Question: Match each of the following ${\mathbf{pk}}_{\mathbf{a}}$ values (3.2, 4.9, and 0.2) to the appropriate carboxylic acid:

(a) ${\mathbf{CH}}_{\mathbf{3}}{\mathbf{CH}}_{\mathbf{2}}\mathbf{COOH}\mathbf{}\mathbf{}\mathbf{}\mathbf{;}\mathbf{}\left[b\right]\mathbf{}{\mathbf{CF}}_{\mathbf{3}}\mathbf{COOH}\mathbf{}\mathbf{}\mathbf{;}\left[c\right]\mathbf{}\mathbf{}{\mathbf{ICH}}_{\mathbf{2}}\mathbf{COOH}$

A low${\mathbf{pk}}_{\mathbf{a}}$ value indicates that the particular acid can easily contribute its protons to other species. A species comprising a high${\mathbf{pk}}_{\mathbf{a}}$ value cannot easily provide its protons to another species.

You can find Carboxylic acids in several pharmaceutical products and food additives. A particular carboxylic acid, citric acid, is a part of food and other beverages.

The presence of the electron-withdrawing group leads the acid to become more acidic, thereby lowering its value.

a. The methyl group in ${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{COOH}$ results in it having a positive inductive effect. It can donate electrons, thereby making difficult the removal of the ${\mathrm{H}}^{+}$ ion. Hence,${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{COOH}$ is the least acidic and will have a ${\mathrm{pk}}_{\mathrm{a}}$value of 4.9.

b.${\mathrm{CF}}_3\mathrm{COOH}$ has three electron-withdrawing fluorine atoms making the molecule most acidic. Hence, the role="math" localid="1648792265071" ${\mathrm{pk}}_{\mathrm{a}}$value of ${\mathrm{CF}}_3\mathrm{COOH}$is 0.2.

c. ${\mathrm{ICH}}_2\mathrm{COOH}$has one electron-withdrawing group that makes the molecule have intermediate acidity. Hence, ${\mathrm{ICH}}_2\mathrm{COOH}$the ${\mathrm{pk}}_{\mathrm{a}}$value of is 3.2.