Organic Chemistry

Electron-withdrawing groups ($-I$ effect) disperse negative charge on the carboxylate ion and enhance acidity. The strength of $-I$ effect follows the sequence: $-\text{NO}_2 \gt -\text{F} \gt -\text{Cl} \gt -\text{H}$.
Corresponding $\text{p}K_a$ values:

• $\text{NO}_2\text{CH}_2\text{COOH}: \text{p}K_a = 1.68$ (strongest acid)
• $\text{FCH}_2\text{COOH}: \text{p}K_a = 2.66$
• $\text{ClCH}_2\text{COOH}: \text{p}K_a = 2.86$
• $\text{HCOOH}: \text{p}K_a = 3.75$ (weakest acid in this set)

Increasing order of acidic strength: $\text{HCOOH} \lt \text{ClCH}_2\text{COOH} \lt \text{FCH}_2\text{COOH} \lt \text{NO}_2\text{CH}_2\text{COOH}$.

Acidic strength is inversely related to $\text{p}K_a$ (lower $\text{p}K_a$ means stronger acid):

• $\text{FCH}_2\text{COOH}$: $\text{p}K_a = 2.66$ (strongest acid due to highest electronegativity of F)
• $\text{ClCH}_2\text{COOH}$: $\text{p}K_a = 2.86$
• $\text{BrCH}_2\text{COOH}$: $\text{p}K_a = 2.90$
• $\text{HCOOH}$: $\text{p}K_a = 3.75$ (weakest acid among these)

Increasing order of acidity: $\text{HCOOH (D)} \lt \text{BrCH}_2\text{COOH (A)} \lt \text{ClCH}_2\text{COOH (B)} \lt \text{FCH}_2\text{COOH (C)}$.

Hydrolysis of esters with a base (such as aqueous $\text{NaOH}$) is known as saponification. The base hydrolyzes the ester to yield a carboxylate salt (carboxylate ion) and an alcohol:

Because the medium is alkaline, free carboxylic acids are not formed unless the reaction mixture is subsequently acidified.