Question

Which of the following groups increases the acidic strength of carboxylic acid?

• A. \(CH_{3}\)
• B. \(-C_{2}H_{5}\)
• C. \(-OCH_{3}\)
• D. \(-NO_{2}\)

Hint:

Remember: \[ \boxed{ \begin{aligned} -I\text{ groups} &\rightarrow \text{Increase acidity}\\ +I\text{ groups} &\rightarrow \text{Decrease acidity} \end{aligned} } \] Examples of strong electron-withdrawing groups are \[ \boxed{-NO_2,\;-CN,\;-Cl,\;-F.} \] Among these, the nitro group produces a marked increase in the acidic strength of carboxylic acids.

Answer 1

The Correct Option is D

Electron-withdrawing groups (EWG) such as $-NO_2$, $-CN$, and halogens increase the acidic strength of carboxylic acids by stabilizing the carboxylate anion ($RCOO^-$) through inductive withdrawal of electron density away from the negatively charged oxygen. Greater electron withdrawal disperses the negative charge more effectively and lowers the $pK_a$.