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

Concept: The acidic strength of carboxylic acids depends upon the stability of the conjugate base (carboxylate ion).

• Electron-withdrawing groups (\(-I\) effect) increase acidity by stabilising the carboxylate ion.

• Electron-donating groups (\(+I\) effect) decrease acidity by destabilising the carboxylate ion.
The stronger the electron-withdrawing effect, the greater is the acidic strength.

Step 1: Identify the nature of each substituent.
\[ CH_3 \] and \[ C_2H_5 \] are electron-donating alkyl groups. Hence, they decrease acidity. The methoxy group, \[ -OCH_3, \] shows a weak electron-withdrawing inductive effect but also exhibits resonance donation. Its overall effect is much weaker than that of the nitro group. The nitro group, \[ -NO_2, \] has a very strong electron-withdrawing (\(-I\)) effect. It stabilises the carboxylate ion effectively.

Step 2: Determine the acidic strength.
Since the conjugate base becomes more stable in the presence of the nitro group, \[ \boxed{\text{Acidity increases significantly}.} \] Thus, \[ -NO_2 \] produces the strongest carboxylic acid among the given options.

Step 3: Choose the correct option.
Therefore, \[ \boxed{-NO_2} \] is the substituent that increases acidic strength. Hence, \[ \boxed{\textbf{Option (D)}} \] is the correct answer.