Identify Compound Giving Strongest Conjugate Acid Among Substituted Anilines

To determine which compound forms the strongest conjugate acid, we use the fundamental concept of acid-base conjugate pairs:

$\text{Base} + \text{H}^+ \rightleftharpoons \text{Conjugate Acid}$

The strength of a conjugate acid is inversely related to the strength of its corresponding base. That is: $\text{p}K_a (\text{Conjugate Acid}) + \text{p}K_b (\text{Base}) = 14 \quad (\text{at } 25^\circ\text{C})$

Thus, the weakest base will yield the strongest conjugate acid.

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Step-by-Step Analysis of the Given Compounds

1. Option A: Aniline ($\text{C}_6\text{H}_5\text{NH}_2$)

   • Standard reference compound without any additional substituents on the benzene ring.

2. Option B: 4-Methoxyaniline ($p$-anisidine)

   • The methoxy group ($-\text{OCH}_3$) exerts a strong electron-donating mesomeric effect ($+\text{M}$) at the $p$-position, increasing the electron density on the nitrogen atom. This makes it a stronger base than aniline.

3. Option C: 4-Nitroaniline ($p$-nitroaniline)

   • The nitro group ($-\text{NO}_2$) exerts very strong electron-withdrawing mesomeric ($-\text{M}$) and inductive ($-\text{I}$) effects at the $p$-position. This drastically decreases the electron density on the nitrogen atom, stabilizing the free base form less and severely reducing its tendency to accept a proton. Thus, 4-nitroaniline is the weakest base among all choices.

4. Option D: 4-Methylaniline ($p$-toluidine)

   • The methyl group ($-\text{CH}_3$) exerts electron-donating hyperconjugative and inductive ($+\text{I}$) effects, increasing the basicity relative to aniline.

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Order of Basicity:

$\text{4-methoxyaniline (Option B)} > \text{4-methylaniline (Option D)} > \text{aniline (Option A)} > \text{4-nitroaniline (Option C)}$

Order of Strength of Conjugate Acids:

$\text{Conjugate acid of Option C} > \text{Conjugate acid of Option A} > \text{Conjugate acid of Option D} > \text{Conjugate acid of Option B}$

Since 4-nitroaniline is the weakest base, its protonated form ($p$-nitroanilinium ion, $\text{O}_2\text{N}-\text{C}_6\text{H}_4-\text{NH}_3^+$) is the strongest conjugate acid.

Correct Option: C