Question

The strongest conjugate acid will result from:

• A.

  

• B.

  

• C.

  

• D.

  

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
According to Brønsted-Lowry acid-base theory, the strength of a conjugate acid is inversely proportional to the strength of its corresponding base. Therefore, the "strongest conjugate acid" will be formed by protonating the "weakest base" among the options.
Step 2: Key Formula or Approach:
Strongest Conjugate Acid $\iff$ Weakest Base.
Basicity of anilines depends on the availability of the lone pair on the nitrogen atom. Electron-withdrawing groups (EWG) pull electron density away from the ring via -M and -I effects, decreasing lone pair availability and reducing basicity.
Step 3: Detailed Explanation:
Let's analyze the given options (aniline derivatives):
- Image a: Aniline. The baseline aromatic amine.
- Imageb: p-Anisidine ($OCH_3$ group at para position). The $-OCH_3$ group is a strongly electron-donating group (+M effect), which increases electron density on the nitrogen, making it the strongest base.
- Image c: p-Nitroaniline ($NO_2$ group at para position). The $-NO_2$ group is a powerful electron-withdrawing group (-M and -I effects). It heavily delocalizes the nitrogen lone pair into the aromatic ring and towards the oxygen atoms, making the lone pair highly unavailable for protonation. This makes it the weakest base.
- Image c: p-Toluidine ($CH_3$ group at para position). The $-CH_3$ group is weakly electron-donating (+I and hyperconjugation), making it a slightly stronger base than aniline.
Order of basicity: p-Anisidine>p-Toluidine>Aniline>p-Nitroaniline.
Since p-Nitroaniline (Option C) is the weakest base, its protonated form (the conjugate acid) will be the strongest acid.
Step 4: Final Answer:
Option (C) represents the weakest base, thus yielding the strongest conjugate acid.