Question

Which of the following compound has most stable conjugate acid? 

• A. \(p\)-Methoxyaniline
• B. \(p\)-Nitroaniline
• C. \(p\)-Methylaniline
• D. Aniline

Answer 1

The Correct Option is A

Concept: Stability of the conjugate acid depends on the basic strength of the amine. More basic amines form more stable conjugate acids. Electron donating groups increase electron density on nitrogen and enhance basicity. Electron withdrawing groups decrease electron density and reduce basicity.
Step 1: Formation of conjugate acid Aniline and its derivatives accept a proton: \[ Ar{-}NH_2 + H^+ \rightarrow Ar{-}NH_3^+ \] Thus stability of \(ArNH_3^+\) depends on electron density on nitrogen.
Step 2: Effect of substituents • \(OCH_3\) : strong electron donating group \((+M)\) • \(CH_3\) : weak electron donating group \((+I)\) • \(NO_2\) : strong electron withdrawing group \((-M, -I)\) Stability order of conjugated acid : [width=0.5]17c ans.png
Step 3: Conjugate acid stability Since \(p\)-methoxyaniline is the most basic, its conjugate acid is the most stable. \[ \boxed{\text{Option (1) is correct}} \]