Three amines are dissolved separately in water at 25 °C: aniline (C₆H₅NH₂), cyclohexylamine (C₆H₁₁NH₂), and p-anisidine (4-methoxyaniline, CH₃O-C₆H₄NH₂). Their conjugate‐acid pKₐ values are as follows:
• aniline: 4.6
• cyclohexylamine: 10.6
• p-anisidinium ion: 5.3
Assume that pKₐ + pK_b = 14.00.
(a) Rank the three amines from weakest to strongest base in aqueous solution. Write the corresponding conjugate‐acid formulas below each amine name in your ranking.
(b) Draw one resonance structure (in addition to the canonical Lewis structure) for the conjugate acid of aniline that helps account for its observed pKₐ value. Clearly indicate all formal charges.
(c) Explain why cyclohexylamine is a much stronger base than aniline even though both contain sp³-hybridized nitrogen atoms.
(d) The methoxy substituent in p-anisidine is electron-donating through resonance. Use resonance arguments to justify why p-anisidine is a stronger base than aniline but still markedly weaker than cyclohexylamine.