Identify the structures: A is aniline, B is anilinium ion, C is cyclohexylamine, and D is p-phenylenediamine.
Understand λmax: λmax refers to the wavelength at which a compound absorbs the most light. It is influenced by conjugation and electron donating/withdrawing groups.
Analyze conjugation: A and D have conjugated systems due to the aromatic ring, while C does not. B has a protonated amine, reducing conjugation.
Consider electron effects: Electron donating groups like NH2 increase λmax by stabilizing excited states. D has two NH2 groups, enhancing this effect.
Rank based on λmax: A has less conjugation than D, B has reduced conjugation due to NH3+, and C lacks an aromatic system. Thus, rank as A < C < D < B.