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Which of the following orderings regarding basic strength of amines is INCORRECT?

1. Ph-NH-Ph < Ph-NH2 < cyclohexylamine
2. pyrrole < pyridine < piperidine
3. pyrrole < pyrrolidine < aniline
4. p-nitroaniline < aniline < p-toluidine

Correct answer: pyrrole < pyrrolidine < aniline

Solution

Pyrrolidine is a saturated secondary aliphatic amine (pKa of conjugate acid ~11) and is far more basic than aniline (pKa ~4.6), so the ordering pyrrolidine < aniline is incorrect; the correct order is aniline < pyrrole (approximately) but pyrrolidine >> aniline.

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