The compound shown is a strong base (pKa of its conjugate acid about 11). Which of the structures shown is the weakest base in water?

AStructure A
BStructure B
CStructure C
DStructure D

Answer: A

The reference is piperidine (pKa of conjugate acid about 11). Pyrrole is protonated only at carbon with loss of aromaticity (pKa of conjugate acid about -4), so it is by far the weakest base. Pyridine is about 5, aniline about 4.6 and 4-nitroaniline about 1. Pyrrole is therefore the weakest.

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