Four heterocycles are shown beside the options (the drawn compound is thiazole, for reference). Which structure is the strongest base?

AStructure A
BStructure B
CStructure C
DStructure D

Answer: A

In N-methylimidazole the protonated form is an amidinium-type cation in which the two nitrogens share the charge, giving pKaH about 7.1. Pyridine is 5.2, pyrimidine 1.3 (the second ring N withdraws), pyrrole about -3.8.

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