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

AStructure A
BStructure B
CStructure C
DStructure D

Answer: C

Guanidine (pKaH about 13.6) is protonated to the guanidinium ion in which the charge is delocalised equally over three nitrogens. Acetamidine (12.4) is next, triethylamine (10.7) and pyridine (5.2) are weaker.

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