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

AStructure A
BStructure B
CStructure C
DStructure D

Answer: C

Basicity falls as the s character of the nitrogen lone-pair orbital rises: sp3 amine (pKaH about 10.7) > sp2 imine (about 7, with CH3CH=NH) and pyridine (5.2) > sp nitrile (pKaH about -10). Ethylamine is therefore strongest.

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