Four cyclic secondary amines are shown beside the options (the drawn compound is piperazine, for reference). Taking pKaH in water, which structure is the SECOND weakest base?

AStructure A
BStructure B
CStructure C
DStructure D

Answer: A

Order of pKaH: morpholine about 8.4 < thiomorpholine about 9.0 < piperidine 11.1 < pyrrolidine 11.3. The oxygen in morpholine withdraws electrons more strongly (-I) than sulfur, so morpholine is weakest and thiomorpholine second weakest.

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