Discussion
Rank the following molecules in decreasing order of basicity :
Answer: A
The order is I \(\gt\) II \(\gt\) III \(\gt\) IV: piperidine \(\gt\) thiomorpholine \(\gt\) morpholine \(\gt\) N-methylmorpholine.
Measured pKa values of the conjugate acids are about 11.1 for piperidine, 9.0 for thiomorpholine, 8.4 for morpholine and 7.4 for N-methylmorpholine.
Piperidine (I) has nothing withdrawing electrons from nitrogen. In II and III a ring heteroatom two carbons away pulls electron density from N by induction; oxygen is more electronegative than sulfur, so morpholine (III) is weaker than thiomorpholine (II). N-methylmorpholine (IV) is a tertiary amine; in water, its protonated form is less well solvated by hydrogen bonding than that of the secondary amine, so it is the weakest base here.
The orders that put III or I last reverse these effects.
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