The compound shown is substituted in the para position with different groups. Which structure is the weakest base in water?

AStructure A
BStructure B
CStructure C
DStructure D

Answer: B

The parent shown is aniline. Para-nitro withdraws electrons by both -I and -R, strongly decreasing the availability of the lone pair; 4-nitroaniline (conjugate acid pKa about 1) is the weakest. Methoxy and methyl donate electrons (more basic than aniline) and chloro withdraws only inductively (pKa about 4).

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