Which of the folllowing has the highest `pK_(b)` ? .

Options

A.
B.
C.
D.

Answer

A

Explanation

Since `pK_(b) = log. (1)/(K_(b))`
Highest `pK_(b)` means lowest `K_(b))` the weakest base Nitro `(NO_(2))` group is electron withdrawing by inductive effect (-I effect) from all positions `(orarrmrarrp-)` but mainly by extended `pi` bonding from the ortho and para positions Therefore all the nitroanilines are less basic than aniline m-isomer is more basic thano- and p-isomers due to operation of only -I effect o-Isomer is less basic than p-because its `NO_(2)` is closer and thus exerts a stronger inductive effect Moreover ortho effect tends to causes ortho isomers to be less basic than para isomers The overall order of basicity is
`C_(6)H_(5)NH_(2)gtm-NO_(2)C_(6)H_(4)NH_(2)gtp-NO_(2)C_(6)H_(4)NH_(2)gtO-NO_(2)C_(6)H_(4)NH_(2)` .