Question

Arrange the following anilines in decreasing order of basicity

$1.$ $C_{6}H_{5}NH_{2}$

$2.$ $o-CH_{3}C_{6}H_{4}NH_{2}$

$3.$ $m-CH_{3}C_{6}H_{4}NH_{2}$

$4.$ $p-CH_{3}C_{6}H_{4}NH_{2}$

• A. 4 > 1 > 2 > 3
• B. 4 > 3 > 1 > 2
• C. 1 > 2 > 3 > 4
• D. 4 > 3 > 2 > 1

Answer 1

Answer: (B)

$-CH_{3}$ group is inductively electron-donating and base-strengthening from all position. Moreover, if $-CH_{3}$ is attached to benzene ring (or if R has benzylic H), it is electron-donating by hyper-conjugation from the ortho and para positions. Because of ortho effect, o-substituted aniline become less basic than aniline. Thus,

$p-CH_{3}C_{6}H_{4}NH_{2}$ (hyper-conjugation and induction) > $m-CH_{3}C_{6}H_{4}NH_{2}$ (induction) > $C_{6}H_{5}NH_{2}>o-CH_{3}C_{6}H_{4}NH_{2}$ (ortho effect).