3

In my worksheet there is a question that asks for the comparison of basicities of the three anisidine isomers and aniline:

enter image description here

After my analysis using:

$\text{Basicity} \propto \text{+M effect (except for meta annisidine)} \propto \dfrac{1}{\text{-I effect}}$,

I could make the following conclusions:

  1. m-anisidine should be the least basic
  2. p-anisidine should be more basic than aniline.

But the main problem is comparing the basicity of aniline and o-anisidine and comparing them with p-anisidine. I doubt the presence of ortho effect in ortho isomer after seeing the ball and stick model which makes the interaction of lone pair of nitrogen with hydrogen atoms of $\ce{-OCH3}$ improbable.

The answer given is:

p-anisidine> aniline > o-anisidine > m-anisidine (I have verified it with $pKa$ values from PubChem)

Gaurang Tandon
  • 9,788
  • 11
  • 65
  • 118
Archer
  • 5,453
  • 9
  • 42
  • 89

1 Answers1

2

Why you considering the methyl group on $\ce{-OCH3}$ to be "locked" in position, facing away from the $\ce{-NH2}$ group? It is free to rotate in all directions, and it will come at a position next to the $\ce{-NH2}$ group, where the ortho effect will apply.

For a visualization, here is a similar image:

(taken from Martin's excellent post)

PS: I believe that Steric Inhibition of Resonance should not be invoked here, because both sides of the $\ce{-NH2}$ group are not blocked by the $\ce{-OCH3}$ group, and $\ce{-OCH3}$ is certainly not as bulky a group as t-butyl.

Gaurang Tandon
  • 9,788
  • 11
  • 65
  • 118