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I have a question where I am required to compare the basicity of Aniline, o-Anisidine, and p-Anisidine. enter image description here

Now here's what I do know:-

In ortho substituted aniline, an ortho effect acts which reduces the basicity of the ortho-substituted aniline. But, my text book (Solomons and Fryhle) states that this effect doesn't act if the ortho substituent is $\ce{-OH}$ or $\ce{-OCH3}$. So I should be able to do the comparison here without taking into account this ortho effect.

Now here's my try:

In Aniline, clearly the lone pair is quite a bit delocalised in the benzene ring which reduces its availability. Now, $\ce{-OCH3}$ is a group that has both $\ce{-I}$ and $\ce{+M}$.

Taking a look at the para variant, clearly there is $\ce{+M}$ acting which reduces the extent of delocalization of the Nitrogen lone pair, thus making it more available. Also, the $\ce{-I}$ effect in the para is lesser than that in the ortho variant, so in the para variant, much more of the electron density of the Nitrogen lone pair is available which helps in easier donation, and thus I find the para variant to be the most basic which is all well and good.

The problem arises in the next part of the comparison, between Aniline, and o-Anisidine.

If the ortho effect isn't acting, due to the $\ce{+M}$ effect of the $\ce{-OCH3}$ group, the lone pair of the Nitrogen should be more available for donation than that of Aniline, (but less than the availability in the para case, due to stronger $\ce{-I}$ in the ortho case), and thus the Ortho variant should be more basic.

But my book states that the basicity order is : p-Anisidine>Aniline>o-Anisidine

I cannot really understand why the basicity order of the last two (Aniline>o-Anisidine) is the way that it is, if no ortho effect is acting and $\ce{-OCH3}$ has a $\ce{+M}$ effect.

PS: Just took a look at Peter Sykes and the order given is same as that in my book. This this due to the fact that in ortho, the molecule goes slightly out of plane, so $\ce{+M}$ effect may not be as effective?

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  • $\begingroup$ @Mithoron No it doesn't. It says that the inductive effect in the Ortho variant is stronger than the resonance but doesn't really explain why or how. $\endgroup$
    – Techie5879
    Apr 22, 2020 at 18:34
  • $\begingroup$ Do believe the basicity order is : p-Anisidine>Aniline>o-Anisidine ? That is easily looked up. $\endgroup$
    – MaxW
    Apr 22, 2020 at 21:41
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    $\begingroup$ @MaxW I know it can be LOOKED UP. I need to be able to explain the order during my examinations $\endgroup$
    – Techie5879
    Apr 23, 2020 at 4:33

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