[ritwik06]

Why is p- toluidine more basic than ortho- toluidine?

I think they should have almost the same basicity because both of these raise elesctron density at the nitrogen centre due to hyperconjugation. On the contrary, ortho position will exert more +I effect.

[aieeee]

but there are cases , when hyperconjugation effect is more than inductive effect or vice verse. i think this is one of those cases.

[Schrödinger]

I'm not too sure, but i think it has something to do with 'ortho effect'.

I'd be glad if someone could explain to me why it arises.

[aieeee]

right. ortho effect says :
if any group present on ortho position of benzoic acid it always increases acidic nature of acid because this group decreases outer resonance of ring towards acid which increases acidic nature for example orthonitrobenzoic acid is most acidic in any nitro derivative of benzoic acid.orthomethyl benzoic acid is maximum acidic than other isomers.similarly if any group present on ortho position of aniline it decreases basic nature.

[ritwik06]

why would ortho substituent decrease basicity of aniline?
In case of benzoic acid, the phenyl group has -I, +R effect. Therefore an ortho substituent rules out +R effect by affecting the coplanarity of benzene ring and -COOH group. But what about aniline?? In case of aniline, the phenyl group should have -I and -R effect. So now if resonance is stopped, it should make aniline more basic rather than less basic.

What do you have to say about this?

[aieeee]

dude , the order of basicity is explained by two opposing effects :
i) inductive effect            ii) solvation effect

the facts i mentioned in my previous posts , is attributed to steric hindrance by the ortho toulidine to the solvation of cation.this steric effect destabilises the cation , making ortho substituent the weaker base.( this is the case, perhaps u were looking for , for aniline )