Basicity Order Questions: Exploring the Ortho Effect and Substituent Influence

[sachin123]

Hey folks.
The orders given are stated in my book.I'm here to ask why.

basicity: p-methyl aniline>m-()>aniline>o-()
p exerts hyperconjugation and +I,m only +I ,but ortho exerts hyperconjugation and higher +I.
Shouldn't o-methyl aniline have high basicity?

basicity: aniline>m-nitro()>p-nitro()>o-nitro()
Why is o- at last?Shouldn't it move NH2 out of plane so that no -R takes place?

And is the ortho effect applicable to all substituents?Can you clear this for me?
Thank You.

 

[sachin123]

PLease tell me if my post isn't clear(and where).

 

[Phrak]

Ask the moderators of this forum to move your thread to the Chem folder. Send a PM to Moonbear.

 

[sachin123]

This is the chem place isn't it?

 

[DeeNos]

Thank you for your questions. The basicity order of substituted anilines can be explained by the electronic and steric effects of the substituents. In the first case, the p-methyl aniline has the highest basicity due to the combined effects of hyperconjugation and the +I effect of the methyl group. The ortho effect, which refers to the increased stability of a molecule due to the presence of a substituent at the ortho position, also contributes to the higher basicity of p-methyl aniline compared to o-methyl aniline. This is because the ortho position is closer to the lone pair of electrons on the nitrogen atom, leading to stronger hyperconjugation and a higher +I effect.

In the second case, the basicity order of substituted nitroanilines can be explained by the -R effect of the nitro group, which withdraws electron density from the nitrogen atom, making it less basic. The m-nitroaniline has the highest basicity because the nitro group is in the meta position, which is farther away from the nitrogen atom compared to the para and ortho positions. The o-nitroaniline has the lowest basicity as the nitro group is in the ortho position, which is closest to the nitrogen atom and has the strongest -R effect.

The ortho effect is applicable to all substituents, but its strength may vary depending on the substituent. For example, the +I effect of a methyl group is stronger than that of a nitro group, leading to a stronger ortho effect in the case of p-methyl aniline compared to p-nitroaniline. Additionally, steric hindrance can also play a role in the basicity order of substituted anilines, as bulky substituents may hinder the approach of a base to the nitrogen atom.

I hope this helps clarify the basicity order and the role of the ortho effect and substituent influence in determining basicity.