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Vishesh Jain
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Homework Statement
According to my textbook (Organic Chemistry by Solomon & Fryhle) "When an o-substituted aniline is protonated (at the N), increase of H-N-H bond angle leads to steric hindrance with the group at the o-position. This makes this protonated conjugate acid unstable and makes the amine less basic." But why dosen't it apply when the o-substituent is -OH or -OCH3 ?
The attempt at a solution
Because this effect operates when a methyl group is at ortho (as shown in textbook photo) but it dosen't for a methoxy (much bulkier) group, i presumed it's due to the O in -OCH3 which forms a H-bond with the protonated amine (thereby stabilizing it). But this steric inhibition effect operates in o-nitro aniline, (according to this image) where also there should be H-bonding, and o-nitroaniline is the weakest base. Why steric inhibition effect applies in case of o-nitroaniline but not o-methoxy aniline ..?
Explanations involving computational chemistry & molecular orbitals are most welcome.