Can NH2 with no formal charge on it be protonated?

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SUMMARY

The discussion centers on the protonation of the NH2 group in an aromatic amine structure containing a methoxy group. It is established that nitrogen (N) is a stronger base than oxygen (O), making the NH2 group more likely to gain a proton compared to the methoxy oxygen. The pKa values of NH4+ (+9.25) and H3O+ (-1.5) illustrate the relative strengths of ammonia and water as bases. Additionally, the presence of the aromatic ring in aniline reduces its basicity due to electron withdrawal, with aniline's pKa being +4.6.

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  • Understanding of acid-base chemistry, particularly protonation reactions.
  • Familiarity with pKa values and their significance in determining base strength.
  • Knowledge of aromatic compounds and their electronic effects on substituents.
  • Basic concepts of amines and their behavior in aqueous solutions.
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ngu9997
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So I have this question that asks which is the strongest base. I have one structure with a benzene ring with an NH2 group on one side and a methoxy group on another side. Automatically I knew that the NH2 group would be what is protonated in the situation that this structure acts as a base and becomes protonated. But the more I think about it, the less it makes sense. N already has 2 H bonds on it and is attached to the benzene ring and has a lone pair giving it a formal charge of 0. How could it form another H bond in the case it becomes protonated (I assume there has to be some logic as to why the N is what I'm looking at to become protonated rather than the oxygen on the methoxy group).
 
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ngu9997 said:
How could it form another H bond in the case it becomes protonated
How does any amine (or ammonia) get protonated? Just the same.
 
Oh wow , I forgot that it was H+ so it wouldn't violate any rules of having more than 8 electrons in N's octet. That makes sense, but how would we determine that the NH2 group gains the hydrogen in protonation rather than the oxygen on the methoxy group?
 
ngu9997 said:
Oh wow , I forgot that it was H+ so it wouldn't violate any rules of having more than 8 electrons in N's octet. That makes sense, but how would we determine that the NH2 group gains the hydrogen in protonation rather than the oxygen on the methoxy group?

Because N is usually a stronger base than O.

If you have NH3 and H2O in ammonia water solution and add acid, then you can protonate either water - forming H3O+ - or ammonia - forming NH4+.
But ammonia is much stronger base. H3O+ has pKa of -1,5, while NH4+ has pKa of +9,25.
Now, in aromatic amines, the aromatic ring actually withdraws some of the free electron pair and makes the amine a weaker base. Aniline, C6H5NH3+ has pKa of +4,6.

But it is still much more basic than water, or presumably phenylether.
How does an ether group across the aromatic ring affect pKa of an aromatic amine?
 

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