Can NH2 with no formal charge on it be protonated?

  • Thread starter ngu9997
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  • #1
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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  • #2
mjc123
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How could it form another H bond in the case it becomes protonated
How does any amine (or ammonia) get protonated? Just the same.
 
  • #3
ngu9997
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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?
 
  • #4
snorkack
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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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