maikelm

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Hi everyone,
I could use some insight onto this example problem from The Berkeley Review (TBR) Organic chemistry book. This is in Chapter 1 (Molecular Structure).

The question is simple: What is observed when histidine is protonated on its side chain?

My thought process: so, this is in reference to amino acid that has an amide group (NH) and an imine grounp (N double bonded). I'm thinking about the role of steric hindrance and resonance that may be occurring here when we are adding a Hydrogen atom. In a normal state, each Nitrogen has a lone pairs of electrons that are possible to move around the ring. Looking at the amine nitrogen, I believe those lone pair of electrons are allowing the entire structure to experience resonance. When I see the lone pairs of electrons on the imine, then I think resonance does not continue - I believe it stops at the amine as bonds are formed, even though the amine becomes positive. So, now we are adding a Hydrogen to the mix. My idea is that it will go to the amine and form a positively charged area without any potential for resonance. I see when a hydrogen is added to the imine, then resonance is still possible in the structure.

Here are the answers:

My answer choice: The amine nitrogen gets protonated, because the imine nitrogen shares its lone pair of electrons with the pi-bonds in the right through resonance, thus it cannot be protonated.
Correct answer choice: The imine nitrogen gets protonated, because the amine nitrogen shares its lone pair of electrons with the pi-bonds in the right through resonance, thus it cannot be protonated.

I'm confused as to how the correct answer choice makes sense. It seems like my thought process makes sense. Am I missing something with how resonance may play a role? I think there is something not clicking in my head, especially with the answer choice.
I've attached a picture of the problem as well.

Any help is appreciated it.
Thanks in advance
Maikel
 

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PlsLetMeIn21

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The lone pair on the amine nitrogen can be shared with the neighboring pi-bonds through resonance, so it is not available to be donated to an H+. The lone pair on the imine nitrogen cannot be shared through resonance, because N is already sharing an electron through through a pi-bond. The lone pair on the imine nitrogen is more available to donate to an H+.

I think their picture is really good at showing how resonance will be less when you protonate the amine nitrogen.
 
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maikelm

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The lone pair on the amine nitrogen can be shared with the neighboring pi-bonds through resonance, so it is not available to be donated to an H+. The lone pair on the imine nitrogen cannot be shared through resonance, because N is already sharing an electron through through a pi-bond. The lone pair on the imine nitrogen is more available to donate to an H+.

I think their picture is really good at showing how resonance will be less when you protonate the amine nitrogen.

Thanks @PlsLetMeIn21 !
So, it seems like we want to have resonance in the final product, which will improve stability. I can see what you mean when it comes to amine lacking the ability to donate a H+. But overall, we want to ensure resonance in the structure.

I think I was having difficulties grasping the concept, but I appreciate your help.
Maikel
 

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