CH3NH2 in the Bronsted-Lowry Theory

[SMc21]

We just got a worksheet about acids and bases today. One of the questions said:

"Using the Bronsted-Lowry theory of acids and bases, write equations for the following acid-base reactions and indicate each conjugate acid-base pair."

One such reaction was CH₃NH₂ + H₂O

I checked a ton of websites, and nearly every answer I can find says that CH₃NH₂ is a base, and the products of this reaction are OH^- + CH₃NH₃⁺.

However, my teacher seeemd to think that CH₃NH₂ was an acid, because if it was a Bronsted base and another H⁺ was added to it, it wouldn't make sense, since NH₃ is a stable compound itself and wouldn't be part of a larger compound.

I know that CH₃NH₂ should be a base, but I also see my teacher's point about it being an acid. How can this compound accept another hydrogen? It'd also help if I could see a Lewis dot structure of CH₃NH₃⁺.

 

[symbolipoint]

The amine group (the -NH₂) can accept a hydrogen ion. Yes, the methylamine is a base. The PROTONATED form of it can be considered an acid; this would be CH₃NH₃⁺¹.

 

[Borek]

However, my teacher seeemd to think that CH₃NH₂ was an acid, because if it was a Bronsted base and another H⁺ was added to it, it wouldn't make sense, since NH₃ is a stable compound itself and wouldn't be part of a larger compound.

NH₃ as a separate entity is different from the same group of atoms in CH₃NH₃⁺. Comparing them make about as much sense as stating H₂O can't exist, because H₂ is a stable compound.

 

[SMc21]

The amine group (the -NH₂) can accept a hydrogen ion. Yes, the methylamine is a base. The PROTONATED form of it can be considered an acid; this would be CH₃NH₃⁺¹.

But couldn't one of the groups in the methylamine also "give up" an H⁺? Why wouldn't they be able to?

I understand how it can be a base now, but I still don't understand why it can't be an acid.

 

[Borek]

Technically speaking it is possible, and in well designed experiments, in anhydrous solutions, you should be able to see CH₃NH^-. However, CH₃NH^- is a very strong base, so in water it will immediately react with solvent, yielding CH₃NH₂ and OH^-

 

[SMc21]

Ah, so basically, any possibility for CH₃NH₂ to behave as an acid is canceled out because the resulting compound would almost instantly "re-react"?

 

[Borek]

Yes.

 

[SMc21]

Ah, I see. Thanks for the help!