TBR: Protonating an Amine

[justadream]

TBR page 206 #10

“To separate an amine from an amide, it would be best to use which of the following for extraction:”

1. Ether and water at pH = 3
2. Ether and water at pH = 7
3. Ether and water at pH = 11
4. Amides cannot be separated from amines

The answer is A because at low pH, you can protonate the amine. I chose B for the same reasoning except that I thought that at pH = 3, the amide would get protonated too.



How do you know that the amide won’t be protonated?

---

Members don't see this ad.

 

[Chrisz]

You sure, think little bit deeper

 

[NextStepTutor_1]

Amines are weak bases - a fact that you remembered (good job!) when thinking about acid/base extraction. To extract one compound using acid/base extraction the goal is protonate or deprotonate that compound creating a charged species that can bind to water and thus be separated.
In this example, you need to protonate the amine - which would happen more readily at pH=3 than pH=7. An amide would be not be protonated at either pH.

Why does this discrepancy exist between the basicity of an amine and the relative lack of basicity of an amide? Short answer - the amide's carbonyl group is electron-withdrawing, delocalizing the nitrogen's lone pair of electrons, and preventing them from acting in a basic manner.




http://forums.studentdoctor.net/forums/next-step-mcat-tutor-office-hours.970/