Help on random o.chem MCAT question.

[Dyingduck]

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The most effective way to remove triethylamine durin the workup of an organic reaction would be to extract the reaction mixture with aqueous:

A> Sodium bicarbonate
B> Sodium bisulfite
C> Sodium sulfate
D> Hydrochloric acid

The answer is D. But I am not sure why or how to approach this question.

Thanks.

 

[TLL]

Since triethylamine has a tertiary nitrogen with a free electron pair, you need an acid to protonate it so it becomes positively charged. When the amine is positively charged, the amine becomes aqueous and is thus separated from the organic layer.

A.) NaHCO3: HCO3 is a weak acid. Would not be strong enough to protonate the amine.
B.) NaHSO2: Weak acid, same deal.
C.) NaSO4: Not even a bronsted acid.
D.) HCl: Strong acid and thus the answer

 

[mtbtx]

One is not like the others...

 

[Dyingduck]

Since triethylamine has a tertiary nitrogen with a free electron pair, you need an acid to protonate it so it becomes positively charged. When the amine is positively charged, the amine becomes aqueous and is thus separated from the organic layer.

A.) NaHCO3: HCO3 is a weak acid. Would not be strong enough to protonate the amine.
B.) NaHSO2: Weak acid, same deal.
C.) NaSO4: Not even a bronsted acid.
D.) HCl: Strong acid and thus the answer

Thanks! I guess my problem is not understanding what the question is trying to ask.

 

[ru4real1666]

One is not like the others...

haha! yes, in this question, that way works. I have used this many times. But try to understand as someone explained above.

 

[SN2ed]

Thread moved. These types of questions belong in the MCAT Study Question Q&A forum.

 

[orgohacks]

Thanks! I guess my problem is not understanding what the question is trying to ask.

The question is essentially about water solubility. By itself, NEt3 is an organic compound and not expected to dissolve in water. How does one make an amine water soluble? By turning it into a salt. How does one turn an amine into a salt? Through protonation. Hence HCl as the choice of workup.

The protonation of an amine to form a salt is a fairly common reaction seen in a broader context, especially with regards to amino acids and also with the impact that basic amino acids (e.g. lysine) can have on the isoelectric point of proteins.

Another question in this vein is one that asks how to make a carboxylic acid water soluble. In that case, one would add a base to make the conjugate base of the acid (i.e. the carboxylate) which is now a salt and thus is water soluble.