[CCC123]

Hello, in my o-chem lab were doing an acid base extraction of a primary amine.  The pKa is around 9.8.

We first take the solution with the amine salt and add NaOH to pH 13, and we calculated the ratio of base to salt from Henderson-Hasselbach.  We mixed it up, let it sit, then added diethyl ether.  We stirred it, then removed the ether layer and back extracted with more ether.  The next step was to add a phosphoric acid solution (pH 2.0) mix, and remove the aqueous layer. 

Here is my question:  My TA said that after you extract the ether which contains your amine in base form, you can add distilled water, mix and keep the ether solution.  He said this helps remove any left over NaOH that came along for the ride.  After thinking about it, won't adding pure water (pH 7.0?) PROTONATE the amine in the ether and severly decrease your yield?  The amine that gets protonated would end up in the aqueous wash and de discarded.  Or does this not happen because theres no salt/counter-anion in solution?  Why would you be so concerned about left over NaOH anyway couldn't you just add more acid or stronger acid? 

Thanks for the replies

[jake.n]

Water is not acidic enough to protonate a significant amount of your free amine, as you are dealing with a buffered system.  By washing away excess, NaOH, you can more effectively protonate your amine when you collect the salt in the aqueous layer later.

[CCC123]

Thanks for your reply.  What you are saying makes sense, but I am not clear on the following concept, could you point out the error in my reasoning....


If the pKa of the amine is 9.8 and water is pH 7, then according to Henderson-Hasselbach the ratio of protonated amine to freebase would be ~10^2.8  ~ 1000:1 , assuming a pure seperation and no residual NaOH in the ether.

Where am I going wrong here?

[Honclbrif]

0.05% w/w solution of NaOH has a pH of ~12 so if there was any NaOH "that came along for the ride", it would probably create a basic enough environment that the amine would remain as the free base. However, since NaOH isn't particularly soluble in ether, this would probably be more to deal with incomplete separation of the liquid layers.

[AC Prabakar]

If the pKa of the amine is 9.8 and water is pH 7, then according to Henderson-Hasselbach the ratio of protonated amine to freebase would be ~10^2.8  ~ 1000:1 , assuming a pure seperation and no residual NaOH in the ether.

Where am I going wrong here?

You are correct,but practically the pH of water is around 5.5 to 6.5!
More over,  generally when we are extracting the amine in basic condition there is a possibility of small amount of carry over of base along with amine.

In order to ensure the complete absence of base or minimum level of base along with amine,water washing is preferable to the organic layer.

[frisbee]

The next step was to add a phosphoric acid solution (pH 2.0) mix, and remove the aqueous layer. 

To what did you add the pH 2.0 phosphoric acid solution?

[CCC123]

Sorry I got it wrong it was sulfuric acid, not phosphoric, and the compound is called ethyl 4-aminobenzonate.

I have another question, what happens to the excess sulfate ions after all the water has evaporated?  You don't have an equal molar ratio of sulfate ions to amino groups so where do they go when the water is gone?  Do they end up in the final product or do they crystalize?