Why does benzocaine precipitate during the neutralization?

Question

During an experiment when I created benzocaine by direct esterification of p-aminobenzoic acid with ethanol why does benzocaine precipitate during the neutralization?

Answer 1

If you look at the mechanism of this reaction the final step involves abstracting an acidic proton and the leaving of water.

The base that you add in the neutralization removes that proton.

The reason that your product precipitates is because it is less soluble in your reaction solvent than the intermediate was.  

So when you add the base you see a precipitate because the product formed by the reaction of the base and the intermediate is not soluble in the solvent that you used to dissolve your reactants and intermediate.

Make sense?

Hope this helped!h

Answer 2

This happens because the esterification reaction is acid catalyzed (usually H2SO4 and ethanol plus heat are involved). The resulting product is the ethyl ester but at the same time, an amine salt is formed because of the acidic conditions used. This amine salt is very polar and is very soluble in ethanol, a polar solvent. However, when you neutralize the product with aqueous base, you are doing two things simultaneously. First, the free base (neutral amine) that forms is much less polar than the salt form. In addition, the solvent system that results from neutralization is much more polar since water has been added to it. The end result is that your product crashes out of solution ...