Organic Chemistry Lab Extraction

[wooj0313]

Please help me with Organic Lab Post Lab question: Extraction based on acid and base reaction

Here's an interesting paradox. Take a solution of benzoic acid in ether, and add an equal volume of water. The partition coefficient is about 100, i.e., benzoic acid is 100 times more soluble in ether than in water. So only about 1% of the benzoic acid goes into the water. Now use aqueous NaOH or aqueous NaHCO3 instead of plain water. The benzoic acid gets deprotonated, and it all goes into the aqueous layer. Why doesn't only the 1% that partitions into the water get deprotonated? It's not deprotonated in the organic layer, so shouldn't the large partition coefficient keep most of it in the ether? Something must be wrong with this reasoning --- what is it?

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[TieuBachHo]

If I understand your question right, then here is how it happens.

When the aqueous base deprotonates the acid, that 1% become neutral and yields its space for more acid to partition into the aqueous base (which is also 1% but this 1% comes from the 99% of acid-ether). So it continues on until most of the acid become deprotonated. Do you see the logic in this?

 

[solitarius]

benzoic acid has a polar end (-COOH) and a non-polar end (C6H5).

In the situation with no base (i.e., 1% solvation in water), the benzoic acid is not deprotonated. It just means that out of 100 benzoic acid molecules, only 1 neutral molecule will randomly find itself in the aqueous layer due to the hydrogen bonds between the -COOH and water.

When you deprotonate and create the benzoate anion, the -COO group is charged and all charged molecules will be strongly H-bonding with the polar H2O molecules, enough to most of them out of the ether layer and into the aqueous layer.