Question

A student performs the following steps to isolate biphenyl from a solid 1:1 mixture of biphenyl and salicylic acid.

Dissolve 2.0 grams of the mixture in 20 mL of ether

Extract the ether layer 2-3 times with an aqueous solution of 5% sodium

bicarbonate

Add 2-3 spatula tips of sodium sulfate to the ether layer

Extract the ether layer with a saturated aqueous solution of sodium chloride.

Isolate the organic layer and evaporate the ether.

The solid they collect at the end weighs 1.23 g and has a wide melting point range. If the student expected to recover 1 gram of pure biphenyl, explain, based on the procedural information given above, why this student did not see their expected results. Hint: The order of steps in an extraction can be important.

Answer 1

Extracting the ether layer 2-3 times with an aqueous solution of 5% sodium bicarbonate will remove the acid, salicylic acid from biphenyl.

The expected results not obtained because extraction of the ether layer with a saturated aqueous solution of sodium chloride should take place after washing with 5% sodium bicarbonate. Washing with a saturated solution of NaCl will remove the intervening emulsions during extraction. Addition of sodium sulfate to the ether layer is the last step for drying.

In the above extraction procedure, the last 2 steps are reversed. Thatis why we did not see the expected result.