Chemistry 29a M. Walker

Note that we are separating benzoic acid (an acid) from benzophenone (a neutral compound), using a variation of the method given in chapter 8, expt. 1 of Williamson.

Often substances are too similar to be easily separated by recrystallization alone. However, if one substance be separated out contains an acidic (or basic) group, there is the possibility of using this to ease the separation.

Benzoic acid in its neutral form is of fairly moderate polarity (as is benzophenone), and it is soluble in medium polarity solvents such as methyl tert-butyl ether (as is benzophenone). However, if a base such as sodium bicarbonate is added, benzoic acid will form its sodium salt (i.e., its conjugate base), and this ionic compound will have markedly different solubility properties. For example, it will soluble in water, unlike both benzophenone and neutral benzoic acid. This sodium salt can be removed as an aqueous solution, from which the neutral benzoic acid can be recovered simply by acidification.

Dissolve about 0.20 g of the benzoic acid/benzophenone (1:1) mixture in 2 mL of methyl tert-butyl ether in a reaction tube. Then add 1 mL of a saturated aqueous solution of sodium bicarbonate to the tube. Use the graduations on the side of the tube to measure the amounts, because they do not need to be exact. Mix the contents of the tube thoroughly by pulling the two layers into a Pasteur pipette and expelling them forcefully into the reaction tube. Do this for about 3 min. Allow the layers to separate completely, and then draw off the lower layer into another reaction tube (tube 2). Add another 0.15 mL of sodium bicarbonate solution to the tube, mix the contents as before, and add the lower layer to tube 2. Exactly what species is present in tube 2? Add 0.2 mL of methyl tert-butyl ether to tube 2, mix it thoroughly, remove the ether layer, and discard it. This is called backwashing and serves to remove any organic material that might contaminate the contents of tube 2.

After this point, follow the procedure as in Williamson starting at the paragraph "To tube 1". Completely omit the paragraph that begins "In exactly the same way..", and go straight to "The neutral compound". Benzophenone can be recrystallized from methanol.

Assigned Problems: 1, 8.