Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: kadmo on September 29, 2009, 12:15:58 AM
-
1. How would you separate a mixture of benzoic acid, phenol, and naphthalene? Benzoic acid (pKa = 4.2), phenol (pKa = 10) and naphthalene (pKa is too high for chemically active extraction to be practical) are soluble in organic solvents like methylene chloride or ether, but insoluble in water. A mixture of the three compounds can be separated by a chemically active extraction using two different bases, sodium hydroxide and sodium bicarbonate (NaHCO3). Prepare a flowchart. Write out a series of equations for the acid-base reactions between each of the acids and each of the bases so that you can determine the correct order of steps in your flowchart. Note that naphthalene is neither acidic nor basic enough to participate in a chemically active extraction. You will also need to consider the fact that when bicarbonate ion (HCO3−) accepts a proton, it becomes carbonic acid (H2CO3, pKa 6.4); carbonic acid decomposes immediately to carbon dioxide and water. Recall also that the pKa of water is 15.7.
Please help I am drawing a blank on some steps
-
So what do you know so far? Which of your compounds is the strongest acid? Which of the two bases is the weakest base?
-
This is an acid base extraction procedure.
Basically, use a base that is only strong enough to form a salt of only one of the molecules.
i.e. it has a conjugate acid with a pkA lower than one molecule, but higher than the other molecules
This will cause this molecule to become water soluble and can be extracted from the organic phase via washing with water.
Repeat this as necessary to isolate the other compounds.