In building a flow chart for the extraction of a mixture of 1:1 of the follwoing
ID: 838983 • Letter: I
Question
In building a flow chart for the extraction of a mixture of 1:1 of the follwoing two compounds:1,1'Biphenyl, and cinnamic acid, and diethyl ether, aqueous NAHCO3, aqueous NAOH and queous HCL. What would the sodium bicarbonate be used for? Based off of the experiment we did in class I would disolve the biphenyl and cinnamic acid in diethyl ether, add NAOH to react with the cinnamic acid, then separate the two layers that would appear. One would be the diethyl ether with biphenyl and the other would be sodium cinnamate. Then for the Sodium Cinnamate you would add HCl to convert it back to cinnamic Acid. In class we used a vacuum to dry the acidic crystals. I just don't know what to do with Aqueous NaHCO3.
Explanation / Answer
In order to remove an acidic compound from a mixture, a base like NaOH or NaHCO3 is used. The carboxylic (or mineral) acid and the base react to form a sodium salt, which is usually exhibits a higher solubility in aqueous solutions due to its negative charge and higher polarity (as indicated by a more negative log Kow value i.e., CH3COOH: -0.17, Na+CH3COO-: -3.72).
Which of the two reagents should be used depends on the other compounds present in the mixture. Sodium hydroxide is usually easier to handle because it does not evolve carbon dioxide as a byproduct.
In addition, the concentration can be increased significantly if is needed. However, if compounds were present that are sensitive towards strong bases or nucleophiles (i.e., esters, ketones, aldehydes, etc.),
sodium bicarbonate should be used. It does not react with these compounds because it is a weaker base and a weak nucleophile (due to its resonance stabilization). Note that the formation of carbon dioxide as a byproduct causes a pressure build-up in the separatory funnel, the centrifuge tube or the conical vial. Thus, additional precautions (i.e., frequent venting) have to be taken to prevent any accidents resulting from the pressure build up in the extraction vessel. The target compound can subsequently be recovered by adding a mineral acid to the basic extract
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