A brand new graduate student made an error in calculating the stoichiometry of t
ID: 918060 • Letter: A
Question
A brand new graduate student made an error in calculating the stoichiometry of the following reaction. Since only half of the needed DCC was used, the student obtained only a 50% yield of the amide product, 50% of each of the two starting materials, and the byproduct of DCC. Draw the byproduct of DCC (when it has completed its role as an amide-forming reagent). Assuming that the DCC byproduct is water soluble, describe and illustrate how you could purify and isolate each of the four compounds in the reaction mixture using acid/base extraction techniques.Explanation / Answer
DCC is a dehydrating agent for the preparation of amides, ketones, nitriles. In these reactions, DCC hydrates to form dicyclohexylurea (DCU), a compound that is insoluble in most organic solvents and in water and hence is readily removed by filtration.
In the above reaction, DCC leads to a by-product DCU. Generally, the DCU will precipitate out from the reaction mixture. Thus, by just filtration one can avoide the DCU from the mixture. After the filtration, the filtrant consists of only three compounds i.e. DCC, Carboxylic acid, amine and amide. Add 1-5 % (v/v) of the mixture AcOH: H2O (1:1) to quench unreacted DCC and stir for 0.5-1h. This process converts DCC into DCU ppt. Then filter off the ppt.
Now, the filtrant consists of the three compounds i.e. two starting material carboxylic acid and amine and the product amide. Now, acidify the solution, the amine get converted into its salt form of amine (water soluble), the extraction with the organic solvent contains carboxylic acid and amide (product). Separate the organic extract and basify the organic extract the carboxylic acid get converted into salt of carboxylate( which goes into water layer). Finally, the organic layer only contains amide (product). Dry off the organic layer and evaporate off the organic layer to get the amide compound.
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