13 Reaction Workup

Before you begin any reaction, you need to think about how you will get product out at the end. The reaction mixture probably contains solvent, product, (hopefully) minimal amounts of by-products, and probably some unreacted starting material. The process of isolating the crude product is called “working up the reaction”, and depending on the nature of the reaction and the product, this can be done by various methods.

If you have never done the reaction before, you need to do some microscale (test-tube sized) tests of how to work up your reaction. Consult the Greener Solvent List to test acceptable solvents.

• The first thing to try is pouring the reaction mixture onto ice water; if you are lucky, unreacted starting materials will be dissolved while your product will crystallize. When crystallization is complete, you can filter off your product. Note: You need to wait until the aqueous solution does not look cloudy before you filter your product; otherwise, you will lose product that is still in the process of crystallizing.
  If after ~5 minutes no crystals have formed, it is improbable that any will form during the lab period, so you need to move on to the next test.
  Also, in many reactions this method will not work for the following reasons:
  • If your product is a liquid at 0°C, it will (obviously) not crystallize.
  • If your reaction has formed by-products, these may prevent crystallization.
  • Your solvent may not be water soluble, and thus your product may be in the organic layer of the reaction mixture.
• If no crystallization is seen on pouring into water, try extracting the mixture with organic solvents. This will separate the desired product (which is in the organic layer) from the water (also called aqueous layer). When you do the full scale (macroscale) work-up, be sure to use two or three small portions of solvent, rather than one large portion. For example, if you have about 100 mL of reaction mixture and water, use three 15 mL portions of organic solvent to extract the product.
  As each organic layer is separated from the aqueous layer, combine it with the others in an Erlenmeyer flask or a beaker. Dry the combined organic layers by adding an anhydrous salt (a drying agent), such as calcium chloride or sodium sulfate. The amount of solid should be enough to cover the bottom of the flask; if the solid seems lumpy and wet, add some more salt until it is dry and powdery.
  Swirl the flask and filter off the drying agent through a gravity filtration apparatus into an already-weighed round-bottom flask. Evaporate the solvent either by blowing compressed air into the flask or on a rotary evaporator if one is available. Weigh the flask and observe the product.
• At this point, it is a good idea to check the purity of your product by TLC. If the product seems pure (one spot), you can obtain an FTIR spectrum and try to crystallize it.