APPENDIX A: MICROSCALE APPARATUS AND TECHNIQUES This Chapter describes and illustrates the apparatus and microscale techniques that are used in the experiments you will perform but are not necessarily arranged in the same order you will use them. A.1 CRAIG TUBES: RECRYSTALLIZATION Recrystallization on a microscale utilizes a Craig Tube. The Craig tube (Figure A.1A) looks similar to a test tube but contains a ground-glass outer shoulder. A Teflon inner plug is inserted to filter out liquid, leaving solid compounds in the Craig tube. A wire is used on the end of the inner plug and you will need a centrifuge tube, to separate liquids from solids. Figure A.1A Equipment for recrystallization with a Craig Tube: (shown left to right: centrifuge tube, inner plug, Craig Tubes (two sizes), wire) A minimum amount of hot solvent is added to the sample to dissolve the solid, and heating may be accomplished either by placing the tube in a small beaker containing hot water, in a sand bath or on an aluminum heating block. After the solid is dissolved, the Craig tube is cooled in an ice water bath to induce and complete the recrystallization. Once the recrystallization is complete, the inner plug is placed in the outer tube, and a thin copper wire is wrapped around the narrow part of the inner tube (Figure A.1B). 1
Figure A.1B: Craig Tube Assembly Solution to be cooled and crystallized in bottom of Craig Tube. Inner Plug fits on top and wire is wrapped around inner plug. Hold the Craig tube upright and place a glass or plastic centrifuge tube over the Craig tube so that the copper wire extends just below the lip of the centrifuge tube (Figure A.1C). While holding together tightly, turn the apparatus upside down so that the centrifuge tube is in an upright position. Centrifuge the Craig tube by spinning in a centrifuge at maximum speed for several minutes; balance the Craig tube with another centrifuge tube that is about two-thirds filled with water on the opposite side. The spinning causes the recrystallization solvent to go to the bottom of the centrifuge tube and the crystals to remain inside the Craig tube, on the end of the inner plug (Figure A.1D). Figure A.1C -The centrifuge tube is placed over the top of the Craig tube and inner plug. 2
Figure A.1D Filtered solids stuck on end of Teflon inner plug while the liquid has filtered through to the bottom of the centrifuge tube. The Craig tube is removed from the centrifuge tube using the copper wire and the inner part of the Craig tube is carefully removed. The crystals may be scraped from the inner tube onto filter paper in a Petri dish to dry. On occasion, it may be necessary to scrape some crystals from the outer tube with a microspatula if some of them remain inside the tube. A.2 HIRSCH FUNNEL: VACUUM FILTRATION Vacuum filtration using a Hirsch funnel (Figure A.2) is a convenient way to isolate small quantities of solid from solvent. It serves the same purpose as vacuum filtration using a Büchner funnel, but the latter is used for collecting larger quantities of material. Our microscale laboratory uses ceramic funnels (and some plastic versions, as well). Figure A.2A Ceramic Hirsch funnel (left) and neoprene rubber adapter. Figure A.2A shows the ceramic Buchner funnel and neoprene adapter required for the vacuum system that will be applied. Figure A.2B shows the difference between the 3
plastic Hirsch funnel and the plastic Hirsch funnels. Both utilize the same 1 cm circular microfilter papers. Figure A.2B Plastic and ceramic Hirsch funnels Figure A.2 demonstrates the complete set up of the Hirsch funnel. Clean the filter flask before using it for these reasons: (a) In the event desired solid is pulled through the filter due to a tear in the paper, the materials in the flask can be refiltered. (b) It may be desire to remove the filtrate (i.e. the liquid that passes through the filter paper, a.k. a. the mother liquors ) and concentrate them to get more solid. (c) Occasions arise when the desired compound is contained in the filtrate while the solid collected is an undesired impurity. Note the filter flask is clamped to prevent it from being tipped over by the heavy walled rubber vacuum tubing. The 1 cm filter paper used in the funnel will just fit over the holes when placed correctly and can be moistened with solvent to help it adhere to the funnel before pouring the solvent-solid mixture onto the funnel. [Caution excessive wetting when using the plastic funnels may cause the filter paper to soften until it is pulled into the slits, lifting the edge of the filter paper and allowing the crystals to slide underneath and into the filtering flask.] If the desired compound is contained on the funnel, it is frequently left on the funnel to dry. The Hirsch funnel should be left in a dry beaker to keep it from falling over in the drawer. Figure A.2 Hirsch Funnel: vacuum filtration set-up 4
A.3 CONICAL VIAL EQUIPPED WITH AN AIR CONDENSER AND SPIN VANE Reactions may be carried out in a conical vial equipped with an air condenser and/or a spin vane, the latter being using to ensure mixing of the reactants and to minimize bumping. A versatile apparatus is shown in Figure A.3. The conical vial containing a spin vane and equipped with an air condenser is immersed in a beaker. If a stirring hot plate is available, a variety of options are available. The beaker may be filled with ice-water or tap water and only the stirring option may be used. The beaker may be filled with water or a higher boiling liquid, such as mineral or silicone oil, and heated to the desired temperature; stirring may be done simultaneously with heating. Note that only the air condenser is clamped to allow the conical vial to be immersed easily in the liquid contained in the beaker. The temperature of the liquid in the beaker may be determined by placing a thermometer in the beaker. Figure A.3 Conical Vial equipped with an air condenser and spin vane A.4 USE OF THE ALUMINUM BLOCK Aluminum blocks are Aluminum blocks are available with various sized holes drilled in them to accommodate different sized microscale glassware and apparatus (Figure A.4A) and may be used for various purposes. 5
Figure A.4A Reactions may be carried out with heating and/or stirring if desired. When stirring is used, the conical vial should be centered on the stirring hot plate. The temperature of the block is controlled by adjusting the temperature of the hot plate (Figure A.4B). The heating block is made of aluminum to permit magnetic stirring and to avoid rusting. While Figure A.4B shows a conical vial equipped with a water condenser, other glassware, such as a Hickman still or any experiment requiring the use of a conical vial, may be used in place of the apparatus shown. Aluminum blocks are also very useful to hold conical vials and sample vials and keep them from tipping over. Figure A.4B Heating with an aluminum block. Stirring may also be done with this apparatus. 6
A.5 APPARATUS FOR PREPARATION OF GRIGNARD REAGENT The apparatus shown in Figure A.5A can be used for reactions when it is desired to maintain anhydrous conditions by attaching the L-shaped drying tube containing a drying agent such as anhydrous calcium chloride. Various reactants may be present in the conical vial, and a variety of liquids may be contained in the syringe. If desired, the conical vial may be heated by immersing it in a beaker containing a suitable liquid that is heated by the hot plate or cooled by placing ice water in the beaker. The contents of the vial may be stirred and/or heated if a stirring hot plate is used. The apparatus shown in Figure A.5A is that used for the preparation of the Grignard reagent, where the conical vial contains the magnesium metal and ether, and the syringe contains a mixture of bromobenzene and ether. Figure A.5A Apparatus for conducting reactions under anhydrous conditions Variations of this apparatus are used through-out the semester. In Figure A.5B, used for the second part of the lidocaine synthesis, the water-cooled condenser is placed directly above the conical vial. Heat can commence and the mini-hood is used above the opening of the water-cooled condenser to remove any potential vapors of diethylamine (See A.6 for mini-hood discussion). 7
Figure A.5B Lidocaine Part B Apparatus This apparatus set-up works particularly well, as it allows the reaction mixture to be heated and stirred while a vacuum system is applied through the Claisen adapter sidearm (Figure A.5C). The solvent of the reaction is heated, vaporized and removed as vapors from the system when the vacuum is turned on. The water is left running in the water-cooled condenser to ensure that the vapors only exit the side-arm and do not enter the lab. Figure A.5C Vacuum removal of solvent during Lidocaine Part B. 8
The apparatus shown in A.5D is used when reagents need to be heated to a reflux and other liquid reagents added in over a period of time, as in the esterification lab. Notice that when liquids are added during the reaction, the liquids are added via syringe through a rubber septum placed directly over the conical vial so the reagents being added go directly into reacting solution, instead of trailing down and around arm of Claisen adapter. This would result in loss of reagent, as not all of it would make it into the reaction flask. Figure A.5D In the final apparatus variation, shown in Figure A.5E, note the drying tube at the top of the water-cooled condenser and the placement of the mini-hood over the opening of the drying tube. This apparatus is used for a reaction that must be refluxed under anhydrous conditions. Any potential vapors exiting the drying tube will be ventilated through the mini-hood system (See A.6 below). Figure A.5E, reflux apparatus, with addition of reagents and drying tube 9
A.6 REMOVAL OF NOXIOUS GASES USING MINI-HOODS Some reactions evolve noxious fumes or gases, such as HCl, SO 2 and others. Assuming that it is not desired collect such gases or fumes, a convenient technique for removing them and keeping them from entering the laboratory is shown in Figure A.6. The houseexhaust mini hood is clamped over the opening where the fumes are being released, and the exhaust fan system removes them. Figure A.6 Removal of gases using mini-hoods in lab A.7 HICKMAN STILL: MICROSCALE DISTILLATION The Hickman Still shown in Figure A.7A can be used to distill small quantities of liquid contained in a conical vial. Depending on the boiling point of the liquid, the conical vial can be immersed in a beaker containing a heated liquid or placed in an aluminum block and heated by a stirring hot plate. Note that the Still and conical vial are held in place by a single clamp. The spin vane eliminates the necessity of adding boiling stones. If desired, the Still may be equipped with a water or air condenser above the Still, with the clamp being placed in the same location as in Figure A.7A. The boiling point of the distilled liquid may be determined by placing a thermometer inside the Still as shown in Figure A.7B. The distilled liquid collects in the trough at the bottom of the widest area of the Still (Figures A.7A and A.7B). The sample is recovered by removing the cap from the side arm and withdrawing the distillate with a pipet. 10
Figure A.7A Hickman Still for microscale Figure A.7B Hickman with distillation thermometer in place 11