Chapter 26: Borohydride Reduction of 2-Methylcyclohexanone
Modification of procedure:
Put a 10 x 100 mm reaction tube in a small Erlenmeyer flask (or beaker) on a balance and zero it. Weigh about 300 mg of 2-methylcyclohexanone. Add ~1.2 mL of methanol to the tube and cool it in an ice/water bath in a small beaker. Weigh ~70 mg of sodium borohydride, and add it in 4 small batches via spatula to the solution. (Do not add all at once.) Remove the tube from the ice/water bath and allow it to stand at room temperature for 10 minutes. Add ~1.2 mL of 3 M sodium hydroxide solution to quench the reaction and decompose the borate ester. Further add 1 mL of water and 2 mL of dichloromethane (CH2Cl2), transfer all contents into a test tube (test tube #1), and use a Pasteur pipette to mix all materials well by repeatedly drawing the liquids up and down. (Note: If you still perform this operation in a 10 x 100 mm reaction tube the liquid could easily overflow once you stick the pipette to the bottom.) Let the mixture sit undisturbed for a few minutes until you see clear layer separation. Remove the bottom organic layer to another clean dry test tube (test tube #2). (Question: Why is the organic layer at the bottom?)
Make sure no visible water droplets get transferred. Add another 1 mL of CH2Cl2 to the residual aqueous solution in test tube #1, mix well, and transfer the bottom organic layer to combine with the previous batch of organic solution in test tube #2. Now add 1 mL of saturated sodium choride solution (NaCl) to test tube #2, mix well, and transfer the bottom organic layer to another clean dry test tube (test tube #3). It is OK to leave a little bit of organic solution behind with the aqueous layer in test tube #2 to ensure that no visible water droplets get into the organic solution. The NaCl wash serves to remove most residual water from the organic layer. Add enough sodium sulfate (Na2SO4) to cover the bottom of test tube #3, shake to make sure that some solids are freely moving, and let the mixture sit for 2 minutes to dry. If your solution still looks cloudy keep adding more Na2SO4 until it becomes clear.
Prepare a sand bath of about 80 oC (actual bath temperature but not the digital temperature reading on your hot plate). Add one piece of boiling stone to a short-necked 5-mL flask and record the weight on your notebook. Carefully decant the dried CH2Cl2 solution to the flask and leave the solid Na2SO4 behind in test tube #3. Assemble a small-scale simple distillation apparatus as shown in Fig. 5.5 on page 93. (Note: We are using the short-necked but not long-necked 5-mL flask here.) Distill for 10 minutes or until no more significant liquid droplets come out of the distillation head, whichever takes shorter time. Discard the distillate in the organic waste container as it is CH2Cl2, but not your product. Use medium strong compressed air to blow the contents of the distillation flask for 3 minutes to remove residual CH2Cl2 and measure the weight of the product. If it is significantly more than 0.32 g, there is likely still a lot of CH2Cl2 left in your flask, and you should further blow it away with compressed air. Submit your product to the instructor for IR measurement.
Cleaning up. Rinse all organic product and solvent with acetone into the organic waste container. Wash all your glassware clean with soap water and finally rinse with a small amount of acetone.
Note: IR is a great method to determine whether the reaction has worked because the carbonyl group (C=O) in the starting material and the hydroxyl group (OH) in the product have clearly distinct IR absorption peaks. You must discuss and compare your own IR spectrum to 2-methylcyclohexanone IR spectrum in your lab report. Unlike NMR, IR will not be able to reveal the cis/trans ratio of the possible products. Discussion of the stereochemistry of the products is thus optional.