Question

Hydroboration-Oxidation (Organic Chemistry Laboratory)

1.Explain the mechanism of the reaction.

2.What was the most important piece of equipment for the success of the reaction?

3.Was there a rate determining step in the reaction?

4.Explain the purpose of the key reagent(s) in the reaction.

Experiment Procedure

1. Flame dry a 5.0 mL conical vial and a Claisen head at the start of the lab period. Place a dry spin vane in the dried conical vial. Assemble these pieces with a calcium chloride (~1 inch) drying tube and a septum cap as shown in Figure 11.2a.

2. Add 1-octene (210 μL by Eppendorf pipette in the hood) to the reaction vial. Take care to do this procedure quickly to limit any moisture from entering the anhydrous reaction apparatus.

3. Cool the reaction vial in ice-water and then using a syringe which the TA has charged with the borane solution (0.5 mL), add the solution to the reaction vial through the septum slowly over 5 min. Rinse the syringe with ~0.5 mL of anhydrous diethyl ether into reaction vial. Disassemble the syringe and immediately rinse with 1 mL acetone.

4. Remove the ice-bath and stir the solution for 45 minutes at room temperature.

5. Remove the Claisen head and add two drops of water to the reaction solution to quench any unreacted borane (H2↑!). (Be sure to empty out the calcium chloride drying tube into the solid waste jar before it hardens.)

6. Attach a reflux condenser to the reaction vial as shown in Figure 11.2b and add to the reaction vial 3 M NaOH (300 μL) followed by 30% hydrogen peroxide (300 μL by Eppendorf pipette), swirling the vial after each addition.

7. Heat the solution (90–100 °C) for 1 h. Allow the solution to cool to room temperature, cap the vial and store it until the following lab period.

8. Add ether (1 mL) to the solution, cap, agitate and carefully vent the solution. Using a Pasteur pipette remove the organic layer to a 4 dram vial, leaving the aqueous portion in the reaction vial. Extract the aqueous portion two more times with ether (1 mL each time) and combine the organic portions in the 4 dram vial. If a solid forms during any of these extractions, add a few drops of 0.1 N HCl. Dispose of the aqueous layer in the liquid waste.

9. Wash the combined organic portion with 0.1 N HCl (0.75 mL). Then wash the organic portion with water (0.5 mL each time) until the aqueous layer is neutral to pH paper.

   10. Remove any residual water from the organic portion using anhydrous sodium sulfate and transfer the water-free solution to a 1 dram vial and remove the solvent using the rotary evaporator (see page 46). Dispose of the sodium sulfate in the solid waste container and the aqueous solutions in the liquid waste bottle.

   11. Prepare a small chromatography column by packing a Pasteur pipette with cotton, sand and silica gel (400 mg), as shown in Figure 11.3. Clamp it over a tared 1 dram vial.

   12. Dissolve the organic residue from step 10 in pentane (1 mL). Add this solution to the silica gel column. Rinse any remaining sample with another 0.5 mL pentane onto the column. Elute the column with 2.0 mL of pentane. Remove the solvent using the rotary evaporator until you obtain a constant weight (liquid starting material). Don’t overdo this or you risk losing any octene.

   13. 13. Place a second tared 1 dram vial under the column and elute with 2.0 mL of ether. Remove the solvent using the filter flask drying apparatus until you obtain a constant weight. This weight gives you the mass of the final product. Dispose of the entire column in the solid waste container.

       14. Turn in the entire product sample (vial 2) to the TA for IR analysis.

Answer 1