CHEM 2122 / Organic Synthesis 1 - Reduction of a Ketone

Which is more reactive and air moisture-sensitive: NaBH₄ or LiAlH₄? Which is then easier to handle in the lab?

LiAlH₄ is more reactive and moisture-sensitive, making NaBH₄ easier to handle in the lab.

NOTE: Adding water to LiAlH₄ leads to immediate generation of H₂ gas, generating enough heat to ignite causing a fire. This same reaction with NaBH₄ is relatively slower and does not lead to a fire hazard.

What are the three stages to this lab?

1. Reduction.

2. Workup.

3. Analysis.

Explain how you are going to perform the reduction reaction.

1. Using a calibrated pipette, measure out 120uL of acetophenone and place in a 25mL Erlenmeyer flask.

2. Use 5mL of methanol to dissolve the acetophenone and place a small stir bar in the bottom of the flask.

3. On a weighing paper, weigh about 40mg of sodium borohydride and slowly add to the solution in the Erlenmeyer flask.

4. Stir this mixture at room temperature for 30 minutes.

Explain how you are going to perform the workup reaction.

1. Slowly add 4mL of water to the Erlenmeyer flask to dilute the reaction mixture.

2. Slowly add approximately 2mL of 6M HCl to make the solution acidic.

3. Add an additional 5mL of water and allow to stir for 2 minutes.

Explain how you are going to extract/wash.

1. After 2 minutes, transfer the reaction mixture to a separatory funnel and extract the aqueous layer with 20mL of EtOAc.

2. Separate the aqueous and organic layers and place the aqueous layer back in the separatory funnel and extract with an additional 20mL of EtOAc.

3. Separate the layers,

   combining the first organic layer with this second organic layer.

4. Wash the combined organic layers with 20mL of saturated NaCl and separate the layers.

5. Dry the organic layer

   with sodium sulfate (Na₂SO₄) to remove any remaining water in the organic layer.

6. Gravity filter the organic layer into a round bottomed flask and remove the organic solvent on the rotary evaporator.

7. Using a glass Pasteur pipette remove a small amount of the reaction product from the

   round bottomed flask for IR analysis.

8. Use a small amount of solvent to transfer the residue in the round bottomed flask to a sample vial. Take one drop of this solution and dilute in more solvent to make a very dilute solution for the TLC analysis