Expt #2. Grignard Synthesis of Hex-2-yn-4-ol

Relevant textbook readings – Karty, Chapter 18.4-18.5

Overview – We will react propynyl magnesium bromide 1 with propanal 2. Acidic work up should yield hex-2-yn-4-ol 3. The product will be used as the starting material for expt #3.

Procedures

Running the reaction.

• Grignard reactions require dry glassware and anhydrous solvents. Take care to avoid water until the work up procedures are begun.
• Assemble the apparatus pictured below. The Claisen adapter functions to provide two entry ports to the reaction flask. The CaCl₂ drying tube ensures no moisture from the air gets into the reaction. The liquid reagents are added by syringe through the rubber sleeve stopper.

• Use an oven-dry 20-mL syringe to add 15 mL of 1 (0.5 M solution in THF).
• Cool the solution on an ice bath with stirring.Then to the stirring solution slowly (dropwise) add 0.55 mL propanal using a 1.0 mL plastic syringe.
  
•  Allow the reaction to warm to room temperature and then continue stirring for 15 min at r. t.

Work-up procedures.

• Slowly add 10 ml 1M HCl to the reaction solution. (At this point the drying tube can be removed and water does not need to be avoided anymore.)
  
• Transfer the solution to a separatory funnel and extract it 15 mLdiethyl ether. Repeat the extraction twice more.
• Transfer the combined ether extracts back into the sep funnel and wash with 45 mL saturated NaCl (aka "brine").
  
• Dry the ether layer over sodium sulfate for 10-15 min.
• Decant the ether into a dry pre-weighed 100 mL rbf and remove the solvent on the rotovap.
  
• Determine the yield and obtain a proton NMR and prepare a sample for GC-MS analysis.