Organic Chemistry II   | Lecture | LaboratoryGeneral Chemistry

Organic Chemistry Laboratory II
Preparation of 2-Phenyl-2-propanol (Grignard Reaction)
Experiment Description

In this two-week experiment, 2-phenyl-2-propanol will be synthesized through a Grignard reaction. Students will individually in to prepare the Grignard reagent  by reacting bromobenzene with solid magnesium in diethylether. The prepared Grignard reagent, phenylmagnesium bromide, will then be combined with acetone to form the desired 2-phenyl-2-propanol  product.

NOTE: All glassware must be extremely dry!  Place glassware in the drying oven for at least 10 minutes before beginning the reaction.

Preparation of the Grignard Reagent
Add 2g of magnesium turnings and a magnetic stir bar to a dry 100 ml round-bottomed flask, and place the flask in the drying oven for 30 minutes. Remove the flask from the oven, clamp it to a ring stand,  fit the flask into a heating mantle and immediately attach a drying tube.  Position the flask on a stirrer/hotplate.  Set the reaction up on the side of the hood near to the faucet labeled for distillation.  Allow the reaction flask to cool to room temperature completely before proceeding.  While the flask is cooling, place a 150ml beaker into the drying oven.

Once the flask has completely cooled to room temperature, remove the drying tube and add 15 ml of anhydrous diethylether and 6 ml of bromobenzene using a glass funnel. If the flask is not sufficiently cool, the ether will evaporate immediately.  If it evaporates, add more ether.  Re-insert the drying tube into the neck of the flask and turn on the magnetic stirrer.  If the reaction does not begin immediately, carefully use a dry stirring rod to crush one of the magnesium turnings against the bottom of the flask. Be careful not to press too hard and crack the flask!  Alternatively, a single crystal of iodine can be added to help initiate the reaction. A cloudy solution is an indication that the reaction has begun.  Once the reaction has started, attach a dry condenser tube, and add an additional 25 ml of ether through the top of the tube using a glass funnel.  Connect the drying tube to the top if the condenser.  Connect the water hoses to the condenser and turn the water on.  Use a permanent marker to mark the level of the contents of the mixture. If the reaction becomes too vigorous, cool the flask for 2-3 minutes in an ice water bath.

Once the reaction has become steady, warm the flask in the heating mantle on low heat for 5-10 minutes until the magnesium has completely dissolved/reacted and the solution has a brown or cloudy appearance. If the mixture has evaporated more than 0.5 cm below the starting line, add an additional 5ml of the ether.  If loss of ether continues, turn off the heat or immerse the flask into an ice bath to cool it down.  Allow the reaction mixture to cool to room temperature (if hot).  Remove the condenser tube and replace it with the drying tube.  Set the flask aside, keeping it clamped securely to the ring stand.  This Grignard reagent will decompose rapidly, so the next step must be started at once.

Preparation of 2-Phenyl-2-propanol
Remove the dry 150 ml beaker from the drying oven and allow it to cool to rt.  Place  20ml of acetone into the beaker. Dispense the acetone into a clamped 100ml rb flask containing a stir bar.  Using a dry glass pipette (from the drying oven, be sure to allow it to cool to rt), add the Grignard reagent solution dropwise to the acetone, with stirring.  Rinse the Grignard reagent flask with 5-10ml of dry ether and add this to the reaction flask. Place a drying tube on the reaction flask after all of the Grignard reagent has been added to the flask.  Allow the reaction mixture to stir at rt for 30min.

For the final step of the reaction, add 20 ml of 3M HCl to the flask. This will hydrolyze the alkoxy/magnesium bromide salt to form the 2-pehnyl-2-propanol. Transfer the reaction mixture to a 200ml beaker.  Add an additional 20 ml of ether to the rb flask to remove any residual product and add it to the beaker.  Two layers should be apparent in the beaker and no solids should remain in the reaction flask.  If solid still remains, add more ether (~10ml).  Transfer the entire reaction mixture to a 125ml separatory funnel (Be sure the stopcock is in the closed position).  Two layers should appear.  The top layer is the ether layer, containing product.  The bottom layer is the aqueous layer containing the magnesium bromide salt by-product.  Label two 100ml beakers or Erlenmeyer flasks as “water layer” and “organic layer".  Drain the lower layer from the separatory funnel into the beaker or flask labeled “water layer”.  Add 10ml of deionized water to the separatory funnel.  Stopper the funnel and shake vigorously for 1-2 minutes.  Set the separatory funnel back onto the ring stand and allow the two layers to separate.  Drain the bottom layer into the beaker or flask labeled “water layer”.  Add 10ml more of the deionized water to the separatory funnel and repeat the process one last time, combining the lower layer again into the “water layer”. Pour the remaining ether layer into the dry 100 ml beaker labeled "organic layer". Add ~100mg of magnesium sulfate drying agent to the beaker or flask labeled “organic layer”.   Swirl the flask, then allow the drying agent to settle to the bottom.  Carefully decant the solution into a pre-weighed, clean, very dry 100 ml beaker, leaving the drying agent behind. Place the beaker containing the ether layer in a warm water bath and heat to remove the ether.   Store your product in an open beaker to allow it to finish drying.

End of Week 1
Weight and Melting Point/Boiling Point  Determination of 2-Phenyl-2-propanol
Inspect your product and if it appears to be dry, weigh it and calculate the percent yield.  Calibrate the thermometer of the melting point apparatus with benzoic acid (lit mp = 122oC) or boiling point apparatus with water.  Record the melting point/boiling point of the product.

IR Spectroscopic Analysis of 2-Phenyl-2-propanol
Prepare a sample of the product for IR analysis.  Run a background scan if one has not yet been for the lab, and then record a spectrum of the product. Set up a table in your notebook and record the major peaks ( and frequencies in cm-1) in the table. 

TLC Analysis of 2-phenyl-2-propanol
Prepare a TLC plate with two tick marks (labeled 1 and 2) to analyze your synthesized product (1) and benzene (2).  Use the EtOH solution from the vial labeled "benzene" for the TLC analysis.  Prepare a solution of your product in ethanol (10mg product/1-2ml EtOH).  Spot the plate with the solutions using two capillary pipettes and allow the spots to dry completely before developing the plate.  Develop the plate using a developing solvent that will provide an Rf of ~0.5.  Start with 25:75 ethyl acetate: hexane and make appropriate adjustments in the developing solvent ratio, if necessary.  View the plate under UV light and in the iodine chamber.  Calculate Rf values for all the spots observed.  What do the relative Rf values tell you about the success of the reaction and the purity of the product?

End of Week 2
Waste Disposal
Dispose of all  organic waste (ether, hexane, ethanol, acetone) in the "non-halogenated" waste container.  The pH of aqueous solutions should be checked.  Acidic solutions should be disposed of in "acidic aqueous waste" and basic solutions in "basic aqueous waste".    Used filter paper, pipettes, TLC plates, melting point cover slips, KBr, MgSO4 and organic solids should be disposed of in the solid waste containers.