Preparation of the Grignard Reagent
All of the glassware needs to be very dry for this
reaction to work, so the instructor will supply
oven-dried kits to each lab group at the start of the
lab. Each kit should contain: a 10-mL round bottom
flask, two large screw top vials, a 50-mL Erlenmeyer, a
100-mL beaker, a Claisen adapter, a drying tube, a
condenser, and a Pasteur pipet. Roughly calibrate the
pipet by making marks with a wax pencil or Sharpie as
shown on page 45 in Mohrig.
Assemble a reaction apparatus suitable for the slow
addition of a reagent under anhydrous conditions
(Mohrig, Chap 7.5a). The basis for this apparatus is
shown in fig 7.5a. However, a drying tube filled with
CaCl2 needs to be placed at the top of the
reflux condenser (similar to the setup shown in fig
7.2b). Also, you will need to use a 10-mL round bottom
flask in place of the conical vial shown in fig 7.3.
(Also note that the syringe does not get inserted until
you are actually ready to begin adding a reagent using
it.)
Temporarily disassemble the reaction apparatus so that
you can add 7.5 mmol magnesium and a magnetic stirbar to
the round bottom flask. Reassemble the apparatus.
Obtain approximately 20 mL anhydrous ethyl ether using
your dry Erlenmeyer. This is your stock of ether for the
first part of the reaction. Keep the flask capped when
not actively removing ether from it.
Weigh a dry vial with cap. Add 7.5 mmol 4-bromotoluene
to it and reweigh it to get the actual mass used. Add 8
mL ether (from your stock vial) and swirl to dissolve.
Add 1 mL ether (by syringe) to the reaction flask and
begin stirring.
Add 0.75 mL of the prepared 4-bromotoluene solution.
Stir with slight warming (cupping the flask with your
hand works well or you can use a warm water bath (40-45
°C) but take care to not get water into the flask!).
Watch carefully for signs that the Grignard reaction
has commenced. These include bubbles coming off the
surface of the metal, gray/brown cloudiness to the
solution, and signs of corrosion on the surface of the
metal. If the reaction has not started on its own within
10 min of warming, then you will need to "jump start"
it.
To jump start the reaction, discontinue warming and
disassemble the rb flask from the rest of the apparatus.
Working quickly, carefully use a dry spatula or glass
rod to crush or break in half some of the Mg pieces.
Doing so exposes unoxidized reactive surfaces on the
metal which then allow the reaction to begin.
Once the reaction has started begin adding the rest of
the 4-bromotoluene solution over a period of about 15
minutes. Maintain a rate of addition such that the
solution refluxes on its own without external heating.
If the addition is done too rapidly then the reaction
may threaten to become too vigorous, in which case you
should cool the flask briefly on a cold water bath (just
enough to slow down the reaction take care not to stop
it entirely). If the addition is too slowly, the
reaction will start to subside as it runs out of
starting material. If this happens to the point of it
appearing that the reaction may stop all together then
it is important that you add more bromotoluene solution
and/or apply gentle warming to get it going again.
After all of the bromotoluene solution has been added,
add 1 mL ether to the vial it was in, rinsing to
dissolve any remaining 4-bromotoluene and then add this
rinse solution ot the reaction flask. The reaction will
naturally start to die out once all of the bromotoluene
has been added. Once it starts to do this, but before it
stops entirely, apply heat from a warm water bath so as
to keep it refluxing. Reflux on the warm water bath for
10 min and then cool to room temperature.
Reaction of the Grignard Reagent with CO2
Weigh out about 2 g dry ice and try to wipe off any
surface condensation with a paper towel. (If the dry ice
is in large chunks then you will need to wrap it in a
towel and use a hammer to crush it into small pieces.)
Place the dry ice in a dry 100-mL beaker quickly add 2.5
mL anhydrous ether and then immediately pour the
Grignard solution into the beaker. Use a glass rod to
stir vigorously for several minutes. Just before the dry
ice has completely sublimed add 7.5 mL 10% HCl(aq) in
small increments stirring vigorously.
Work-up
(Note: for the remaining procedures the ether does not
need to be anhydrous.)
Transfer the reaction mixture into a separatory funnel
and allow the layers to separate. Drain the lower
aqueous layer into a flask or beaker and then transfer
the organic layer through the top of the sep funnel into
a dry 125-mL Erlenmeyer flask.
Return the aqueous layer to the separatory funnel and
extract it three times with 5 mL ether. For each
extraction, first drain off the aqueous layer and then
transfer the organic layer into the dry Erlenmeyer flask
containing the first ether layer.
Extract the combined ether layers with three 5-mL
portions of 1M NaOH(aq).
Incrementally add enough 1M HCl(aq) to make the
solution acidic to litmus. You should be able to
calculate the approximate volume of HCl necessary based
on the amount of 1M NaOH used in the previous step.
Chill the mixture on an ice bath to maximize
precipitation of the water-insoluble product then
collect the product by vacuum filtration.
Characterization. Obtain the mp as well as 1H
NMR and IR spectra of the final product.