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
25-mL round bottom flask, two large screw top
vials, a 50-mL Erlenmeyer, a 100-mL beaker, a
Claisen adapter, a drying tube, 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 25-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 15 mmol magnesium and a
magnetic stirbar to the round bottom flask.
Reassemble the apparatus.
Obtain approximately 40 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 15 mmol
3-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 2 mL ether (by syringe) to the reaction
flask and begin stirring.
Add 1.5 mL of the prepared 3-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 3-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 3-bromotoluene solution and/or
apply gentle warming to get it going again.
After all of the 3-bromotoluene solution has
been added, add 2.0 mL ether to the vial it was
in, rinsing to dissolve any remaining
3-bromotoluene and then add this rinse solution
ot the reaction flask. The reaction will
naturally start to die out once all of the
3-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 4 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 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
15 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
10-mL portions of 5% NaOH(aq).
Acidify the basic extracts through the addition
of 10% HCl(aq). (Use litmus paper to test the
pH.) (You should be able to calculate the
approximate volume of HCl necessary based on the
amount of 5% 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 and IR spectra of the final
product.