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
starte 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 alltogether 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.
Recrystallization.
Run mixed solvent tests to determine a solvent
pair that works well for the recrystallization
of the product. Also test water and ethanol
each by itself as potential solvents. (See
page 188-189 in Mohrig for the procedures
involved in mixed solvent recrystallizations.)
Use the solvent mixture determined to be most
effective to recrystallize the remainder of
your crude product.
Characterization. Obtain
the mp as well as 1H and IR
spectra of the final product.