Essay About Grignard Reaction Of Phenyl Magnesium Bromide And Ml Separatory

Experiment – Grignard Reaction of Phenyl Magnesium Bromide with Benzophenone to Form Triphenylmethanol
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CH224 –Organic Chemistry Laboratory II
Experiment #6 –Grignard Reaction of Phenyl Magnesium Bromide
With Benzophenone to form Triphenylmethanol
BACKGROUND
In this experiment, we will be synthesizing triphenylmethanol from bromobenzene and
benzophenone by preparing the Grignard reagent phenyl magnesium bromide (I) from
bromobenzene and magnesium metal. The Grignard reagent will then be reacted with
benzophenone to form triphenylmethanol (II), the same compound you used in Experiment 1
earlier in the semester.
then H
In a Grignard reaction, an organohalogen compound is reacted with magnesium metal
to form a Grignard Reagent, which is carbanion with a Mg2+ Brcounterion.
The Grignard
reagent then attacks a carbonyl compound (aldehyde, ketone, or ester) to form an alkoxide
STEP 1: Formation of the Grignard Reagent
R-Br + Mg R MgBr
STEP 2: Nucleophilic Attack of the Grignard Reagent on the Carbonyl Group
R-Br + Mg R MgBr
an alkoxide ion
STEP 3: Protonation of the Alkoxide Ion to the Alcohol
O MgBr OH
+ H+, H2O + MgBrOH
The generation of the Grignard reagent and its reaction with benzophenone must be
carried out using dry glassware (dried with a Bunsen Burner first) and in an atmosphere where
the water vapor has been removed by a drying reagent, in this case calcium chloride. If water
is permitted to enter the reaction flask, it will react with the Grignard reagent (a very strong
Bronsted base) in a side reaction to form benzene.
_ MgBr H
+ H2O + MgBrOH
Benzene
If bromobenzene is added too quickly to the magnesium metal, another side reaction
will occur, where the Grignard reagent will react with excess bromobenzene to form biphenyl.
This reaction is called a Wurtz Coupling Reaction.
Biphenyl
To minimize the extent of this coupling reaction, the bromobenzene is usually added very
slowly to the magnesium metal, so that there is never an excess of bromobenzene present.
This is not very practical in this experiment, so your final Grignard product will contain some
biphenyl side product. This will be removed by washing the final triphenylmethanol product
with petroleum ether (hexanes). The petroleum ether will dissolve the non-polar biphenyl,
while the product triphenylmethanol does not.
PROCEDURE
Make sure that each of the following pieces of glasware is clean and DRY (no
noticeable water droplets). If the glassware is wet, then wash it with acetone and use suction
to draw air through the glassware to evaporate the acetone.
100 mL rb flask Claisen Head Adapter
Condenser Thermometer Adapter
Drying Tube 125 mL Separatory/Addition Funnel
10 mL Graduated Cylinder 50 mL Erlenmeyer Flask
100 mL Graduated Cylinder A large magnetic stirring bar
Assemble the glassware according to the picture below (Note: you are using a 125 mL
separatory/addition funnel, not the 50 mL one shown):
Preparation of the Grignard Reagent. Weigh out 0.45 g of magnesium turnings and
place them in the 100 mL round-bottom flask. Add two crystals of iodine and a large magnetic
stirring bar to the flask, reassemble the apparatus, and prepare a drying tube by placing some
glass wool in the drying tube and loosely add enough calcium chloride pellets fill the round part
of the drying tube. The drying tube should not be packed too tightly. Do not attach the water
hoses to the condenser just yet.
Light your Bunsen burner and adjust the air intake to a cool uniform blue flame between
two and three inches high. With the burner in hand, quickly direct the flame over the lower
portion of the round-bottom flask, observing the displacement of water from the glass and
quickly move the flame up the glassware so the water vapor is pushed toward and out the
drying tube. Do not heat excessively around the ground-glass joints and the Teflon stopcock
of the separatory funnel. The added iodine crystals should have vaporized during this
procedure