Experiment - Grignard Reaction of Phenyl Magnesium Bromide with Benzophenone to Form Triphenylmethanol

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.

Br

+ Mg

MgBr

MgBr

+

O OH

then H

+

(I)

(II)

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

ion.

STEP 1: Formation of the Grignard Reagent

R-Br + Mg R MgBr

- +

STEP 2: Nucleophilic Attack of the Grignard Reagent on the Carbonyl Group

O

O

R-Br + Mg R MgBr

-

+

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.

_ Br

+

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

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