CHEM 310 L Summer 2
Experiment 1
Organic Chemistry Laboratory
Name: _________________________________________________________________
Grignard Synthesis: Preparation of Benzoic Acid
Br
1. Mg / Et2O
2. CO2
3. H3O+
O
OH
A Classical method for making new carbon-carbon bonds uses reactants containing magnesium
carbon bonds, known as Grignard reagents. Grignard regents react with a variety of classes of
organic compounds such as aldehydes, epoxides, esters, acid halides, and nitriles. Compounds
containing metal-carbon bonds are called organometallics.
Background: Victor Grignard, a French chemist at the turn of the twentieth century discovered that
organic halides (R-X, X=Cl, Br, or I) and magnesium react to form organomagnesium halides (R-Mg-X,
X=Cl, Br, or I). In 1912 he was awarded the Nobel Prize for this discovery. Grignard reagents may be
formed by reacting elemental magnesium with primary, secondary, and tertiary alkyl halides, or aryl and
vinyl halides.
Assorted Grignard Reagents:
Mg Br
Br
Br
MgBr
H3C
H3C
CH 3
Mg
Metal
CH 3
H3C Br
CH 3MgBr
Br
MgBr
Br
Br
MgBr
MgBr
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CHEM 310 L Summer 2
Experiment 1
Preparation of Benzoic Acid:
Phenylmagnesium bromide will be used to produce benzoic acid via reaction with solid carbon
dioxide. The resulting salt will be quenched with hydrochloric acid to give the final organic carboxylic
acid.
You must keep the reaction apparatus and reagents completely dry because water functions as
an acid and would kill the Grignard reagent, to produce benzene. All glassware must be clean and dry.
Do not wash your glassware at the start of the experiment. When you have finished the experiment,
please clean your glassware and place all pieces back in the oven.
Reaction:
Br
bromobenzene
Mg
Br
Mg
O
C
O
phenylmagnesium bromide
Grignard Reagent
O
+MgBr
O
O
H+
OH
magnesium bromide
carboxylate salt
benzoic acid
Reagent Table:
Name
Structure
Magnesium
Mg
MW
(g/mol)
Mp.
Bp.
Density
(˚C)
(˚C)
(g/ml)
34-40
0.7134
Br
bromobenzene
Hydrochloric
Acid
Amount
Needed
HCl
36.46
(need 6M)
Ether
Et-O-Et
Magnesium
Sulfate
MgSO4
74.12
Drying
Agent
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CHEM 310 L Summer 2
Procedure:
Experiment 1
Making the Grignard Reagent
 Weigh out 1.0g magnesium ribbon or turnings, scrape the sides of the ribbon or crush the
turning to expose a clean metal surface (beneath the outer metal oxide coating).
 Place the magnesium into a 100ml r.b. flask.
In the separatory funnel:
♦ Add 4.5ml dry bromobenzene
♦ Add 15ml dry diethyl ether
 Add approx. 3ml of this bromobenzene/Et2O solution from the sep funnel to the magnesium
 As the reaction proceeds, you will observe the development of cloudiness and darkening of the
solution. This may take 5-10 minutes. It depends on how fresh the metal is and if it has a clean
surface.
 Once the reaction has started, turn on the cooling water in the condenser and manually mix the
solution. Add the remainder of the bromobenzene/ether solution SLOWLY and DROPWISE over
30 minutes. Adding the bromobenzene too rapidly may increase the formation of biphenyl by
product. Once all of the bromobenzene/ether solution has been added, rinse the separatory
funnel with 3ml ether.
 Heat the reaction flask while maintaining a reflux for 10 minutes (after all of the
bromobenzene/ether and 3ml wash have been added to the reaction vessel).
 After heating, allow the flask to cool to room temperature and add 20 ml of ether.
 As quickly as possible, weigh out 10g of dry ice and place it in a clean dry beaker. Cover the
beaker with a watch glass to minimize the condensation of water on the surface of the dry ice.
The Grignard Reaction
 Transfer the Grignard solution to the beaker containing dry ice. Swirl and mix the reagent and
dry ice. Do not worry if you have an excess amount of dry ice. It will sublime.
 Neutralize the solution with enough 6M HCl to make the solution acidic. Any extra and
unreacted magnesium metal should dissolve in the hydrochloric acid.
 If any solids are present in the beaker – other than Magnesium, add a little more ether to
dissolve the residual solid (this is your product).
 Decant and/or use a pipet to transfer the upper ether layer to an Erlenmeyer flask. Dry the
ether with magnesium or sodium sulfate.
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CHEM 310 L Summer 2
Experiment 1
 Gravity filter the solution into a tared Erlenmeyer flask and boil off the solvent using a hot water
bath.
 Keep your sample until tomorrow.
 Recrystallize your compound from a minimal amount of water
 Collect the product on a Büchner funnel and air dry the solid as best as you can
 Determine the m.p. of your sample the next day.
Reaction Apparatus:
Drying Tube
100ml round bottom flask
Claisen adapter
Addition (separatory) funnel
Separatory Funnel
West Condenser
Condenser
Drying tube with CaCl2
Claisen Tube
Round Bottom Flask
Reaction Apparatus
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CHEM 310 L Summer 2
Experiment 1
Name:
Data Sheet
Mass of the benzoic acid recovered: ____________________g
Theoretical Yield calculation:
Theoretical Yield: ____________________%
Percent Yield: ______________________%
Post Lab Question 1) Draw the product for each reaction:
1. Mg / ether
Br
O
2.
1.
H
3. H3O+
OH
2.
MgBr
H3C
CH3
Post Lab Question 2) Propose an experimental procedure you could utilize to physically separate
benzoic acid from a possible byproduct (biphenyl) in a laboratory setting.
biphenyl
O
OH
reagent ?
benzoic acid
in Et2O
Discussion (continue on the back of this page):
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