Chemistry 201
Clark College
Carboxylation of an Aromatic Bromide
Read:
F&F Technique 3.4: Chemically Active Extractions
F&F Technique 14.4 and 14.5: Adding Reagents and Excluding Moisture.
Introduction
In this experiment, you will be assigned an aromatic bromide, either bromobenzene
or p-bromoanisole. From that compound, you will prepare a Grignard reagent (an
organomagnesium bromide compound), and then react it with carbon dioxide to for the
corresponding aromatice carboxylic acid. The product will be characterized by melting
points, IR and NMR spectroscopies (literature). You will not be told which of the two
aromatic bromide compounds you are using, but you will be asked to identify your acid
product from the physical and spectroscopic data produced.
Br
Br
H3C
O
p-bromoanisole
Bromobenzene
Formation of a Grignard reagent is, in essence, a redox reaction. The starting
organic halide is reduced and receives two electrons per molecule, whereas the
magnesium metal is oxidized and loses two electrons per atom:
Br
Mg2+Br-1
Mg0
The end result is that the organic portion of the Grignard reagent now behaves like a
potent base or nucleophile. The inorganic portions are now in their favored oxidation
states now, and are thus inert to further reactions.
The organic portion can react as a base by abstracting a proton, so Grignard
reagents are extremely sensitive to –OH groups, such as carboxylic acids, alcohols and
water.
Mg2+Br-1
RO-Mg2+Br-
ROH
In the absence of –OH groups, the Grignard reagent acts as a strong nucleophile
and can add to the electrophilic carbon of carbonyl groups. In this experiment, the
grignard reagent will be added to CO2. Subsequent acid work-up yields the carboxylic
acid.
Mg2+Br-1
O
C
O
O
O-Mg2+Br-
HX
OH + MgBrX
O
Fall 2004
Carboxylation of an Aromatic Bromide
Page 1 of 3
Chemistry 201
Clark College
Experimental Procedure
If your last name begins with A – H, use aromatic bromide A
If your last name begins with L – Y, use aromatic bromide B
Drying Tube
Clean and dry a 100-mL
round-bottom flask, a
condensing column, a
separatory funnel, a 10-mL
graduated cylinder, a ground
glass stopper, a claisen
adapter and a thermometer
adapter. All glass pieces
should be drying in an oven for
at least 30 minutes. Be sure
to not put any plastic in the
oven! Carefully remove your
glassware, and assemble the
reaction apparatus while the
glassware is still warm.
Crush some magnesium turning with a mortar and pestle, then weigh out 0.5 g of
the crushed magnesium turnings. Place these turnings in the round-bottom flask, with a
stir bar. Using a pre-weighed 10-mL graduated cylinder, measure approximately 2.0 mL of
your assigned aromatic bromide (ArBr), and reweigh the cylinder to determine the exact
mass of the ArBr. Transfer the ArBr to the addition funnel, rinse the graduated cylinder
with 2 portions of 5-mL of anhydrous ether (10 mL ether total), and add this to the addition
funnel. If you have not done so already, turn the water on in the condenser. Add
approximately half of the ether mixture from the addition funnel to the magnesium in the
round bottom flask, and begin stirring the magnesium-ArBr mixture. Add an additional 7
mL of anhydrous ether to the addition funnel, and seal the funnel with a ground glass
stopper. Turn the hot plate on low heat. At this point, the reaction should begin, and the
solution will become cloudy and grey/green in color. If the reaction does not start, remove
the addition funnel and carefully crush the magnesium turnings with a stirring rod. If it still
does not start, seed the reaction with a small crystal of iodine, and/or heat the reaction with
a heat gun.
Once the reaction is self-sustaining, insert a small piece of filter paper between the
ground glass stopper and the neck of the addition funnel, to prevent a vacuum. Add the
remaining ArBr-ether solution slowly over 5 minutes, keeping the reaction mixture at a
gentle boil. Rinse the addition funnel with 1 – 2 mL of additional ether, and add this to the
reaction mixture. Remove the addition funnel and replace it with a ground glass stopper
(you will need it again later, so clean it while the reaction proceeds). Continue heating the
Fall 2004
Carboxylation of an Aromatic Bromide
Page 2 of 3
Chemistry 201
Clark College
reaction gently for 15 minutes, until most of the magnesium dissolves. Remove the
solution from heat and cool to room temperature.
Pour the cooled grignard reagent onto 10 g of crushed dry ice in a beaker, leaving
any unreacted magnesium in the flask. Rinse the round-bottom flask with 2 mL of ether
and add to the dry ice mixture. Place a watch glass over the beaker and place it in the
hood, until the dry ice has completely reacted. Slowly add 8 mL 6M HCl while stirring the
solution with a stirring rod. If any Mg is still present, add a few additional mL HCl. Add
ether, a few milliliters at a time, until most of the solid has dissolved (approx. 8 – 10 mL
ether). The bottom layer should be a thick emulsion. Stir the solution well, scraping the
solid off of the beaker. Allow the mixture to settle for a few minutes, and pipet off the
organic layer. Rinse the emulsion with 5 mL of additional ether, stir, and pipet off the
resulting organic layer. Repeat this wash once more, combining all organic extracts.
Transfer the organic extracts to the separatory funnel and extract with 3 x 5 mL 5%
NaOH. Combine the aqueous layers, and heat them, with stirring, for 5 – 10 minutes to
remove any ether. Cool the solution, then add 10 mL 6M HCl to precipitate the solid.
Filter and wash with ice-cold water. Dry the product and obtain a yield, IR and melting
point.
Fall 2004
Carboxylation of an Aromatic Bromide
Page 3 of 3