Grignard Reagent / Reactions
Preparation of a Grignard Reagent
(Phenylmagnesiumbromide) and reaction with
Carbon Dioxide to form Benzoic Acid
through an Electrophilic Addition reaction
References:

Slayden – p. 99 - 102

Pavia – p. 303 – 309; 313 – 314

Schornick
http://classweb.gmu.edu/~jschorni/chem318
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Grignard Reagent / Reactions
Synthesis of Benzoic Acid – Electrophilic Addition reaction
between a freshly prepared Grignard Reagent
(Organomagnesium Halide) acting as the Nucleophilie (Lewis
Base) and the Carbonyl group of Carbon Dioxide acting as the
positively charged Electrophile
 The Grignard Reagent must be prepared and reacted with the
solid Carbon Dioxide before the end of the day
 Sonication of mixture of Magnesium, Bromobenzene &
Ether (Ether acts as solvent and Grignard stabilizer)
 Reaction between Grignard Reagent and Carbon Dioxide
 Hydrolysis of Acid Salt
 Product Separation – Liquid/Liquid Extraction
 Precipitation
 Vacuum Filtration
 Product Purification– Recrystallization
 Product Verification– Melting Point

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Grignard Reagent / Reactions

Laboratory Report:

Synthesis Experiment


Mass, Moles, Molar Ratio, Limiting Reagent,
Theoretical Yield
Procedures

Title – Concise: Simple Distillation, Dry Sample, IR
Spectrum, etc.

Materials & Equipment (2 Columns in list (bullet)
form)
Note: include all reagents & principal equipment used

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Description:

Descriptions must be detailed, but concise

Use list (bullet) form

Use your own words (don’t copy book)
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Grignard Reagent / Reactions

Laboratory Report: (Con’t)

Results – Neat, logically designed template to present
results

Summary


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Paragraph summarizing experimental observations,
computed results, and principal absorptions from IR
& NMR
Analysis & Conclusions

Limiting reagent

Discuss the reaction in context with the results you
obtained; what evidence do you have to indicate
you got the intended product?

Yield – How could the experimental process be
improved?
4
Grignard Reagent / Reactions

Background:

Grignard Reagents, aka, Organomagnesium Halides

Prepared by Reaction of an Organic Halide and
Magnesium metal turnings, usually in an Ether solvent

Relative Reactivity of Halides is important
RI > RBr > RCl > RF
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
Very few Organomagnesium Fluorides are produced.

Aryl Grignards are more easily prepared from Aryl
Bromides and Aryl Iodides than Aryl Chlorides

Grignard Reagents are seldom isolated, but they can
be stabilized briefly in Ether solution and then used in
the preparation of other compounds

The preparation of a Grignard Reagent usually
produces a high yield (85 – 95%)
5
Grignard Reagent / Reactions

Reactions:
–
–
–
–
–
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–
+
+
–
+
–
+
+
–
–
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Grignard Reagent / Reactions

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Reactions (Con’t):

The Alkyl portion of the Grignard reagent behaves as
if it had the characteristics of a Carbanion – the
carbon atom bears a formal negative charge

The Magnesium portion bears the positive charge

Grignard Reagent, thus, acts as a Strong Base and is a
strong Nucleophile that acts as a Lewis Base donating
an Electron pair to a positively charged Electrophile

Especially useful in Nucleophilic Addition reactions of
the Carbonyl group, which has electrophilic character
where the positive charge of the carbonyl carbon
atom is attractive to the Grignard Nucleophile
7
Grignard Reagent / Reactions

Reactions (Con’t):
Grignard & CO2 to produce Benzoic Acid
Grignard & Ketone to Produce Tertiary Alcohol
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Grignard Reagent / Reactions

Reactions (Con’t)
Grignard & Aldehyde to Produce Secondary Alcohol
Grignard & Formaldehyde to Produce Primary Alcohol
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Grignard Reagent / Reactions

Grignard with Benzophenone to form
Triphenylmethanol 



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

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Grignard Reagent / Reactions

Grignard with Dry Ice (Solid CO2 ) to form Benzoic Acid
Salt, which is then hydrolyzed with HCL to form Benzoic
Acid
Today’s Experiment
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Grignard Reagent / Reactions

Preparation of Benzoic Acid:

Experimental Precautions

The Grignard reaction is very reactive. It must be
used in a reaction as soon as it is created.

Moisture must be avoided. Water acts as an Acid in
this reaction; thus the acid proton competes for the
Magnesium in an acid – base reaction.
Therefore, moisture could result in the formation of
a Hydrocarbon – R-H
R:MgX + H:OH  R-H
Stronger
Base

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Stronger
Acid
+
Weaker
Acid
HO-
+ Mg+2 + X-
Weaker
Base
Anhydrous Ether is used to stabilize the Grignard
12
Grignard Reagent / Reactions

Use as little heat as possible to avoid formation of
Biphenyl
Note: The Biphenyl produced from excessive heat is not
the same as the Hydrocarbon formed in an acid - base
reaction above.
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
The Organic Ether layer contains Benzoic Acid,
Biphenyl and Inorganic Salts.

The Benzoic Acid is separated from the Biphenyl by
converting it to an aqueous soluble form by
washing with NaOH
13
Grignard Benzoic Acid Procedure

Procedure (Ultrasonic Bath Version)
NOTE: Water/Moisture must be avoided:
“All glassware must be “dry”
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
Weigh a little more than 0.5g of Magnesium (Mg) (MW –
24.31) to the nearest 0.001 g and place the Magnesium
in a large oven-dried Test Tube or Centrifuge Tube

Obtain a vial of Bromobenzene (MW – 157.01) and
weigh it to the nearest 0.001g

Transfer the Bromobenzene to the test tube containing
the Magnesium filings

Reweigh the empty vile to obtain the net mass of
Bromobenzene
14
Grignard Benzoic Acid Procedure

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Procedure (Ultrasonic Bath Version) (Con’t)

Add about 10 mL of Anhydrous Ether to the test tube
containing the Bromobenzene and Magnesium

Add a very small piece of “Iodine” to the mixture

Add a cotton ball to the top of the test or loosely
screw a cap on the Centrifuge tube (if used)

Place the tube in the Sonication Bath to stimulate the
reaction. The solution will begin to turn brownish/gray
as the reaction proceeds

After about 5 minutes in the Sonication bath, remove
the tube and gently swirl the mixture to insure
maximum exposure of the Magnesium filings to the
Bromobenzene
15
Grignard Benzoic Acid Procedure

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Procedure (Ultrasonic Bath Version) (Con’t)

Replace the tube in the Sonication bath for about 40
minutes or so

Remaining in the Hood, remove the tube containing
the Grignard reaction mixture from the Sonication
Bath

Pour the solution over approximately 10 g of Dry Ice
(solid CO2) in a labeled 150 mL beaker. Leave the
unreacted Magnesium particles in the Centrifuge tube

Rinse the tube with a few mL of Ether and add the
Ether to the beaker containing the product

Store labeled beaker containing your product in the
hood until next lab session
16
Grignard Benzoic Acid Procedure
 Week 2 - Hydrolysis & Purification of Benzoic Acid

Slowly add about 10 mL of 6 M HCl to the beaker
containing the product

Stir and scrape contents of the beaker with the spatula
until the solids and liquid form a uniform mixture

The mixture may show some reaction activity, such as
frothing and increased temperature, which might include
the reaction between any remaining Magnesium and the
HCl to evolve Hydrogen

add 10 mL Methyl –t-Butyl Ether to the beaker

Continue to stir and scrape the mixture with a spatula

If solids still persist in the solution, use a medicine
dropper and add small additional amounts of Methylt-Butyl Ether and stir
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Grignard Benzoic Acid Procedure

Week 2 - Hydrolysis & Purification of Benzoic Acid

If some solids are still insoluble in either the ether
layer or the aqueous layer, add small amounts of the
6 M HCl and stir.
Keep track of the volume of HCL added

There should be two distinct clear layers in the beaker
Note: The Benzoic Acid (and any by-product
Biphenyl) is soluble in the Ether
The inorganic compounds (MgX2) are soluble
in the aqueous phase
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Grignard Benzoic Acid Procedure

Week Two - Hydrolysis & Purification of Benzoic Acid

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Liquid/Liquid Extraction:

Transfer the mixture to a 125 mL Separatory funnel

Rinse sides of beaker with additional ether and add
to Separatory funnel

Insert stopper and secure it with your finger

Shake the mixture gently, invert funnel, point stem
up and away from your face and slowly open
stopcock to vent any gas

Allow the layers to separate (remove stopper)

Discard the aqueous (lower) phase into a separate
waste beaker and put aside
19
Grignard Benzoic Acid Procedure

Week Two - Hydrolysis & Purification of Benzoic Acid
 Liquid / Liquid Extraction with Distilled Water (Con’t)
● Add 5 mL distilled water to the organic solution
● Shake the mixture gently, invert the funnel, and
slowly open stopcock to vent any gas
● Allow the layers to separate (remove stopper)
● Discard aqueous phase
● Repeat the liquid/liquid extraction with an
additional 5 mL of distilled water
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Grignard Benzoic Acid Procedure

Week Two - Hydrolysis & Purification of Benzoic Acid

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Liquid / Liquid Extraction with Sodium Hydroxide

To the organic layer, which contains the Benzoic
Acid and by-product Biphenyl, add 5 mL 5% NaOH

Shake the mixture gently, invert funnel pointing
away from your face, and slowly open stopcock to
vent any gas

Allow the layers to separate (remove stopper)

Drain aqueous (bottom) layer into clean, dry 100 ml
beaker

Repeat liquid/liquid extraction with another 5 ml
5% Sodium Hydroxide, draining bottom layer into
your beaker

Repeat liquid/liquid extraction a 3rd time with
5% Sodium Hydroxide
21
Grignard Benzoic Acid Procedure
 Week Two - Hydrolysis & Purification of Benzoic Acid

Liquid / Liquid Extraction with Sodium Hydroxide (con’t)
Note: The Sodium Hydroxide converts the Benzoic
Acid to the water-soluble Sodium Benzoate salt
The Sodium Benzoate is thus transferred
to the aqueous phase

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Discard the organic Ether layer into the waste jar in
the hood
22
Grignard Benzoic Acid Procedure
 Week Two - Hydrolysis & Purification of Benzoic Acid

Remove Excess Ether

Gently heat the combined aqueous extracts (about
15 mL) containing the Benzoic Acid salt on a hot
plate (set setting at 3-4) until formation of small
bubbles (Ether) ceases. This removes any residual
Ether, which is slightly soluble in water
Note: Don’t overheat; heat just enough to boil off
Ether
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Grignard Benzoic Acid Procedure

Week Two - Hydrolysis & Purification of Benzoic Acid

Cool the solution to room temperature

Add 10.0 mL 6 M HCl and stir (a precipitate forms in
the beaker)
Note: If little or no precipitate forms add an additional
3 ml HCL. If no precipitate forms inform instructor
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
Cool mixture in water/ice bath

Vacuum Filtration

Vacuum filter the product using a Buchner Funnel

Wash crystals on the filter 2 times with 5 mL cold
distilled water
24
Grignard Benzoic Acid Procedure

Week Two - Hydrolysis & Purification of Benzoic Acid

Recrystallize product from hot (near boiling) water

Transfer solid from Buckner Funnel to 100-150 mL
beaker using cold Distilled Water to insure complete
transfer

Add an additional 15 mL of distilled water to
mixture

Heat solution to just below boiling point, swirling
the mixture occasionally until all solids go into
solution
Note: The Benzoic Acid solution gives off a strong
pungent vapor; avoid proximity to nose

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Check with instructor if you feel additional Distilled
Water is needed to effect total solution
25
Grignard Benzoic Acid Procedure

Week Two - Hydrolysis & Purification of Benzoic Acid

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Recrystallize Product (Con’t)

Allow solution to cool slowly to room temperature
effecting recrystallization of the purified product
(Soluble impurities remain in solution

If the cooled solution does not appear to have
sufficient water to suspend the purified crystals as a
slurry, add 10 ml of Distilled Water, redissolve the
mixture, and cool again slowly to room temperature

After mixture has reached room temperature, place
the beaker in an ice/water bath for about a minute
26
Grignard Benzoic Acid Procedure

Week Two - Hydrolysis & Purification of Benzoic Acid

2nd Vacuum Filtration

Clean and dry the Buckner Funnel, add a new filter,
and weigh it to the nearest 0.001g
● Assemble the Buckner Funnel and vacuum filter the
product
● Wash crystals on the filter with 5 mL cold Distilled
Water
● Dry the product in a pre-weighed weighing tray in
the class drawer until next week
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Grignard Benzoic Acid Procedure
Week Two - Hydrolysis & Purification of Benzoic Acid

Weigh the dried product & weighing tray to the
nearest 0.001g

Compute the mass of product (Benzoic Acid) by
difference

Determine the % yield

Determine the melting point of the product

(MP of Benzoic Acid – 122.4oC)
NOTE: If little or no yield was obtained, see next
slide
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Grignard Benzoic Acid Procedure
The Experiment was a Bust!!

Occasionally students get little or no yield for the
Grignard / Benzoic Acid experiment

The principal reason for this is the failure of the original
reaction between Bromobenzene and Magnesium to
form the Grignard Reagent

There are two main indications of a failed reaction:
3/10/2016

No precipitate forms upon the addition of the 6 M HCL
to the aqueous NaOH washings

The precipitate largely disappears after the first
vacuum filtration
29
Grignard Benzoic Acid Procedure
The Experiment Was a Bust

3/10/2016
In order to give the students full exposure to the
complete procedure, the following steps should be
performed:

Weigh out to the nearest 0.001 g approximately 3
grams of Benzoic Acid supplied by the instructor

Dissolve the Benzoic Acid in 15 mL Anhydrous Ethyl
Ether.

Wash the Ether phase 3 times with 5 mL 5% NaOH
as before combining the 3 aqueous washings in a
beaker

Discard the Ether
30
Grignard Benzoic Acid Procedure
The Experiment Was a Bust
3/10/2016

Gently heat the NaOH solution, approximately 15
mL, to drive off any residual Ether

Cool the solution to room temperature

Add 10 mL HCL, as before, and place in water-ice
bath

A precipitate should form

Separate the precipitate by vacuum filtration

Wash product on filter with 5 mL cold water

Recrystallize the product from hot distilled water

Vacuum Filtration

Dry the Product

Determine the Melting Point
31
Grignard Benzoic Acid Procedure
Note: Lab procedure sequence vs. Lab report procedure
sequence
3/10/2016

The writing of the laboratory report does not always
follow the exact physical sequence of procedures used
in the lab

The measurement of the Mass of a reagent may be
immediately followed by an apparatus set-up or
transfer of the sample to a piece of apparatus

This same process could be repeated with the second
or a third reagent used in the experiment

In your report, however, all determinations of Mass
are followed immediately by all calculations that can
be based on the Mass values, i.e. moles, molar ratios,
limiting reagent, theoretical yield, etc.
32
Grignard Benzoic Acid Procedure

3/10/2016
In the Grignard experiment the following calculations
are carried out before any procedure dealing with the
mixing of the reagents and subsequent processing of
the reaction mixture

Mass of first reagent

Mass of second reagent (HCL – Vol & Density)

Moles of both reagents (1 procedure)

Set up Balanced Stoichiometric Equation

Determine Molar Ratio

Determine Limiting Reagent

Compute Theoretical Yield
33
Grignard Benzoic Acid Procedure

Use the following table form (handout) as a substitute
for the table in the Lab Manual Data Report.
Balanced
Equation
Mg
Ether
CO2
HCl
Mixed
Salts
Mol Wgt
Grams
Moles
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Grignard Benzoic Acid Procedure
Hydrolysis of
Benzoic Acid Salt
Ether Layer
8 mL 6 M HCL
20 mL Anhydrous Ether
Separatory Funnel (Shake Gently)
Benzoic Acid
Biphenyl
Aqueous Layer
Inorganic Salts (Discard)
Add 5 mL 5% NaOH
Aqueous Layer
(Repeat 3 Times)
Biphenyl (Discard)
Soluble Benzoic Acid Salt
(Trace of Ether)
Heat on Hot Plate in Hood to Drive off Ether
Cool to Room Temperature
Add 10 mL 6 M HCl
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