CARBON-CARBON BOND FORMATION:
GRIGNARD ADDITION TO CARBON DIOXIDE
SHAUN LYNN
J9106574
Abstract
A solution of bromo benzene in ether and magnesium were subjected to a nucleophilic
addition reaction to produce the Grignard reagent phenyl magnesium bromide which was
then reacted with crushed solid carbon dioxide diluted with sodium hydroxide and acidified
with dilute hydrochloric acid to produce benzoic acid. The benzoic acid product was purified
by recrystallisation then an infra red spectrum was obtained as well as a melting point for
comparison to published literature.
Introduction
Born May 6th 1871, François Auguste Victor Grignard was awarded the Nobel Prize in
Chemistry in 1912 for creating a new method of producing carbon – carbon bonds, using
magnesium to couple ketones and alkyl halides. (The Nobel Foundation 1912, 2011)
The Grignard reagent is prepared from a reaction of a halogenalkane with a suitable R-group
(typically an alkyl or aryl group) and magnesium turnings with the use of dry diethyl ether as
a solvent.
Grignard reagents are versatile intermediates in the synthesis of organic functional groups
and their reactions are extremely useful in organic synthesis, as they produce high yields of
carbon – carbon bonds which are highly specific. Grignard reagents are strong bases as well
as nucleophiles.
Aim
To prepare benzoic acid via the reaction of the Grignard reagent phenyl magnesium
bromide with carbon dioxide.
Hazards
Ethyl ether – extremely flammable and highly volatile
Solid carbon dioxide – may cause frostbite with prolonged contact
Bromo benzene – flammable irritant.
Hydrochloric acid – harmful, irritant may cause burns with prolonged contact
Handling advice – This should be carried out in a fume cupboard.
Method
A three neck adapter was attached to a dry 250cm³ round bottomed flask. A water cooled
condenser was dried, fitted with a CaCl2 drying tube and attached to the central neck of the
3 neck adapter.
A solution of dry bromo benzene (10.5cm³) was prepared in dry ethyl ether (50cm³), poured
into a dropping funnel and then attached to the 3 neck adapter.
Magnesium turnings (2.5g) were weighed and added to the round bottomed flask. 10cm³ of
the bromo benzene solution was then added to the round bottomed flask (just enough to
cover the magnesium turnings).
The round bottomed flask was then warmed over a water bath. After 10 minutes the
solution turned a cloudy pale yellow. The remaining bromo benzene was then added drop
wise. When all the solution had been added, the reaction vessel was left over the water
bath for a further 20 minutes.
Crushed solid carbon dioxide (20g) was weighed and added to the mixture, it was stirred
until all the carbon dioxide disappeared. Dilute hydrochloric acid (2M) in excess was then
added.
The mixture was then poured into a separating funnel (250cm³) and lightly shaken until all
the solid material dissolved. The liquid separated into two layers, an ethereal layer and an
aqueous layer. The aqueous layer was discarded and dilute sodium hydroxide (2M) was
added to the ethereal layer. The extracts were then collected in a conical flask and slowly
acidified with dilute hydrochloric acid (2M)
The precipitated benzoic acid was collected via filtration using a Buchner funnel. The sample
was then transferred to a conical flask to be recrystallised, however and unknown organic
solid contaminant was found in the bottom of the solution. The solution needed to be
decantered to separate the two.
The sample was then recrystallised again. Once cool, the sample was then filtered in a
Buchner funnel and the purer benzoic acid product was transferred to a watch glass and left
in a preheated oven (105°C) until dry.
Discussion
Mass of watchglass
Mass of Watchglass & product
Mass of product
= 36.43g
= 41.52
= 5.09g
Melting point range
= 121.9°C – 122.8°C
The Dictionary of organic compounds, 6th edition, Chapman and Hall, London, Volume 3(&
Volume 6), 1996 states that Pure benzoic acid has a melting point of 121.1°C – 122.35°C
Which suggests the sample of benzoic acid is quite pure.
Calculations
Mr of C6H5Br = 157.0079
𝟏𝟎. 𝟓𝒈𝑷𝒉𝑩𝒓 𝒙 (
𝟏
) = 𝟎. 𝟎𝟔𝟔𝟖𝟕 𝒎𝒐𝒍𝒆𝒔
𝟏𝟓𝟕. 𝟎𝟎𝟕𝟗
𝟏
𝟐. 𝟓𝒈 𝑴𝒈 𝒙 (𝟐𝟒.𝟑𝟎𝟓 ) = 𝟎. 𝟏𝟎𝟐𝟗 𝒎𝒐𝒍𝒆𝒔
As bromobenzene was the limiting reagent in the production of phenylmagnesium bromide,
0.06687 moles of PhBr will produce 0.06687 moles of PhMgBr.
Mr of PhMgBr = 181.3129
0.06687𝑚𝑜𝑙𝑒𝑠 𝑥 181.312 = 12.124𝑔
𝟏𝟐. 𝟏𝟐𝟒𝟑𝒈𝑷𝒉𝑴𝒈𝑩𝒓 𝒙 (
𝟐𝟎𝒈 𝑪𝑶𝟐 𝒙 (
𝟏
) = 𝟎. 𝟎𝟔𝟔𝟖𝟕 𝒎𝒐𝒍𝒆𝒔
𝟏𝟖𝟏. 𝟑𝟏𝟐𝟗
𝟏
) = 𝟎. 𝟒𝟓𝟒𝟒 𝒎𝒐𝒍𝒆𝒔
𝟒𝟒. 𝟎𝟎𝟗𝟓
For the overall reaction Bromobenzene is the limiting reagent.
Moles are then converted to grams
0.06687𝑚𝑜𝑙𝑒𝑠 𝑥 157.0079 = 10.5𝑔
Theoretical yield = 10.5g
Percentage yield =
5.09
10.50
𝑥100 = 48.476%
The Mechanism of the Grignard reactions is as follows:
R- +MgBr
+
O
O
C
C
O
H3O+
R
C
O- +MgBr
O
R
OH
This reaction takes advantage of the high nucleophilicity of the phenyl portion of the
Grignard reagent and the electrophilicity of the carbon atom of CO2. The Grignard reagent
attacks the carbon of CO2 to form a carboxylate salt that is readily converted to benzoic acid
by acidification with hydrochloric acid. The use of dry ice as the source of carbon dioxide
helps regulate the reaction, because the extremely low temperature of this solid moderates
the usual high exothermicity of Grignard additions.
IR interpretation
An O-H group is present in the molecule (identified by the region between 2500-3300 cm-1)
Absorption at 1677.55 shows that a C=O is present.
Absorption at 1288.67 cm-1 again shows the presence of an OH group.
Absorption at 931.65 cm-1 could be due to the benzene ring or to the carbon-oxygen bond of
the acid group.
The spectrum was obtained using a Diamond ATR spectrometer. The spectrum obtained
supports the statement that the sample is benzoic acid.
WORKS CITED
Clark, J. (2011, 04 04). Grignard Reagents. Retrieved 04
http://www.chemguide.co.uk/organicprops/haloalkanes/grignard.html
04,
2011,
from
chemguide:
OrganicChemistry.org. (2011, 04 04). Grignard Reaction. Retrieved 04 04, 2011, from Organic Chemistry Portal:
http://www.organic-chemistry.org/namedreactions/grignard-reaction.shtm
Unknown. (2011, 04 01). Microsoft Word - GRIGNARD_INSTR.doc. Retrieved 04 01, 2011, from
http://itech.pjc.edu/tgrow/2211L/grignard_instr.pdf
Dictionary of organic compounds, 6th edition, Chapman and Hall, London, Volume 3(& Volume 6), 1996
McMurry, J. (2008). Organic Chemistry. J. McMurry, organic Chemistry . London: Thompson Brooks/Cole.