The lab report below was submitted as part of the coursework for CM1121 Basic Inorganic Chemistry. Please do not plagiarise from it as plagiarism might land you into trouble with your university. Do note that my report is well-circulated online and many of my juniors have received soft copies of it. Hence, please exercise prudence while referring to it and, if necessary, cite this webpage.
R—MgBr + O═C═O RBrMg+O—C═O R—COOH +
Mg2+ + Br-
The
lab report below was submitted as part of the coursework for CM1101
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The
lab report below was submitted as part of the coursework for CM1101
Basic Analytical Chemistry. Please do not plagiarise from it as
plagiarism might land you into trouble with your university. Do note
that my report is well-circulated online and many of my juniors have
received soft copies of it. Hence, please exercise prudence while
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The
lab report below was submitted as part of the coursework for CM1101
Basic Analytical Chemistry. Please do not plagiarise from it as
plagiarism might land you into trouble with your university. Do note
that my report is well-circulated online and many of my juniors have
received soft copies of it. Hence, please exercise prudence while
referring to it and, if necessary, cite this webpage. - See more at:
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The
lab report below was submitted as part of the coursework for CM1101
Basic Analytical Chemistry. Please do not plagiarise from it as
plagiarism might land you into trouble with your university. Do note
that my report is well-circulated online and many of my juniors have
received soft copies of it. Hence, please exercise prudence while
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The
lab report below was submitted as part of the coursework for CM1101
Basic Analytical Chemistry. Please do not plagiarise from it as
plagiarism might land you into trouble with your university. Do note
that my report is well-circulated online and many of my juniors have
received soft copies of it. Hence, please exercise prudence while
referring to it and, if necessary, cite this webpage. - See more at:
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The
lab report below was submitted as part of the coursework for CM1101
Basic Analytical Chemistry. Please do not plagiarise from it as
plagiarism might land you into trouble with your university. Do note
that my report is well-circulated online and many of my juniors have
received soft copies of it. Hence, please exercise prudence while
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The
lab report below was submitted as part of the coursework for CM1101
Basic Analytical Chemistry. Please do not plagiarise from it as
plagiarism might land you into trouble with your university. Do note
that my report is well-circulated online and many of my juniors have
received soft copies of it. Hence, please exercise prudence while
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The
lab report below was submitted as part of the coursework for CM1101
Basic Analytical Chemistry. Please do not plagiarise from it as
plagiarism might land you into trouble with your university. Do note
that my report is well-circulated online and many of my juniors have
received soft copies of it. Hence, please exercise prudence while
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The
lab report below was submitted as part of the coursework for CM1101
Basic Analytical Chemistry. Please do not plagiarise from it as
plagiarism might land you into trouble with your university. Do note
that my report is well-circulated online and many of my juniors have
received soft copies of it. Hence, please exercise prudence while
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The
lab report below was submitted as part of the coursework for CM1101
Basic Analytical Chemistry. Please do not plagiarise from it as
plagiarism might land you into trouble with your university. Do note
that my report is well-circulated online and many of my juniors have
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1. Aim:
To first
synthesize the Grignard reagent,p-tolyl
magnesium bromide, and use it for the subsequent synthesis of p-toluic acid.
3. Discussion:
3.1
Synthesis of Grignard Reagent:
During the first part of the experiment, Grignard reagent, p-tolyl magnesium bromide, was
synthesized by adding p-bromotoluene
and anhydrous tetrahydrofuran to magnesium turnings. To protect against
atmospheric moisture, this was carried out in a round bottom flask with two
guard tubes connected to the reflux condenser and dropping funnel. The guard
tubes were packed with anhydrous calcium chloride, which is hygroscopic [1].
The anhydrous calcium chloride prevented atmospheric water vapour from entering the
setup by acting asa drying agentas follows:
CaCl2 + 2H2O → CaCl2∙2H2O
This prevention was crucial asthe Grignard reagents were
highly sensitive to water[1] and would react rapidly and exothermically
with water, to produce alkanes as shown below.The hydrocarbon and magnesium
salt formed might have coated the surface of the unreacted magnesium turnings
and inhibited further formation of the Grignard reagent [2].
For example: R—MgBr +
H2O
R—H + Mg(OH)Br
R = alkyl chain
Tetrahydrofuran (THF) was used as a solvent for a
number of reasons. Firstly, THF was an organic solvent which dissolved organic
compounds including the Grignard Reagents. Also, the solvent was a neutral
aprotic solvent which did not participate in the reaction for it contained no
dissociable H+ ions that would react with the Grignard reagent to
form toluene. Thirdly, THF can be easily separated in subsequent steps to
obtain the crude product. While neutral, THF was still polar and helped to
stabilize the Grignard reagent through the formation of ligand bonds. The Grignard
reagents were unstable and highly reactive; any stabilization at the
covalent-ionic bond between the polarized carbon (more electronegative) and
magnesium would favor the forward reaction.
However, with THF as a solvent, another
consideration must be factored in as well. THF is a volatile solvent and the
synthesis of Grignard reagent is exothermic; these factors caused THF to
vaporize rapidly. The reaction mixture was thus refluxed and a condenser was
connected to cool the organic vapor back to the solvent state. This prevented
the Grignard reagent from “drying up” and reduced the probabilities of
byproducts forming.
As a good
measure, all reactants were added quickly and the guard tubes, replaced as soon
as possible. Besides reacting with water, Grignard reagent might have
participated in other side reactions, as shown below[3]:
With Oxygen: R—MgBr +
O2 ROO—MgBr RO—MgBr
The aforementioned steps sought to minimize the entry of
atmospheric oxygen and the subsequent decomposition to undesired side products.
The introduction
of p-bromotolueneand THFinto the
round bottom flask from the dropping funnel was done slowly in a drop-wise
manner. Asthe reaction between magnesium and the alkyl halide was strongly
exothermic, the significant amount of heat produced might evaporate the
solvent, forming byproducts. Drop-wise addition of the reagents reduced the
occurrences of side reactions[4].
Because
magnesium turnings were covered with an unreactive oxide layer, the reaction
was expected to be slow. Iodine crystals were added into the mixture to
catalyse the process by reacting with the magnesium turnings to form
iodine-activated magnesium, magnesium (I) iodide.Magnesium(I) iodide functioned
as an active agent in the formation of Grignard reagent as it was more soluble
and reactive than the magnesium metal alone. Since a small amount of iodine was
used and was regenerated at the end of the reaction, iodine served as a
catalyst [5]. The regeneration of iodine was indicated by the
reappearance of the brown colouration. While some side reactions occurred with
the use of such activating agents, the amount of byproducts formed were
considered to be insignificant relative to the expected mass of the Grignard
reagent; there was a slight compromise between maintaining the yield and rate
of reaction. The Grignard reagent had to be used immediately as it was unstable.
3.2Grignard reaction with Carbon Dioxide to synthesize p-Toluic
Acid:
In this second
part of the experiment, the freshly prepared Grignard reagent was added to
crushed solid carbon dioxide (dry ice) to form a carboxylic acid. Dry ice
served as both a reagent and a cooling agent[5]. The mechanism is
displayed below:
H+/H2O
|
The Grignard reagent attacked the
electrophillic carbonyl C atom to form a conjugate base of the p-toluic acid. Grignard reagent was
added quickly to thebeaker of dry ice as the dry ice sublimed quickly. The
resulting semi solid substance in the beaker was stirred continuously to ensure
maximum reaction between the reagents.
In
order to recover the carboxylic acid from its salt, concentrated hydrochloric
acid (HCl) and ice water was added to the beaker of bromomagnesium salt. As a
strong inorganic acid, HCl would dissociate completely in the presence of water
to give hydrogen (H+) and chloride ions (Cl-). Then, the
hydrogen ions protonated the salt to recoverp-toluic
acid; for once protonated, the acid was no longer soluble in water and would be
precipitated. De-ionized water was used as it contained no undesirable contaminants
such as fluoride compounds and trihalomethanes which might cause side reactions
to occur and reduce the percentage yield. A cold aqueous work-up with low
temperature caused further precipitation and compensated for the significant
heat of neutralization released when the concentrated acid was added. The
solution was tested with a Congo Red paper, which turned blue, to ensure that
the solution was acidic, and that the toluic-salt had been completely
protonated and precipitated[6].
3.3 Separation by solvent extraction
Prior
to separation, the separatory funnel was filled with de-ionized water and
checked for leakage from the stopcock. It was then inverted to confirm that the
stopper was not leaking. This ensured that there would be no loss of reactants
due to equipment flaws as the solvent extraction progressed.
In order to separate the solvent from
the crude product,dichloromethane (DCM) was used to completely dissolve the
crude product. To ensure that all crude product was collected, the beaker was
swirled with DCM and the washings added to the separatory funnel.The theory of
solvent extractionsuggested that additional solvent and multiple extractions
would result in a greater separation, as described in the equation below[7]:
where: q is the fraction remaining after one
extraction
n is the number of extraction carried
out
V1is the volume of solute in
phase 1
V2is the volume of solute in
phase 2 (the extraction solvent)
K is the partition coefficient
DCM, being an organic solvent, dissolved
the organic p-toluic acid due
to favorable solute-solvent interactions.
However, DCM dissolved organic impurities too, including THF, unreacted p-bromotoulene and undesired byproducts.
To solve this problem, sodium hydroxide (NaOH) was added to form a conjugate
base of p-toluic acid, as shown below:
R-COOH + NaOH → R-COO-Na+
+ H2O
This acid-base neutralization allowedp-toluic acid to
form an ionic salt whichwas soluble in water and thus, be extracted into the
aqueous phase. The organic impurities would remain dissolved in the organic
phase.
The separatory
funnel was shakenvigourously in the fumehood. Shaking ensured that there was
even mixing between the reagents such that most of the p-toluic acid came into contact with NaOH, reacted to produce a
water-soluble conjugate base and was extracted into the aqueous phase. As DCM
was volatile, the mechanical agitation would cause it to vaporize. Built-up
pressure in the funnel was released by slowly opening the stopcock after a few
shakes. The separatory funnel was left to stand to allow the mixture to
separate into two immiscible layers.
DCM, being
denser, would be collected at the bottom of the separatory funnel, while the
aqueous layer, containing the conjugate base of p-toluic acid, would be collected at the top.The DCM layer was
collected and multiple extractions were repeated to ensure maximal separation
of compounds. The resultant aqueous layers were combined and cooled in an
ice-water bath.
In
order to recover the p-toluic
acid
from its salt, concentrated hydrochloric acid (HCl) to the beaker containing
the aqueous layers. As a strong acid, HCl would dissociate completely in the
presence of water to give hydrogen (H+) and chloride ions (Cl-).
Then, the hydrogen ions protonated the conjugate salt and precipitated p-toluic acid. A cold aqueous condition with low
temperature caused further precipitation and compensated for the significant
heat of neutralization released when the concentrated acid was added. The
solution was tested with a Congo Red paper, which turned blue, to ensure that
the solution was acidic, and that p-toluic
acid
had been completely protonated and precipitated.
3.4 Purification by recrystallization
The most common method of purifying solid organic compounds is by
recrystallization[8]. In this technique, an impure solid compound is
dissolved in a solvent and then allowed to slowly crystallize out as the
solution cools. As the compound crystallizes from the solution, the molecules
of the other compounds dissolved in solution are excluded from the growing
crystal lattice, giving a pure solid.
Crystallization of a solid is not the same as precipitation of a
solid. In crystallization, there is a slow, selective formation of the crystal
framework resulting in a pure compound. In precipitation, there is a rapid
formation of a solid from a solution that usually produces an amorphous solid
containing many trapped impurities within the solid's crystal framework. For
this reason, experimental procedures that produce a solid product by
precipitation always include a final recrystallization step to give the pure
compound.
The process of recrystallization relies on the
property that for most compounds, as the temperature of a solvent increases,
the solubility of the compound in that solvent also increases. p-toluic acid is insoluble in ethanol at room temperature, soluble
in the boiling solvent and has a higher boiling point than the solvent. The
heating rate was controlled such that ethanol was not completely evaporated
off, thereby preventing the product from decomposing.
After p-toluic acid
dissolved in boiling ethanol, water was added to the mixture until the mixture
became cloudy, indicating that p-toluic
acid was precipitating from the solution. A few drops of ethanol was added to
redissolve the precipitate, producing a clear solution. This ensured that the
solution was saturated. The solvent-pair of ethanol-water was chosen as the two
solvents were miscible with each other, but have opposite abilities to dissolve
p-toluic acid. The acid was soluble
in ethanol and was relatively insoluble in water.
The
mixture was left to cool to room temperature for 10 minutes before being placed
in an ice-bath for 10 more minutes. If crystal formation occurred too rapidly,
impurities may become trapped in the crystals. The p-toluic acid crystals were then obtained by vacuum filtration and
tested for its purity by determining its melting point.
3.5 Discussion on experimental and theoretical yield
From
the results obtained, the experimental yield was only 55.3%. This yield, while
reasonable, could be higher if not due to several factors. One reason might be
due to the destruction of Grignard reagent by atmospheric water vapourand oxygen
while it was prepared, despite the use of calcium chloride guard tubes. Also,
some reactants dripped out from the inverted separatory funnel’s stopper due to
internal accumulated pressure, before the stopcock could be opened.
Melting point
range was used to determine the purity of the compounds. Pure crystalline
compounds possess characteristic melting points; therefore any deviations in
the experimental melting points of the compounds would mean the presence of
impurities. From the results below, the experimental melting point range was
within its given literature value:
Compound
|
p-Toluic
Acid
|
Theoretical
melting point range
|
178°C – 182°C
|
Experimental
melting point range
|
178.8°C – 179.5°C
|
Table comparing experimental and theoretical melting point ranges of
p-toluic acid
This translated to a high
percentage purity of p-toluic acid.
4. Conclusion:
Grignard reagents are extremely
useful in extending the carbon chain on organic compounds. However, as they are
reactive and unstable, the process of synthesizing and using Grignard reagents
has to be conductedunder strict experimental conditions. In this experiment, a
Grignard reagent, p-totylmagnesium bromide, was
synthesized from p-bromotoluene and
this reagent was subsequently reacted with dry ice, solid carbon dioxide, to
form p-toluic acid. The final
purified product obtained was 3.01g of white needle-like crystals with a
melting point range of 178.8°C – 179.5°C and a percentageyield of 55.3%. The
experimental melting point range of p-toluic
acid compared favorably to its literature values, suggesting that the eventual
product was of a high purity.
5. References:
[1]: Jim Clark. 2003. An Introduction to Grignard
Reagents. [Internet][cited 2010 February 7th] Available from: http://www.chemguide.co.uk/organicprops/haloalkanes/grignard.html
[2]: Cotton, Wilkinson, Gaus,1995: Basic Inorganic
Chemistry, 3rd Edition, Pg309
[3]: Daniel
C. Harris, 2003: Quantitative Chemical Analysis, 6th edition
[4]:
John McMurry, 2004: Organic Chemistry, International Student Edition, 6th
Edition
[5]:
Experiment 5: General Grignard Procedure. [Internet][cited 2010 February 7th]
Available from: http://www.cs.moravian.edu/~rdlibby/_211-212Chem-PDF/Laboratory/Experiments/212-09Lab/Expt5-Alkene-Syn/_Expt_5_Alkene-syn-wk1-gen-proc.pdf
[6]:
Gary S. Silverman. Philip E. Rakita. 1996: Handbook of Grignard Reagents.
Marcell Dekker.
[7]:
Peter Taylor. 2002. The Molecular World: Mechanism and Synthesis. The Royal
Society of Chemistry.
[8]:
Recrystallization Techniques. [Internet][cited on 2010 February 10th]
Available from: http://www.sas.org/E-Bulletin/2002-08-09/features2/body.html
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