Experiment 5 Preparation of Benzoic Acid using a Grignard Reagent PDF

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Experiment 5: Preparation of Benzoic Acid using a Grignard Reagent

By: (NAME) TA: (NAME)

CHM 1321 Section Z01

Due Date: March 19, 2019

Department of Chemistry University of Ottawa

Introduction:

The purpose of this lab is to prepare benzoic acid using a Grignard reagent. The general structure of a Grignard reagent is RMgX. R can be any alkyl, vinyl or aryl group. X can be any halogen (Venkateswaran, 2014). A Grignard reaction is a coupling reaction where and alkyl gets added to a carbonyl group (in an aldehyde or ketone). For Grignard reactions, the materials must be dry. This is due to the fact that the Grignard carbon is a nucleophile. Water and alcohols would protonate it to form a hydrocarbon and the reaction would not work if it was not dry. Suction filtration is a technique that is going to be used in this lab. A filter is a contraption that separates compounds by size. Filtration by gravity allows smaller molecules to pass through a paper and does not allow larger molecules to pass through. Suction filtration uses a pressure gradient to filter materials and is more efficient than filtration by gravity. In order to perform an effective extraction, the two solvents must be immiscible, the compound getting extracted should have a much higher solubility in one solvent relative to the other. Also, polar compounds tend to be soluble in polar solvents and non-polar compounds in non-polar solvents. In this reaction bromobenzene will react and produce benzoic acid in a Grignard reaction. This is depicted in the following reaction mechanism:

At the end of this laboratory experiment, thin layer chromatography (TLC) techniques are used. TLC is the process in which c ompounds are separated on a thin layer of adsorbent material. In order to determine how accurate the amount of solid precipitate obtained is, the percent yield will need to be calculated. Percent yield is used to calculate how close the reaction is, to what it is supposed to be. To calculate percent yield, the actual yield is needed (how much solid was obtained), divided by the theoretical yield (how much was supposed to be obtained or how much was started with) and then multiplied by 100.

Materials: -bromobenzene

-water

-funnel

-anhydrous diethyl ether -calcium chloride -cotton -iodine crystals

-dry ice -ice -hydrochloric acid -ordinary diethyl ether

-TLC plates -1:9 Ethyl Acetate: Hexanes -sodium hydroxide -suction filter

Procedure: Refer to procedure in lab manual for “Experiment 5: Preparation of Benzoic Acid using a Grignard Reagent” (Venkateswaran, 2014). Please note that in Step 12, four pellets of dry ice were used. Observations: Table 1: Qualitative and Quantitative Observations Compound

Observation

Anhydrous Diethyl Ether

Clear, colourless solution

Magnesium Turnings

Metallic, silver solid

Iodine Crystals

Black, small powdery substance

Bromobenzene

Clear, colourless solution

Organic Solution Layer

Yellowish, clear solution

Aqueous Solution Layer

Clear, colourless solution

HCl

Clear, colourless solution

NaOH

Clear, colourless solution

Mass of Precipitate

1.56 g (Powdery substance) Melting point 125°C

-it was observed that when co-spots were added, they made spots on the TLC paper 0.5 cm in diameter -it was observed that the appearance of co-spots disappeared a few seconds after putting them on -when spots were observed under a UV light, they appeared as white ovals on the silica (which appeared green under the UV light) To calculate the Rf value, the d1value must be divided by the ds value. TLC Plates:

Rf Values: TLC Plate 1: Reference (bromobenzene) and Organic Layer in Solvent 1:9 EtOAc:Hexanes Rf values: Ref: 0.55 CS: 0.15, 0.55, 0.77 Org:0.15, 0.55, 0.77

TLC Plate 2: Reference (bromobenzene) and final product in Solvent 1:9 EtOAc:Hexanes Rf values: Ref: 0.50, 0.75 CS: 0.17, 0.50, 0.75 Prod: 0.16

Flowcharts: Mechanism of the Reaction

Mechanism of Extraction

Percent Yield: Initial amount of bromobenzene = 3.0 mL Density of bromobenzene= 1.50 g/mL # of moles of bromobenzene = (3.0 mL)(1.50 g/mL)(1/157.01 g/mol)= 0.0287 mol (Convert moles of benzil to moles of benzoic acid (the final product)) Moles of Ben = 0.0287 mol (122.12 g/mol) = 3.50 g Final amount of Benzoic Acid: 1.56g Percent Yield = (Final amount / Initial amount) x 100 % = (1.56 g / 3.50 g) x 100% = 44.57% Table of Reagents:

Reagent

Amount (g/ml)

Molecular Weight (g/mol)

Moles (mol)

Density (g/ml)

Magnesium Turning

0.80g

24.31g/mol

0.0033

N/A

Bromobenzene

3.0 ml

157.01 g/mol

0.0287

1.50 g/ml

Diethyl Ether (Anhydrous)

20.0 ml

74.12 g/ml

0.270

0.706 /ml

Iodine

A few crystals (2-3)

126.904 g/mol

N/A

N/A

Dry Ice

20-30g

44.01g/mol

1.51

1.56 g/mol

HCl

25ml

36.36 g/mol

0.688

1.19 g/mol

18.05 g

44.01 g/mol

0.41

N/A

Carbon dioxide

Table of Results: Reagent

Amount (g/mol)

Molecular Weight (g/mol)

Moles (mol)

% Yield

Melting Point

Benzoic Acid

1.56 g

122.12 g/mol

0.0287 mol

44.57%

125C

Questions: 1) A small amount of benzene is formed due to the magnesium turnings. They bind to the carbon-bromine bond in the bromobenzene, creating a magnesium-bromide bond. Then, water molecules can interfere, causing a proton displacement in the solution. The hydrogens from the water replace the magnesium bromide bond, isolating the magnesium bromide cation and the hydroxide anion. The cation and anion may bond together ( sometimes they do, sometimes they do not.) Hence, explain the reason for the low yield of benzene in the product. (Please refer to flowchart above) 2) A small amount of biphenyl is formed because when a bromine magnesium benzene complex undergoes electron displacement, the electrons go to the magnesium bromide

bond, hence causing a carbocation. During the second complex, electrons go to the carbon located in the benzene ring and form a carbanion. The carbanion and carbocation bind together to form the biphenyl. Our amount of biphenyl depends on whether or not these process can form through the breaking and forming of bonds. (please refer to flowcharts above) 3) The moles of phenylmagnesium bromide are equal to the number of moles of bromobenzene. Moles of bromobenzene = 0.0188 =number of moles of water Mass of water = nM(water) = (0.0188 mol)*(18.02 g/mol) = 0.339 g Density of water is 1 g/mL. 1g is equal to 1 mL. (Mass= Volume) Therefore 0.339 mL is required to kill the reaction. 4) The reason that bromobenzene is added to the flask in two portions (rather than one) is because when liquids boil, bubbles of vapour form in the liquid. These bubbles form around solids in the liquid or imperfections in the vessel, in a process called nucleation. Glassware in the laboratory will not contain enough defects to ensure that there is proper nucleation. This can cause heated liquids to superheat (above the boiling point). Vapour bubbles can form then cause the liquid to violently leave the vessel. If it is added in separate portions, the chances of this occurring is less. 5)

The TLC plate would look like the one pictured above if taken before adding dry ice. Discussion:

The main purpose of this lab was use the synthesis of Grignard reagent technique to transform bromobenzene into benzoic acid. The Grignard reagent method consists of an alkyl group bonded to a magnesium halogen. In this reaction, an alkyl halide and magnesium metal react to form this Grignard reagent by the magnesium metal inserting into the alkyl halide bond. Since, magnesium has a relatively highly positive (stronger) oxidation state of 2+, it repels electrons resulting in the carbon on the alkyl group gaining a negative charge due it the fact that it causes the electrons of the bond to unequally be shared with the carbon on the alkyl group. Resulting in the carbon on the alkyl being a nucleophile and has the ability to donate electrons. This ensures that the formation of the complex on the benzene ring occurs. Following this, Anhydrous ether was added into the reaction mixture to allow the Grignard reagent to form stable benzene rings.It is important to note that the heating of the glassware materials were used to eliminate any traces of water present, in order to prevent a different reaction from occuring in the solution, which would ruin our results, as well a drying tube with cotton and CaCl2 is used to decrease the possibility of water entering the solution and causing a different reaction than desidered. Moving on, in regards to the experiment, the process began by combining magnesium and iodine in a solution of bromobenzene and the organic solvent, Anhydrous diethyl ether. The purpose of the iodine was to present the magnesium oxide layer where the bromobenzene could react. The resulting organic Grignard reagent consisted of an alkyl magnesium halide with an alkyl magnesium bond, this interaction demonstrated nucleophilic properties due to the electrons from the bond favoring the alpha carbon. Ergo, the reagent was able to react with the carbon dioxide exhibiting an electrophilic site of a carbon double bond to an oxygen atom.Then, carbon dioxide was added into the reaction mechanism through the use of dry ice (its reactive form) and in addition, dry ice was used to preserve the bromobenzene as the limiting reagent. The addition of the electrons from the alpha carbon to the carbonyl carbon caused the breaking of the pi bonds towards the oxygen. Thus, resulted in the formation of a bond between the alpha carbon and the carbonyl carbon producing an alkoxide. The oxygen anion on this alkoxide attracted the magnesium bromide cation forming magnesium alkoxide, this was the brown sticky substance that was visible on the dry ice. . Moving on, the use of concentrated HCl and water was added to the sticky substance to convert the magnesium alkoxide intermediate to benzoic acid and ensure that protonation of the formed salt. The concentration of HCl protonated the alkoxide, causing the magnesium - oxygen bonds to break. Then, the electrons from the breaking of the magnesium - oxygen bonds were transferred to the carbonyl carbon. The pi bonds was displaced in order to move the electrons to the oxygen of the hydroxyl group , causing the formation of benzoic acid (Benzoic Acid).

During the extraction process, diethyl was used to perform the extraction of the product which was remained in the organic phase of the solution and then the use of an ice bath to allow the precipitate to form in the solution. A gravity and suction filtration techniques were used to dry the product and allow for the collection of the final product. In terms of our results, two TLC plates were made. The first TLC consisted of the organic phase (sample) and the reference being bromobenzene (reference). Based on our results, it was determined that the organic layer contained bromobenzene as the Rf value of the bromobenzene and the organic layer were very similar and an additional spot on the organic layer line indicated that another unknown substance is present. The second TLC plate consisted of the final product (sample) and bromobenzene (reference). The results from this plate tells us whether or not bromobenzene is present inside of the final product, since the Rf values are different, then it means bromobenzene is not present in the sample due to the fact that they have different polarities meaning that they are composed of different molecules. At the end our melting point for the final product was 125°C, since the theoretical melting point of benzoic acid is 122°C, we can infer that the substance had high purity and the product is what we expected, with minimal impurities present. In the end our percent yield is 45%. (Please refer to the mechanisms drawn above to visualize the reactions) Conclusion: The percent yield is 45%. The final product’s melting point is 125°C and our TLC plates indicated that the reaction went to completion with some impurities. The conversion of bromobenzene to benzoic acid using a grignard reagent was successful. Sources of Error:

● The poor performance of the separation portion of the lab would alter the amount of solution to be acquired in the aqueous phase, thus changing the results for the melting point because there would be more impurities localized in the product. To improve better separation equipment and more practice and demonstrations. ● Final product mass may have been lost when the suction filter let some of the particles through its holes, resulting in a lower yield. Better filtration process can fix this issue. ● If the compound was not completely dry before it was measured, the extra water or solution could have added to the mass of the compound disrupting our results. References:

Venkateswaran, Rashmi. Experiment 5: Preparation of Benzoic Acid using a Grignard Reagent , 2014, Exp. 5, p. 7 

“Benzoic Acid.” National Center for Biotechnology Information. PubChem Compound Database, U.S. National Library of Medicine, pubchem.ncbi.nlm.nih.gov/compound/benzoic_acid. Raw Data:...