“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”

Written by:

CHM 1332 Section Z4

TA: Jatinder Singh

Date: Tuesday, March 26, 2019

Introduction: In the formation of benzoic acid, a grignard reagent is first prepared. In the first steps, solid magnesium is added to form the grignard which will then react with carbon dioxide to get a

carbonyl group. From there, HCl is added to protonate the reagent and water will be added to get rid of the aqueous ions. Mechanism:

Procedure: Refer to the lab manual, experiment 5, “Experiment 5: Preparation of Benzoic Acid using a Grignard”, Dr. Rashmi Venkateswaran, 2019, Exp 5, p. 4-5

Table of reagents: Product

Reagents used

Grignard

● Bromobenzene ● Solid Mg

Benzoic Acid

● H2O, Phenylmagnesium bromide

Catalyst:

● HCl

Observations: ● When mixing the reagents, the substance was red at first, then turned a foggy yellow which turned to white ○ Bubbles of hydrogen gas began to form and as the reaction went on, the solution went dark brown ● After mizig with dry ice, we got a very sticky substance ● When adding acid and cooling after separating the solution, a white precipitate came out and was purified ● The precipitate was white, sticky powder TLC:

Flow chart:

Table of Results: Product

Molecular Mass (g/mol)

Amount (g) Moles (mol)

Percent Yield (%)

Theoretical Melting Point

Experimental Melting point

Benzoic Acid

122.12

4.78g

137%

118-122

N/A

Calculations:

0.039

Rf = di/ds For TLC 1: ds=3.3 cm di=2.8cm Rf= 2.8/3.3 =0.85 *Repeat this calculation for all Rf’s Percent Yield: =3 ml bromobenzene x 1.495 g/mol x 1 mol 157g =0.02857 mols bromobenzene : 0.02857 mols benzoic acid 0.02857 mols benzoic acid x 122g /mol = 3.49g % yield =

Actual x 100% Theoretical = 4.78g x 100% 3.49g = 137%

Discussion: ● We have to make sure our glassware does not have any water in it because it can protonate and ruin our grignard reagent.. ● When removing the glassware from the oven, we have to assemble it quickly with the drying tube or else water vapour from the air could condense in our glassware. ● We use anhydrous diethyl ether because ethers are aprotic and it is removed of all traces of water. ● We added the the bromobenzene in two portions because adding it all at once would create an increase in a product we don’t want and produce little product that we do want to synthesize the reaction. It also maintains a low temperature which favours the grignard reaction. ● The reaction has to boil on its own first because it is an exothermic reaction. ● We add dry ice to the grignard reaction because it yields benzoate. ● We add ice afterwards to manage the temperature and prevent diethyl ether evaporating. ● We rinse the beaker with diethyl ether because it’s an organic solvent. ● We re-extract the aqueous phase so we can clear it of all organic extracts.

● We add the HCl dropwise so we can control the amount we put in. If we put too much it will cause the NaCl to precipitate out. And too little wont acidify the aqueous extracts. ● We use suction filtration after all the visible liquid has been filtered by gravity, so we can have the driest substance possible. ● We find that our first TLC had presence of organic and aqueous solvents but our final TLC did not have any aqueous solvents showing that it is more pure than the first one. ● We did not have time to find the melting point of our product but we suspect it to be fairly close to the theoretical melting point which is between 118-122℃. ● Water will inevitably get into our reaction because we are not working in a vacuum sealed area, therefore we are constantly exposed to water vapours. ● With every step of our reactions, we lose a little bit of compounds each time. So in the end, we won’t have the exact results we were anticipating. ● Our data ended up being higher than the theoretical yield. Since time was rushed, the product did not have time to full air dry, so the excess water in product resulted in a higher mass since there was water in there as well ● Refer to the introduction and questions 1 and 2 for reaction mechanisms that occured during this experiment. Questions: 1. The grignard reagent reacts a bit with water which will give benzene

2. Unreacted bromobenzene will combine with the grignard reagent, giving biphenyl

3. The mass of water to destroy the phenylmagnesium bromide prepared will be equimolar to the amount of phenylmagnesium bromide prepared.

3ml bromobenzene x 1.5g/mol = 4.5g 4.5g bromobenzene = 0.028 mol 157 g/mol 0.028 mol H2O x 18 g/mol = 0.515g V=0.515g x 0.998 g/ml V= 0.514 ml H2O

*Density of water= 0.998g/ml

So, 0.514ml of H2O is required to destroy the phenylmagnesium bromide 4. By adding all of the bromobenzene at once, a side product will form and less phenylmagnesium bromide will be produced, so we add it in different portions to get the desired product. 5. The grignard would eventually destruct, so the water in the air will ruin the reaction. The spots on the TLC that correspond to the grignard would not show up without dry ice.

Conclusion: For our melting point we would have received around 120℃ to prove the purity of our substance. The TLC’s show that the final product is pure benzoic acid. We yielded 4.78g which gave us a 137% percent yield, however expected 3.09g, this is because there was too much water in our substance.

Reference:

Summers, Vincent. “What Is Anhydrous Diethyl Ether?” Sciencing, 2 Mar. 2019, sciencing.com/anhydrous-diethyl-ether-6025498.html....