Lab 4: Preparation of Benzoic Acid using a Grignard Reagent PDF

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Preparation of Benzoic Acid using a Grignard Reagent...

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CHM 1321 Work Sheet – Experiment 4 Experiment Title: Preparation of Benzoic Acid using a Grignard Reagent

Name

TA (Demonstrator)’s Name: Date Experiment Report Submitted: March/08/2021

Sample Data Set Number: L4-Données7

(Please note that if the sample data set numbers are not included, you will not receive a grade for the results as the TAs who are correcting the reports will not know which data sets you received)

Attach here (if required, indicate the appropriate document(s)): Medical or other Acceptable Document: Introduction (6):

_____

A Grignard reactant are made by reacting magnesium with an alkenyl or alkyl halide. They are strong bases that will react with hydrogens and are very good nucleophiles so it can create nucleophilic displacement with double bonds in ketones to create a carboxylic acid. is a reaction where a Grignard reagent made up of organomagnesium halide reacts with an ketone or an aldehyde.. It is used to create carbon-carbon bonds and.

Figure 1.1 Equation of of creating Grignard reactant with 1-bromo-4-isopropylbenzene and magnesium metal and the reaction of the result with carbon dioxide to create a benzoic acid.

Figure 2: Mechanism of the creation of 1-bromo-4-isopropylbenzoic acid. The magnesium benzyl bromide acts as the nucleophile and reacts with the carbon dioxide (in the form of solid dry ice) that is the electrophile. This breaks a pi bond to create the ether. HCl and water are than added to create the end product.

Procedure (1):

As outlined in the Lab Manual. “Laboratory Manual for Introductory Organic Chemistry, Experiment 4, (Durst, Flynn, Ogilvie, Scaiano, Venkateswaren 2021, p.1-6)’’ Table of Reagents (13): Reagent.

Molecular Weight (g/mol) 24.305

Density (g/mL)

Amount

Moles

Equivalent

Magnesium n/a 0.9 g 0.037 2.05 Turnings 1-bromo-4199.09 1.286 2.8 mL 0.0181 1 isopropylbenzène Anhydrous Diethyl 74.12 0.706 20 mL 0.191 N/A Ether Iodine N/A N/A 0.23 g N/A N/A Dry Ice 44.01 n/a 3.8 g 0.086 4.75 Table 1: Table of reagents showing each reagent. Note, iodine is not considered as a limiting reagent in this experiment as it is used to accelerate a reaction and not directly involved in the resulting product and is therefore also not shown in the equation or calculations. Diethyl Ether is also not included as it is the solvent and is not involved with any reaction shown in figure 1. Calculations (2): Yield: Mass of 1-bromo-4-isopropylbenzène= n*M = 0.0181*199.09 = 3.60 g Yield =

𝑓𝑖𝑛𝑎𝑙 𝑎𝑚𝑜𝑢𝑛𝑡 𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑎𝑚𝑜𝑢𝑛𝑡

× 100%

2.4 × 100% 3.6

= 66.67% Rf Value: Example of Figure 3 Rf = d1/ds Rf (P)= 1.0 cm/4.0 cm = 0.25

Flowchart (6):

Results and Discussion (17): (no more than one page of text, single or 1.5 space, 11 point font) The purpose of this lab is to determine how a Grignard reagent are created and used to create benzoic acid. Water must be avoided in this lab because it will protonate any Grignard reagent in an acid-base reaction, destroying it. To prevent this, all glassware that was used was dried in an oven and placed in a drying tube with CaCl2 to get rid of the microscopic layer of water. Acetone must be avoided because the Grignard reactant will react with the double bonded oxygen on ketones to create a tertiary alcohol as an undesired side product. This can be prevented by adequately drying and cleaning any glassware. Iodine was added to the magnesium to help remove any MgO on the surface of the magnesium turnings which would prevent the reaction between Mg+ and the halogen, bromine in this lab. Carbon dioxide is used to create a carboxylic acid as a product. Anhydrous diether ether was used to as it has no water and is aprotic so it will not compete with the reaction. After the carboxylic acid was created it was still in a solution with many other impurities. It was isolated through a series of reactive extractions as demonstrated in the flowchart. In a Grignard reaction, the pi bond is displaced by a carbon in the Grignard reactant to create a carboxylic acid in this lab. However, impurities are also created and filtered out. The first extraction only removed any inorganic ions in the solution as benzene, benzoic acid, and biphenyl and bromobenzene are all soluble in the organic phase in diether ether. This resulted in the TCL in figure 3 with reference lane has only one comparatively non polar spot that contains only the bromobenzene. After the first extraction with diethyl ether there are two spots in the lane organic layer lane which means that the organic phase has at least 2 substances with different polarities, one that is more polar than the starting solution. This is consistent with the flowchart in fig.2. The fact that there are two spots in the O lane means that it has substances that are not the benzoic acid. As the organic phase should contain benzoic acid, benzene, bromobenzene and biphenyl. Benzoic acid has one potential hydrogen bonding location while the other have none and benzene and biphenyl are symmetrical. This means that benzoic acid would have a higher affinity for the polar silica gel and remain in the stationary phase on the TLC as seen by to distinct dots. Table 1 shows that the different spots have significantly different Rf values and suggest the resulting compounds have different polarities. A second extraction with NaOH was used to separate the other organic compounds from the acid. The strong base will react with the acid only creating an benzoate ion that is negatively charged and will be soluble in water only. The other compounds will remain soluble in the diethyl ether only and can be filtered out. After it was filtered HCl was added to the aqueous phase that contains the basic benzoate ions. The acid base reaction converts it back to the desired 4isopropyl benzoic acid and precipitates out NaOH, once precipitate stops forming, all the acid has been converted back. The second TLC shows that the final product after two extractions is in a purer form compared to after the first extraction with just water and diether. Comparing it with figure 3 , figure 4 has only the one spot with the smaller Rf value; with the highest polarity which is benzoic acid which suggests that the other organic compounds were filtered out and only remains the acid and possible impurities with a similar polarity. This conclusion is consistent when comparing melting points. The theoretical melting point of 4-isopropyl benzoic acid is 117°C to 118°C (4-isopropyl benzoic acid n.d). The experimental value is 114°C to 117°C. Since these two ranges are less than 10% apart, it can be concluded that the final product is mostly benzoic acid with some impurities. This matches the percentage yield that was obtained at 67%. The reaction can be improved by making sure that the reaction is done in a better sealed beaker. It was not stated if stoppers were used with the beakers that held the benzene solution or the magnesium iodine solution. This means that it is possible the moisture from the air entered prior to or during the reaction. As discussed, water in a Grignard reaction is extremely detrimental and could explain the lower yield.

Spot R Rf 0.77 Table 1 : Rf values for the TLC plate of figure 3

Co 0.77, 0.25

O 0.77, 0.25

Compose R Rf 0.87 Table 2 Rf values for the TLC plate of figure 4.

Co 0.87, 0.27

O 0.87, 0.27

Questions: (please answer as briefly as possible) 1. During your Grignard formation, a small amount of benzene is formed. Provide a brief explanation and mechanism to explain this observation. (2 points) (use software to draw mechanism)

If water is introduced, the Grignard reagent will react with it in an acid base reaction, protonating the Grignard reagent into benzene. 2. During your Grignard formation, a small amount of biphenyl is formed. Provide a brief explanation and mechanism to explain this observation. (2 points) (use software to draw mechanism)

Any unreacted bromobenzene, and the Grignard reagent will react. Since there is an electronegativity difference between carbon and both magnesium and bromine and an adjacent pi bond, a pair of lone electrons will displace creating two oppositely charged benzene ions that will react with each other. 3. What mass of water would be required to destroy the Grignard reagent that you prepared in this experiment? What volume does this represent? (2 points) Grignard reagents react with water in a 1:1 ratio. Assuming that all the bromobenzene is reacted which is 0.0181 mols than 0.0181 mols of the phenylmagnesium bromide was made and 0.0181 mols of water are needed. Which is 0.326 g and 0.326 mL. 4. Why is the bromobenzene added to your flask in two portions rather than one? (2 points) It was added in two portions because this reaction is extremely exothermic and releases heat. If all the bromobenzene was added at the same time than the reaction would occur too quickly. A higher concentration of the bromobenzene with a higher temperature will also increase the amount of biphenyl that is created from the bromobenzene. 5. If you were to take a TLC of your Grignard solution before you add it to the dry ice, what would your TLC look like? (2 points On a TLC before adding the dry ice, there is just the Grignard reagent and possible biphenyl and benzene. The grignard reagent is extreamly polar as a result of the Magnesium metal so it would have a very low Rf Value. Also, the Grignard reactant would continue to destroyed by water moisture in the air which would create benzene accounting for the additional spots. Benzene and Bi phenyl would be comparatively much more non polar with higher Rf values similar to those in table 3.

R C S

Reference: National Center for Biotechnology Information (2021). PubChem Compound Summary for CID 10820, 4-Isopropylbenzoic acid. Retrieved March 10, 2021 from https://pubchem.ncbi.nlm.nih.gov/compound/4-Isopropylbenzoic-acid....