Grignard Reaction Lab Report PDF

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Grignard Reaction Lab Report...

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PREPARATION OF TRIPHENYLMETHANOL BY GRIGNARD REACTION

Lead Author: Elijah Marsh Reviewer: Hannah Strickland Editor: Brad Wurth Lab Section: G5 Date: 2/16/2017 !

Isoborneol Oxidation and Camphor Reduction

Lead Author: Hannah Strickland Editor: Reviewer:

Chemistry 238 Section G6

Experiment 3

Introduction:

Grignard reagents are created by reacting magnesium with either an alkenyl or alkyl halide. The halide group must be bromide, iodide, or chloride. Grignard reagents are strong bases that will react with acidic hydrogens, and they are excellent nucleophiles. They are often used to form alcohols by reaction with aldehydes and ketones. First, the Grignard forms the carbon-carbon bond. This creates an alkoxide. This alkoxide must be reacted with an acid so that it can become an alcohol. Secondary alcohols are produced by reacting a Grignard reagent with an aldehyde. To create a tertiary alcohol, the Grignard reagent must be reacted with a ketone.1 This is the type of reaction that takes place in the experimental portion of this report. The reaction forms triphenylmethanol. Creation of an alcohol will be unsuccessful in the presence of water, because Grignard reagents are extremely reactive in its presence. If a Grignard reagent reacts with water, it will form an alkane instead of the desired tertiary alcohol. Therefore, all glassware involved must be dried before the procedure.2 The procedure described below is the production of triphenylmethanol by reacting a Grignard reagent (phenylmagnesium bromide) with Benzophenone. Figure-1: Below is the mechanism for the preparation of triphenylmethanol. Mechanism: First, the Grignard reagent must be formed.

Mg

!

Br

MgBr

MgBr

Second, the Grignard reagent must react with the ketone to form triphenylmethanol (a tertiary alcohol). Br

O MgBr

Mg +

Anhydrous Ether

OH

O Cl

H

! Table-1: Below is the table of reagents used throughout the procedure. Table of Reagents: Substance

MW (g/mol)

BP (°C)

MP (°C)

Density (g/mL)

Magnesium

24.36

1,090

89

0.889

Bromobenzene

157.02

156

-31

1.491

Benzophenone

182.22

305

47 to 51

1.11

Hydrochloric Acid

36.46

57

-35

1.2

Sodium Bicarbonate

84.01

851

300

2.16

Calcium Chloride

110.98

1,600

772

1.086

Anhydrous Ether

86.14

146

109 to 112

1.00

Anhydrous Diethyl Ether

74.12

34.6

-116

0.714

Petroleum Ether

112

40 to 60

-101

0.75

Triphenylmethanol

260.34

360

160 to 163

1.2

Phenylmagnesium Bromide

181.31

78.8

N/A

1.134

Experimental: A 5-mL round bottom flask, a stirring rod, and two reaction vials were dried in an oven. The round bottom flask was removed from the oven and capped with a septum before being allowed to cool to room temperature. 0.25-g of heat dried magnesium were added to the flask. The septum remained off of the flask for a very short period of time. 0.7-mL of ether were then drawn into a syringe with a needle attached to it. This syringe was used to inject the ether through the top of the septum. A clean syringe was then used to transfer 0.7-mL of ether and 2-mL of bromobenzene into one of the two reaction vials after the rest of the glassware was removed from the oven. The contents of this vial were mixed with a pipette, and 8 drops of it were added to the round bottom flask with the first syringe. No reaction occurred. The round bottom flask was then heated in a warm water bath at less than 40°C for approximately 15 minutes. Several drops of ether were added occasionally throughout this process to prevent drying. The contents of the flask became light-brown and cloudy. The flask was removed from heat, and the rest of the bromobenzene mixture was added dropwise to the flask. The flask was heated again at the same temperature for 10 minutes. During this 10-minute time-period, the syringe that was used to transfer the bromobenzene mixture and the reaction flask that it was transferred to were rinsed four times with ether. 7.52-g of benzophenone was

mixed with 2-mL of ether. A small quantity of 3M HCl was also diluted with 0.67-mL of water to a concentration of 1M. The round bottom flask was removed from heat, and the benzophenone mixture was added dropwise to the round bottom flask with the syringe. The contents within the flask became a yellowish-brown color. The flask was then heated at above 40°C for 20 minutes. The contents in the round bottom flask were poured into a small beaker. 2-mL of the 1M HCl were then added to the beaker. Then, 3mL of water and 3mL of ether were added. All of the contents were moved to a separatory funnel and mixed with a pipette. The aqueous layer that formed at the bottom was removed with the pipette and transferred to a waste beaker. 5-mL of 1M sodium bicarbonate were mixed into the separatory funnel with a pipette, and the resulting aqueous layer was again removed with the pipette. This process was repeated with 5-mL of water. Several balls of calcium chloride were used to decant the mixture to a 25-mL Erlenmeyer flask. By this point the mixture turned to more of a pinkish-brown color. The ether was evaporated away on a hot plate, and 8-mL of petroleum ether was added after the flask was cooled to 25°C. The mixture was a very dark brown color. This indicates a charred product. The flask was cooled in an ice bath and suction filtered for 5 minutes. No product formed. Because of this, product data had to be obtained from another source. Results: Equation-1: The calculation used to determine the amount of water need to dilute the HCl to 1M is shown below.

Equation-2: The equation used to determine the theoretical yield of triphenylmethanol is shown below.4 First, the limiting reagent must be determined.

Benzophenone is the limiting reagent. Now the number of moles of benzophenone must be converted to the number of moles of triphenylmethanol in order for the theoretical yield to be obtained.

Equation-3: The actual yield of the product was determined by converting the number of grams of product to the number of moles of product.4

Equation-4: The equation used to determine the percent yield is as shown.4

Table-2: All relevant product data is shown below.4

Product

Actual Yield

Theoretical

(Moles)

Yield

Percent Yield

Melting Point (°C)

(Moles) Triphenylmethanol

0.00004225

0.0041159

1.03%

162.4-163.3

Discussion: The procedure began with the creation of the Grignard reagent. This was the purpose of reacting magnesium with bromobenzene in the presence of ether. The carbon that bromine was attached to in the aryl halide was electrophilic. This caused the magnesium atom to pop off the bromide anion. This magnesium halide then bound to the carbon. The carbon atom became nucleophilic once the Grignard reagent formed. Ether was added constantly while this reaction took place to ensure that the mixture did not dry. This is because ether is extremely volatile5 This nucleophilic Grignard reagent reacted with the benzophenone during the second warm water bath and formed an alkoxide. This alkoxide was reacted with HCl acid and formed triphenylmethanol. All of the equipment and reagents were kept dry to ensure that this would happen. One of the main reasons that the reagents remained relatively dry throughout the procedure is that a septum was placed on the round bottom flask during most of the reactions. The third water bath was performed to evaporate any excess ether. The bath was heated to much too high of a temperature. This caused the product to become charred. This resulted in a zero percent yield. Because of this, data had to be obtained from another experiment. This procedure was successful because proper heating guidelines were followed. The percent yield was 1.03% (equation-4). The percent yield was low because the Grignard reagent was exposed to some water during the procedure. The melting point from table-2 proves that the product was triphenylmethanol. The melting point range was very close to the literature value, and it was quite narrow. Conclusion:

Grignard reagents are very useful for creating secondary and tertiary alcohols from aldehydes and ketones. Grignard reagents must be kept from reacting with any water for the desired product to be formed. Triphenyl methanol was not produced because the product was charred during the process of evaporating away the ether. None of the water baths should have exceeded 40°C. The results of the other group were far better due to the evaporation of ether taking place at a slower rate. 0.011-g of product were formed. The percent yield of the product was 1.03%. It was probably so low because there was so much exposure to water vapor throughout the procedure. The caps being left off of the reagent containers constantly for short periods of time could have easily exposed the reagents to water vapor as well. Also, the decanting process may not have lasted long enough. Enough time must be taken to properly dry a solution with calcium chloride so that all excess water can be removed. The product was rather pure, however. This was shown by the melting point of the product (162.4 to 163.3°C). This melting point is not very broad and is very close to the literature value. One way to improve the experiment would be to better monitor the exposure of reagents to the air. Another way would be to cover all of the glassware with parafilm when it is not being used. The most important improvement would be to avoid overheating the water baths. References: 1. J. Reagent Friday: Grignard Reagents http://www.masterorganicchemistry.com/ 2011/10/14/reagent-friday-grignard-reagents/ (accessed Feb 19, 2017).

2. Clark, J. Grignard Reagents http://www.chemguide.co.uk/organicprops/haloalkanes/ grignard.html (accessed Feb 19, 2017). 3. ChemicalBook http://www.chemicalbook.com/ (accessed Feb 19, 2017). 4. Kerianne.; Enrique.; Laila., Preparation of Triphenylmethanol. 5. Formation of Grignard Reagents from Organic Halides http://research.cm.utexas.edu/ nbauld/grignard.htm (accessed Feb 19, 2017)....