Organic Chem Lab 7: Grignard Synthesis of Triphenylmethanol PDF

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Organic Chem Lab 7: Grignard Synthesis of Triphenylmethanol...

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Grignard Synthesis of Triphenylmethanol

Abstract The objective of this experiment is to prepare triphenylmethanol using Grignard synthesis. The product obtained, which produced a 19.5% yield was characterized using Infrared Spectroscopy and by analyzing the melting point.

Introduction Grignard reagents belong to a large class of organic compounds that are called organometallic reagents that has the formula R-M, with M being the metal. Grignard reagents are created by reacting magnesium with either an alkenyl or alkyl halide. Grignard reagents contain a carbon magnesium bond and their general formula is RMgX where X is bromide, chloride or iodide. Because of the large difference in electronegativities of carbon and magnesium, Grignard reagents are very reactive compounds and play an important role in organic synthesis.

Grignard reagents are strong bases and must be prepared under anhydrous conditions. The preparation of the Grignard reagent takes place via an oxidation/reduction reaction of the metal and the halide with ether as a solvent and reactant. RX + Mg → RMgX The carbonyl bond of the carbonyl compound is polarized thus making the oxygen the nucleophile. This allows for the nucleophilic addition of the Grignard reagent to the carbonyl carbon of the compound.The reaction of a Grignard reagent with a carbonyl compound allows for the formation of primary, secondary, and tertiary alcohols, as well as carboxylic acids depending on the nature of the carbonyl compound used. Grignard reagents react: (i) with formaldehyde to give primary alcohols, (ii) with other aldehyde to give secondary alcohols, (iii) with ketones to give tertiary alcohols and (iv) esters to give tertiary alcohol. In this experiment, the Grignard reagent created, phenylmagnesium bromide reacted with the ester, methyl benzoate via nucleophilic addition to obtain the tertiary alcohol, triphenylmethanol. Table1. Physical and Chemical Properties of Chemicals Used Throughout the Experiment Compound

Molecular Weight (g/mol)

Melting Point (°C)

Boiling Point (°C)

Phenylmagnesium bromide

157.02

-

-

Methyl Benzoate

136.15

-13.0

198-199

Triphenylmethanol

260.34

163

>360

Bromobenzene

157.02

-31

156

Experimental Procedure: All glassware needed for this experiment were cleaned and dried prior to the start of the synthesis. Magnesium was mashed using a mortar and pestle in order to expose the surface of the metal to increase the rate of the reaction and were immediately placed inside the 250 mL round-bottomed flask with a sidearm. The Grignard apparatus was assembled using the vacuum and Claisen adapter, west condenser, and separatory funnel. The vacuum adapter was filled with anhydrous calcium chloride to absorb any water. Five milliliters of bromobenzene and ten milliliters of anhydrous diethyl ether were added to the separatory funnel. The mixture was then slowly added into the flask. After the complete mixture was added, an additional 15 milliliters of anhydrous diethyl ether was added into the separatory funnel. The flask was warmed by rubbing the bottom of the flask to initiate the reaction. To increase the reaction time, a stirring rod was used to mash the magnesium in the presence of the mixture. Once the reaction began, the water flow into the west condenser was started and the additional ether in the separatory funnel was added. Once the refluxing stopped the reaction was over, the product made was the Grignard reagent. Three grams of methyl benzoate and ten milliliters of anhydrous diethyl ether were added to the separatory funnel. This mixture was added slowly to the Grignard reagent at a rate that maintained a gentle reflux. Once all of the methyl benzoate solution was added, the reaction mixture was allowed to cool to room temperature. The reaction mixture was poured into a 250 mL beaker that contained 25 milliliters of 10% sulfuric acid and 25 grams of ice. Additional diethyl ether was added to dissolve the magnesium salt of the triphenylmethanol. This mixture was added to the separatory funnel and the organic layer was extracted. The organic layer was washed with 25 mL of saturated NaCl and the organic layer was then dried over anhydrous

sodium sulfate. The dried layer was decanted into a 125 mL Erlenmeyer flask. Twenty-five milliliters of ligroin was added to the flask and the mixture was concentrated on a hot plate in the hood until the diethyl ether was removed. The flask was then removed from the hot plate and allowed to cool to room temperature which was then saved until the next lab period. After a week, the crystals were filtered via vacuum filtration, washed with ice cold ligroin, air dried, and weighed. The recrystallized product was characterized by IR and the melting point range was observed.

Results Yield Report Weight of magnesium

1g

Moles of magnesium

0.042 mol

Weight of bromobenzene

7.5 g

Moles of bromobenzene

0.047 mol

Weight of methyl benzoate

3g

Moles of methyl benzoate

0.022 mol

Limiting reagent

Methyl benzoate

Theoretical yield of triphenylmethanol

0.022 mol

Theoretical yield of triphenylmethanol

5.73 g

Actual yield of triphenylmethanol

1.12 g

Percent yield of triphenylmethanol

19.5 %

Melting point (range) of product

160.2 - 162.7 °C

Yield Calculation: 1 g Mg x

1 mol M g 24.31g M g

= 0.041 mol Mg

5 mL bromobenzene x

1.495g 1mL

x

1 mol bromobenzene 157.02 g bromobenzene

= 0.048 mol bromobenzene

3 g methyl benzoate x

1 mol methyl benzoate 136.15g methyl benzoate

Yield: 0.022 mol methyl benzoate x Percent yield:

1.12g 5.74g

= 0.022 mol methyl benzoate (limiting reagent)

1 mol triphenylmethanol 1 mol methyl benzoate

x

260.34g triphenylmethanol 1 mol triphenylmethanol

= 5.73 g

x 100 = 19.5%

IR Results: Peak observed at 3465.02 cm-1  : This peak corresponds to the –OH group attached to the three

- 

phenyl groups. -

Peak observed in the 3063.29 cm-1  : This peak corresponds to the sp2 hybridized carbons 

around the aromatic rings.

Discussion: The objective of this experiment was to prepare triphenylmethanol using Grignard synthesis. By analyzing the melting point of the obtained product and using infrared spectroscopy we were able to confirm the successful formation of triphenylmethanol. The melting point of the product obtained fell within the range of the expected melting point. The expected melting point for triphenylmethanol 163 °C and the observed melting point of the product fell within the range 161.2 - 163.7 °C which is not a very broad range and very and is also very close to the expected value confirming the purity of the obtained triphenylmethanol. The peaks obtained in the IR correspond to the structural components of triphenylmethanol. The OH on a typical triphenylmethanol IR is a broad peak that can be found around 3550-3200 cm-1  . The main peak observed at 3465.02 cm-1  is representative of the alcohol

present in triphenylmethanol  which has a three benzene ring, a C=C (aromatic) peak around 1600-1585 cm-1 and 1500-1400 cm-1 was also seen. The peak found at 3063.29 cm-1 could be classified as the section where the three benzene rings were present. From these peaks seen in the IR analysis, it can be assumed that the correct product was formed in this experiment. Although triphenylmethanol was successfully obtained , the percent yield was 19.5%; this low yield can be due to a number of factors. Exposure to water vapor, leaving the product unattended for a week, the extraction, or washing of the product throughout the reaction could have lead to the lower than expected yield.

Conclusion:

The results obtained support that the product formed, triphenylmethanol, was obtained from grignard synthesis. The Grignard reagent created, phenylmagnesium bromide reacted with the ester, methyl benzoate via nucleophilic addition to obtain the tertiary alcohol, triphenylmethanol. triphenylmethanol. References and Notes Huddleston, Alexis. "Using the Grignard Reaction to Prepare Triphenylmethanol." Organic Chemistry Labs. Odinity, n.d. Web. Oct. 2019. Roberts, R.M. et al. “Experimental Organic Chemistry” Howard University Edition, Academx Publishing Services, 2014-2015. Pages 173-185....