Triphenylmethanol Lab PDF

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Triphenylmethanol Lab Report...

Description

Preparation of Grignard Reagent, Phenylmagnesium Bromide, and Formation of Triphenylmethanol through Addition Reaction with Carbonyl Compound, Benzophenone

Results In this experiment, a Grignard reagent, phenylmagnesium bromide, was prepared and utilized in a carbonyl addition reaction to produce a tertiary alcohol, triphenylmethanol. As shown in Table 1, while the theoretical yield of triphenylmethanol was 3.436 g the actual yield of the crude product was 1.755 g. Therefore, the percent yield of the crude product was calculated as 51.077 % respectively. Through recrystallization of the crude product, 1.285 g of purified product was collected from the crude product. Therefore, there was a 73.219% recovery of the purified product from the crude product. The literature value of triphenylmethanol is 162 °C . However, the melting point of the purified product determined experimentally was 160.7-161.8 °C . Table 1: Theoretical, Actual and Percent Yield of Crude Product, Percent Recovery of Purified Product, and Melting Point Ranges of Purified Product Theoretical Yield (g)

3.436

Literature Value of Triphenylmethanol (°C)

162

Crude Product

Actual Yield (g)

1.755

Percent Yield (%)

51.077

Melting Point Range (°C)

N/A

Purified Product Actual Yield (g)

Discussion

1.285

Percent Recovery (%)

73.219

Melting Point Range (°C)

160.7 - 161.8

In this experiment, a Grignard reagent, phenylmagnesium bromide, was prepared via Grignard reaction. The reaction was conducted in a reflux apparaturs that was used to continually condense the highly volatile ether solvent. The reflux apparatus ensured the constant heating of the reaction so that the ether creates a barrier above the reaction to keep atmospheric moisture away from the reaction since Grignard reagents react readily with water. The Grignard reagent was then utilized to demonstrate the nucleophilic ability of the Grignard reagent through a carbonyl addition reaction with the carbonyl electrophile, benzophenone. The completion of the addition reaction was checked by the extraction and recrystallization to separate and purify the tertiary alcohol product, triphenylmethanol. A Grignard reagent is a type of organometallic, which contains of a bond between a metal and a carbon. Grignard reagent is usually prepared by adding a halogenoalkane to bits of magnesium in a flask containing ether as shown in the mechanism in Figure 1. It is ideal to keep everything that comes in contact with the reagent dry because Grignard reagents react with water to produce alkanes.

Figure 1: Mechanism for Formation of Grignard Reagent One of the main uses of Grignard reagents is to synthesize complicated alcohols with ease. For instance, In this experiment, the prepared Grignard reagent reacted with a carbonyl compound, benzophenone to synthesize a tertiary alcohol, triphenylmethanol. In the first stage of reaction, the Grignard reagent was added across the C=O bond. Then, dilute acid, HCl

specifically in this experiment, was added to hydrolyze it. Ultimately, tertiary alcohol, triphenylmethanol, was formed, as shown in Steps 1 and 2 of Figure 2 and in Figure 3.

Figure 2: Generic Grignard Reaction

Figure 3: Synthesis of Triphenylmethanol Using Grignard Reagent Grignard reagents generally react with carbonyl compounds because the C=O bond of the carbonyl compound is highly polar, with a significant amount of positive charge on the carbon atom while the carbon atom of the carbon atom and magnesium bond has a slight negative charge, also due to polarity. The negative charge of the Grignard reagent makes it a good nucleophile which is attracted to the positive charge of the carbonyl compound as shown in Figure 4. Because of the nucleophilic ability of the reagent, it is ideal to keep everything that comes in contact with the reagent dry because Grignard reagents react with water to produce alkanes.

Figure 4: Positive Charge of the Carbonyl Compound Relatively low percent yield of the crude product was observed at the end of the experiment. Theoretically, the maximum amount of crude product would have been collected if the prepared Grignard reagent reacted completely with the carbonyl compound, benzophenone. However, this process may have been hindered by several factors. First of all, the Grignard reagent may have not been properly prepared so that not all of the benzophenone may have reacted with the reagent to form triphenylmethanol. This was suggested by the pieces of magnesium was still left in the separatory funnel when the aqueous phase was extracted from the ether layer. The leftover magnesium pieces suggest that not all of the phenylmagnesium bromide have formed at the end of the reflux period. The lack of Grignard reagent would have led to the incomplete addition reaction to form the tertiary alcohol consequently. On the other hand, assuming the successful formation of the Grignard reagent, the addition of an ethereal solution of benzophenone to the Grignard reagent would produce the desired tertiary alcohol, triphenylmethanol. This assumption takes into account the glassware, ether, and benzophenone are all dry because Grignard reagents are strongly basic and therefor are quick to react with any protic solvents. Therefore, if the ethereal solution of benzophenone was contaminated with water, the Grignard reagent would rapidly react with the water to form benzene and a basic bromide instead of the tertiary alcohol which would have ultimately led to the imcomplete formation of triphenylmethanol that contributes to the loss in the overall yield of the crude product. Also, there may have been physical loss of product in the process of extracting the

aqueous layer during separation. The remaining magnesium pieces were extracted with the aqueous layer. This may have stopped a reaction that may have went to completion if the magnesium pieces were left in the organic layer, which would have led to the formation of more products. In addition to the small amount of crude product that was obtained through crystallization, the quality of the crystallization and the purity of the final product was also affected. This was determined by comparing the experimental melting point ranges of purified product to that of the literature value. The experimental melting point range of the purified product was lower than that of the literature value of the pure triphenylmethanol most likely due to the incomplete removal of impurities through recrystallization. However, the experimental melting point range of the purified product and the literature value was very proximate to one another which implied that although an incomplete purification occurred through the recrystallization, the reaction went to completion and actually synthesized triphenylmethanol. Finally, the removal of these impurities, such as such as biphenyl as shown in Figure 5, that were present with the crude product was removed through initial vacuum filtration with petroleum ether and recrystallization with isopropyl alcohol. The removal of byproducts and impurities in the crude product ultimately led to difference in mass between the crude product and the purified product.

Figure 5: Formation of Biphenyl Byproduct...