Synthesizing secondary alcohol diphenylmethanol through the Grignard reaction

The aim of this experiment is to synthesize the secondary alcohol diphenylmethanol, from phenyl magnesium bromide and benzaldehyde using the Grignard reaction. The Grignard reaction is important in organic chemistry as it allows for the controlled formation of carbon-carbon covalent bonds, which is an asset in developing pharmaceuticals and industrial chemical compounds. During the reaction, a combination of bromobenzene, magnesium and ether were used to synthesize the Grignard reagent (phenyl magnesium bromide), which was then mixed with benzaldehyde for the synthesis of diphenylmethanol.

The goal for this lab is to test the efficiency and effectiveness of the Grignard reaction in producing the desired compound by calculating the percent yield and testing the product’s melting point. The result is expected to be crystalline diphenylmethanol which would have a melting point close to 69°C and would have a mass similar to the theoretical mass which was 2.71 g. In Lab 5, the goal was to use the Grignard reaction to synthesize diphenylmethanol using bromobenzene, ether, magnesium and benzaldehyde. This was accomplished by synthesizing an organomagnesium compound from alkyl/vinyl/aryl halides (specifically bromobenzene) and magnesium metal. It formed a cloudy mixture that started off a yellow colour but darkened to dark brown black.

The subsequent Grignard reagent is a strong nucleophile and will then react with the electrophilic carbon on the benzaldehyde (a carbonyl) to form an alkoxide. When adding benzaldehyde and anhydrous ether to the mixture, the solution turned back to a yellow colour before splitting into a white layer (top) and light pink layer (bottom). Water or dilute acid is then used neutralize the alkoxide, with the final product being diphenylmethanol (an alcohol) after the two homogenous layers were separated and recrystallization.

The results obtained from this experiment were compared with current literature data. It was found that the sample’s melting point was 66.5 – 67.5 °C, which indicates that the sample of diphenylmethanol produced was impure, as diphenylmethanol has a recorded melting point of 69 °C. Secondly, the sample’s melting point was displayed over a wide range, which indicates that an impure substance was produced. The amount of diphenylmethanol that was recovered at the end of the experiment was 1.52 g. In comparison to the theoretical yield of 2.71 g, this experiment had a percent yield of 56.1%, which is comparatively lower than expected.

There were many possible sources of error that could have affected the experiment. The percent yield may have affected because the ether extract may not have been thoroughly washed completely, thus leaving some residual hydrochloric acid. This would result in the alcohol being dehydrated, which would have produced diphenylmethylether. The aqueous solution could also have caused contamination if it was not completely filtered out. When adding other compounds during the reaction, the stopcock was open which could have allowed some evaporated solution to escape. Loss of solution could also have occurred during the transfer of materials between equipment or when separating the separatory funnel mixture. Lastly, the glassware being air-dried could have resulted in some leftover water, which would have caused a loss of product.

While the Grignard reaction was able to produce diphenylmethanol, this experiment demonstrated that it was not the most efficient method. Overall, very little diphenylmethanol was recovered in real life compared to the theoretical mass that would have been produced. The final product was also impure, as demonstrated by the resultant melting point. Thus, the Grignard reaction was not effective in this experiment due to the large number of ways the experiment could be flawed such as solvent contamination or loss of materials.

The greatest advantage in employing the Grignard reaction is the synthesis of secondary of tertiary alcohols through the use of an organomagensium halide, which forms ketone and aldehyde intermediates. Its capability to produce carbon-carbon bonds and build more complex reagents from simple compounds is very useful in the workforce. However, using the Grignard reaction in real-life applications could be problematic as percent yield and purity were both lower than expected. In this lab, 1.52 g of diphenylmethanol was synthesized, resulting in a percent yield of 56.1%. The percent yield is considerably low and can be attributed to the possible synthesis of undesired products or other human errors. The melting point obtained was 66.5 – 67.5 ºC, which is a clear indication of the presence of impurities and thus demonstrating that the final product was not pure.

In this particular experiment, the Grignard reaction is not effective in relation to the yield or purity of the product. If diethyl ether was replaced with the acetone as the solvent, it would produce a tertiary alcohol as the major product. This is because the carbonyl group is acetone could be attacked by alkyl/vinyl/aryl halides whereas diethyl ether, being an aprotic compound, will not react. Sodium bisulfite is very polar and is used to remove polar impurities from the reaction. Adding sodium bisulfite washes the ether layer and removes any trace of the remaining aqueous layer that was left from the separation stage, thus allowing for complete removal of any trace acid from the ether layer. If there was some leftover HCl, it would result in the alcohol undergoing dehydration.

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