Nucleophilic Addition to a Carbonyl: the Grignard Reaction Assignment

Nucleophilic Addition to a Carbonyl: the Grignard Reaction Assignment Words: 1147

Procedure: The experimental laboratory procedures were divided into two categories. First the formation of phenylmagnesium bromide, and second the reaction of the phenylmagnesium bromide with the carbonyl compound. However, before any of this could be done, the refluxing apparatus for the Grignard reaction was to be flame dried until no moisture remained inside because any water would cause the reagent to decompose and an alkane to form. The reaction would subsequently fail.

Drierite was placed inside a plastic drying tube as a drying agent, absorbing all the moisture from the solvents that would later be refluxing in the apparatus and coming out into the atmosphere. After setting this up, researchers continued to crush magnesium turnings into smaller pieces in order to expose fresh metal by removing possible magnesium oxide that was on the surface of the turnings. The turnings were added to the round bottom flask as well as an iodine crystal, which facilitates the reaction by cleaning the surface of the magnesium metal.

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Bromobenzene and anhydrous diethyl ether were then added to the flask. Anhydrous ether was used because at this point, no water was to enter the reaction. The reaction was triggered by swirling and the heat of a human hand. From here, the refluxing began and ended once the bromobenzene addition was completed. The second phase of the experiment involved the reaction of the phenylmagnesium bromide with assigned carbonyl compound. Appropriate amounts of benzaldehyde, benzophenone, or methyl benzoate based on the amount of bromobenzene used should be added to a dry beaker along with anhydrous diethyl ether.

The mixture was poured into the addition funnel, added to round bottom flask drop wise, allowed to reflux for an additional 15 minutes, and then cooled it to room temperature. Once the reaction mixture was cooled, it was poured onto 25g of ice and H2SO4. Diethyl ether was added to completely dissolve any remaining solids. The ether did not have to be anhydrous due to the fact that ice had already been added to the mixture and the refluxing was complete. Using a separatory funnel, the organic layer was extracted and washed with 5% sodium bicarbonate and deionized water.

In addition, the ether was dried over anhydrous magnesium sulfate which was removed through gravity filtration. The remaining ether was evaporated under reduced pressure and ligroin was added to the flask in order for the product to undergo vacuum filtration. The crude product was then recrystallized in boiling 95% ethanol. Results and Discussion: As the title of this report states, The Grignard Reaction simply involves the addition of a carbonyl reagent to another compound. It involves an important carbon-carbon bond forming technique that results in the formation of an alcohol.

In this experiment, the researchers investigated the formation of the reagent, phenylmagnesium bromide, its reaction with benzaldehyde, benzophenone, and methyl benzoate, and the products that would then form (Benzhydrol and Triphenylmethanol). Before beginning the experiment, the apparatus for the Grignard reaction was to be thoroughly flame dried. This is because any water or moist air adhered to the walls of the glassware would lower all yields or, in some cases, prevent the reaction from occurring. This would result in the decomposition of the reagent and in the formation of an alkane.

For example: Phenylmagnesium bromide was formed by means of a specialized apparatus involving the primary use of an addition funnel. Once the production of the reagent was complete, subsequent reactions with an aldehyde, ketone, and ester were executed. The first product formed by the reaction of phenylmagnesium bromide with benzaldehyde, is benzhydrol. In this experimental trial, the amount of crude sample recovered was 1. 04g with a percent yield of 27. 43%, and the amount of recrystallized sample recovered was 0. 74g with a percent recovery of 71%.

As shown in the mechanism below, the benzaldehyde is attacked by the reagent and later treated with sulfuric acid in order to form benzhydrol. The ketone and ester reagents were also treated with phenylmagnesium bromide in order to drive their subsequent reactions and form triphenylmethanol. The mechanisms below each depict these two reactions: While the benzaldehyde and benzophenone both react using only one phenylmagnesium equivalent, the methyl benzoate needs to react with two equivalents of phenylmagnesium bromide equivalents in order to produce triphenylmethanol.

This is because a Nucleophilic substitution reaction with one molar equivalent of the original phenylmagnesium bromide reagent produces an intermediate ketone that goes on to an addition reaction with a second molar equivalent of the Grignard reagent to produce the tertiary triphenylmethanol alcohol. Due to this, the relative speed of the Grignard reaction with the ester, methyl benzoate, is slowest when in comparison to the other benzaldehyde and the benzophenone reagents. The fastest reaction occurred with the benzaldehyde reagent.

When comparing aldehydes and ketones in nucleophilic addition reactions, aldehydes react faster than ketones. This is because of the fact that aldehydes do not have bulky alkyl groups attached to them as do ketones, which takes more effort to manipulate during a reaction. Ketones show some steric hindrance because of the presence of these alkyl groups. In addition, alkyl groups are electron releasing, creating an inductive effect that causes the carbonyl carbon to be less favored in a nucleophilic attack.

Therefore, the benzaldehyde reagent reacted the fastest in the nucleophilic addition, followed by the benzophenone, and then the slowest, methyl benzoate. As the reaction follows its standard path, a common byproduct, biphenyl, forms as the phenylmagnesium bromide reacts with the unreacted bromobenzene. Biphenyl, was physically detected when 15 ml of ligroin was added to the mixture and then vacuum filtrated. The byproduct ended up inside the 500 ml Erlenmeyer filter flask as a precipitate. The structure of biphenyl is as shown: The benzaldehyde reagent in the nucleophilic addition produced benzhydrol.

Benzhydrol’s HNMR spectrum clearly depicts the compound’s structure, displaying the presence of aromatic rings around 7. 1-7. 7 ppm. The presence of the hydroxyl group is collectively depicted from 2-6 ppm with significant peaks around 5. 8ppm and 2. 8 ppm. An IR spectrum of Benzaldehyde was also taken in order to analyze the functional groups present in the compound. For instance, at 1696. 69 cm-1, a carbonyl group is observed. Completing the aldehyde properties, a C-H bond can be observed at 2738. 09 cm-1 and 2817. 46 cm-1. The additional alkyl C-H bonds from the phenyl ring can be seen at 3000 cm-1.

In an additional IR spectrum of benzhydrol, its hydroxyl group is visible at 3269. 68 cm-1 where a broad peak may be observed. This proves that the appropriate Gringard alcohol product was indeed produced. Conclusions: All in all, the three independent Grignard reactions proved to be true successes. The benzaldehyde, benzophenone, and methyl benzoate reacted in with the phenylmagnesium bromide to form the Grignard products, benzhydrol and triphenylmethanol as displayed in the physical recovery of the products and the H NMR spectra.

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