Grignard Reaction

Brittany Vrindten

G01024513

Chemistry 318-212

2 April 2018

Grignard Reaction

Introduction

This two-week lab demonstrated a Grignard Reaction through the synthesis of benzoic acid. A Grignard reaction is the addition of an organomagnesium halid (Grignard reagent) to a ketone or aldehyde to create a tertiary or secondary alcohol (Mulcahy). The Grignard reagent in this experiment was phenylmagnesium bromide which was created by adding magnesium to bromobenzene in ether conditions. By completing this part of the reaction in ether conditions, the lone pairs of electrons on oxygen help to stabilize magnesium in the reagent (tigerweb). During this first part of the experiment, the formation of by products such as bi-phenyl could be present and could appear in the final IR spectra.

Organometallic compounds have reactivities that allow chemists to overcome the challenged involved with forming C-C bonds. It wasn’t until the 1920’s when Victor Grignard started working with organometallic chemistry (Mulcahy). A key technique of this time period was the acceleration of chemical transformations by ultrasound. It was believed that ultrasonic waves passed through a liquid cause the formation of bubbles that collapse with production of powerful shock waves. These shock waves are intended to clean and disperse the metal, so a clean surface is present for the reaction to occur. This experiment used the ultrasound modification, so the Grignard reagent does not need to be oven dried and cooled under dinitrogen atmosphere before use (Slayden, 115). This particular reaction used an aryl bromide, to create the Grignard reagent, which acts as a base and reacts with dry ice, carbon dioxide, to form the benzoate salt. The benzoate salt is hydrolyzed and creates the final product of benzoic acid.  

Methods

The overall mechanism for this experiment was given in the prelab found on page 66; however, there are several individual reactions that will occur. The first part of this experiment is to create the phenylmagnesium bromide, this was done by adding magnesium to bromobenzene in anhydrous ether conditions. In the lab, this was done by combining diethyl ether and bromobenzene in one vial and then adding this solution to a test tube that contained magnesium fillings. A cotton ball was placed on top of the test tube with the purpose of keeping out atmospheric water and oxygen and absorb ether that could evaporate during the reaction. This test tube was then placed in an ultrasonic water bath, in which the purpose of this step was mentioned in the introduction. If the reaction failed to start, Iodine crystals were added.

The second part of this reaction was the reaction between the Grignard reagent and carbon dioxide. Here the product from the first reaction, phenylmagnesium bromide, is poured over dry ice into a beaker. Dry ice is a form of carbon dioxide, and when added to phenyl magnesium bromide, will create benzoate salt. This benzoate salt will be saved in the beaker with a watch glass on top, until the following lab period, where benzoic acid will finally be created.

The next part of this experiment is the hydrolysis of the benzoate salt with hydrogen chloride and methyl-tert-butyl ether. This solution will be used to isolate the benzoic acid, through extraction procedure in a separatory funnel. The first extraction is done with water, then with a base sodium hydroxide. The aqueous layer will be heated, and then more hydrogen chloride is added to the beaker. If the correct layer was used a precipitate will form, if not add hydrogen chloride to the other layer and seeing a precipitate form. The product will then be vacuum filtered to get rid of any excess liquid present in preparation for recrystallization.

The last part of this experiment is the recrystallization of benzoic acid, which is done by adding distilled water to the product. The solution will be heated, and then cooled, which should dissolve the crystals. This solution will again be vacuum filtrated, and the final product is expected to be benzoic acid. This can be tested by completing an IR of the product dissolved in acetone. It is important to keep in mind that Grignard reactions are susceptible to several by-products that could appear in the IR spectra.

Results

In this section, two data tables will be provided each with IR wavelengths and corresponding functional groups for the product of the overall reaction, benzoic acid. In the IR spectra, some of the byproducts could be present such as biphenyl.  Benzoic acid is the product, and it will show peaks between 2500 and 3300 cm-1, 1680-1750cm-1, and 900-1100 cm-1 (IAC publishing).

Table 2. 1st IR spectra

Table 3. 2nd IR spectra

Calculation 1: Number of moles of reactants

Bromobenzene: 2.920 g * (x mole/157.000 g) = 0.0186 mole bromobenzene

Magnesium: 0.576 g * (x mole/24.305 g) = 0.0237 mole of magnesium

Calculation 2: Limiting reagent – bromobenzene

Bromobenzene: 2.92 g * (1 mole / 157 g) * (122.12 g/mole) = 2.271

Magnesium: 0.576 g * (1 mole / 24.305 g) * (122.12 g/mole) = 2.894

Discussion: Data analysis

Figure one, the first IR spectra, corresponds with table two which gives the major functional groups. The peaks in this figure are not all that accurate or defined compared to the theoretical IR spectra of benzoic acid given on the NIST chemistry webBook.

Figure two, the second IR spectra, corresponds with table three, which gives the major functional groups. This IR spectra is much more defined and resembles the theoretical IR spectra of benzoic acid. This spectrum also shows the major peak, at 3000 cm-1, which is an identifying peak for the by product of biphenyl. Both figures contained an OH peak, which indicates the presence of water. This means the product was not fully dried, however after the product was dried again, the IR peak that corresponds with water, became more of a medium peak instead of a strong peak seen in figure one.