Grignard Synthesis

Objective:
The goal of this lab is to synthesize a Grignard reagent from bromobenzene and magnesium metal in diethyl ether. This same Grignard reagent would then be used to prepare a tertiary alcohol and then purify and characterize the product.

Procedure:
Refer to the microscale lab in pages 124-130 (15.1 and 15.2) in Modern Projects and Experiments in Organic Chemistry: Miniscale and William Microscale.

Observations:
Mixture of magnesium with bromobenzene turned into a yellow coloration after the bubbling. After much heating, the mixture turned into a brownish-mahogany color.

The mixture did not recrystallize after being placed in an ice water bath (this was after the separation of diethyl ether). It had to be heated once again in order for crystals to form.

Data:
Mass of magnesium: .074 grams
Mass of 1,1-diphenylethanol: .037 grams
Bromobenzene used: .32 mL
Acetophenone used: .23 mL
Melting range of 1,1-diphenylethanol: 80°C-81°C

Calculations:

acetophenone is the limiting reagent

Analysis:
The main goal of this lab was to create a Grignard reagent and then use that same reagent to produce a tertiary alcohol.

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This was accomplished by reacting the Grignard reagent with acetophenone. The end product was 1,1-diphenylethanol. Its recorded melting point range was 80°C-81°C, which is the same as its literature value. This means that there was no or very little impurity in the recrystallization process

From the IR, it is evident that the hydroxyl group (-OH), around 3400 cm-1, is present in this product. There is also a presence of –CH bonds that come from aromatic ring structures at around 3000 cm-1.

C-C bonds from aromatic structures can be seen at around 1500 cm-1.

The percent yield 22.35%. The reason for such a low yield could be due to the quenching of the Grignard reaction and the isolation of the product. It is possible that the product was lost during the filtration process.

1. If phenylmagnesium bromide were prepared, and any water was present in the reaction mixture, phenylmagnesium bromide would be quick to react with water, stealing one of its hydrogens. This hydrogen would therefore replace the attached –MgBr to form benzene.

Ph -MgBr + H2O —> Ph-H + HOMgBr

2. The Grignard synthesis would not proceed because Grignard reagents react quickly with alcohols due to their acidity, which would make them extremely good nucleophiles. Using ethanol would ruin the Grignard reagent before the carbonyl is added, which would consequently reducing the yield.

3. Mg + I2  MgI2

4. The reason why bromobenzene substrate is added in two portions, rather than one full portion, is because adding it all at once would cause an increase of the product 1-phenylethanone and very little phenylmagnesium bromide would be produce. Without phenylmagnesium bromide, the synthesis of 1,1-diphenylethanol would not be able to occur.