Question

Based on the grinard reagent/reaction,

1. During the preparation of phenylmagnesium bromide, an unavoidable byproduct is formed; what is this side product? Illustrate its formation by a chemical reaction.

2. What precaution should be taken to minimize the formation of the above mentioned byproduct?

3. During the preparation of phenylmagnesium bromide, what other byproduct will form if you expose your reaction to CO2? Illustrate its formation by a chemical reaction.

4. If you don’t oven-dried your glassware or used a drying tube, what byproduct will you produce? Illustrate its formation by a chemical reaction.

5. You have air-dried your glassware and used a drying tube during the experiment. However, you used a wet methyl benzoate would the byproduct formed be different from that of question 4 above? Explain your answer

6. Triphenylmethanol (the final product to be obtained during this experiment) can also be prepared by the reaction of PhMgBr with diethylcarbonate (CH3CH2O)2C=O, followed by H+ workup. Draw the corresponding chemical reaction, and explain your answer.

Answer 1

1) Biphenyl is formed as a by-product during the formation of phenyl magnesium bromide from bomobenzene and magnesium metal. The initially formed phenyl magnesium bromide reacts with unreacted or excess bromobenzene to form biphenyl. Another possibility which may lead to biphenyl side product occurs when the reaction is carried out in presence of air (oxygen). The mechanism of reaction of phenyl magnesium bromide with oxygen is no exactly known; however, the proposed mechanism for the reaction of phenyl magnesium bromide with bromobenzene is shown below.

2) The formation of biphenyl may be prevented by using stoichiometric excess of magnesium metal. This will ensure that all the bromobenzene available bromobenzene is converted to phenyl magnesium bromide and the phenyl magenisum bromide so formed will have no bromobenzene to react with and form biphenyl.

Another way to mitigate the formation of biphenyl is to ensure that the reaction takes place in inert, air-free atmosphere. Therefore, the reaction flask must be charged with nitrogen to make sure no air is present in the reaction vessel.

3) Phenyl magnesium bromide reacts with carbon dioxide (CO₂) of air to form benzoic acid; hence, benzoic acid is a by-product of the reaction if the Grignard reaction is carried out in presence of CO₂. The formation of benzoic acid from phenyl magnesium bromide is well documented and the mechanism is shown below.

4) Benzene is formed as a by-product of the reaction if the reaction is carried out in presence of water. The reaction can be demonstrated as below.

PhMgBr + H₂O ---------> Ph-H + Mg(OH)Br