Question

A Grignard reaction was performed using the following steps:

Step 1: Addition of Grignard Reagent: Phenyl Magnesium Bromide

A solution of phenyl magnesium bromide (2.7 mL of a 3.0 M solution in anhydrous diethyl ether) and 5 mL of anhydrous diethyl ether was dispensed into a round-bottom flask apparatus and stirred with a stir bar.

Step 2: Addition Formation: Reaction with Benzophenone

A solution of 0.72 g (mp = 48-49 degrees C) of benzophenone and 2.0 mL of anhydrous diethyl ether was prepared and added in 1.0 mL portions to the reaction flask. It was then left to sit for 13 minutes with occasional stirring.

Step 3: Acidification and Hydrolysis

6.0 mL of 3 M hydrochloric acid was added to the reaction flask to neutralize the reaction mixture.

Step 4: Isolation

Two distinct clear layers formed and using a separatory funnel the lower aqueous layer was removed and discarded. An equal volume of saturated sodium chloride solution was added to the separatory funnel and the lower aqueous layer was removed and discarded again. The remaining organic layer was transferred to a centrifuge tube and a small amount of anhydrous sodium sulfate was added and left to sit for 5 minutes. The ether solution was decanted into a flask and a small amount of methyl tert-butyl ether was added to the sodium sulfate to remove any remaining product. The additional ether was decanted into the flask as well.

Step 5: Purification

Since a by-product produced in this reaction is biphenyl, it is removed with pet ether. 6 mL of pet ether was added to the flask and swirled for 3 minutes. The solid was collected in a Hirsch funnel.

According to Step 5, biphenyl is an expected impurity for the reaction performed here. The formation of biphenyl from the reaction of bromobenzene with phenyl magnesium bromide may proceed via nucleophilic aromatic substitution of aryl halides (S_nAr).

There are two proposed mechanisms for S_nAr; the Meisenheimer complex intermediate mechanism for aryl halides with strong EWGs, and the benzyne intermediate mechanism for aryl halides without strong EWGs.

A) Write a balanced equation for the formation of biphenyl from bromobenzene and phenyl magnesium bromide.

B) Provide a method to reduce the formation of this impurity.

C) Explain the use of petroleum ether in separating this impurity from the product.

Answer 1