Can you help me the blue question SYNTHETIC PROCEDURE PERFORM THIS REACTION IN T

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Can you help me the blue question

SYNTHETIC PROCEDURE PERFORM THIS REACTION IN THE HOOD In a mortar and pestle, grind 375 mg (approximate amount) of magnesium metal to provide a fresh surface. Place the magnesium in a dry 25 mL round bottom flask equipped with a Claisen adapter and condenser with a calcium chloride drying tube. Add 5 mL of anhydrous diethyl ether. In a separate vial, measure out 2.5 g of bromobenzene Take a drop or two of the bromobenzene and add it below the ether close to the magnesium metal. This is so you have a high concentration of the halide at the surface of the metal. Warm the flask gently with a warm water bath. If you do not see any evidence of reaction (bubbling or the formation of turbidity) after about 5 minutes, you will need to use other means to initiate the reaction. Add a very small crystal of iodine to the reaction. Iodine reacts readily with magnesium and will serve to produce a "clean" surface on the metal. If this does not work, consult with your TA for other suggestions Once the reaction has initiated, dissolve the remaining bromobenzene in about 5 mL of ether and add it in a drop wise fashion to the reaction at a rate that will maintain the reflux. Once the addition is complete, warm the reaction to reflux and continue for 20-30 minutes Why do you think it would not be a good idea to add all of the bromobenzene at once prior to initiating the reaction with the magnesium metal?

Explanation / Answer

This is Grignard method.

The role of bromobenzene is to react with magnesium to make grignard reagent. The grignard reagent is strong base and strong nucleophile and it reacts with all protons that are found on water, bromobenzene, amines, thiols. If we add all of our bromobenzene then phenylmagnesium bromide (Grignard reagent) reacts with bromobenzene and will make Biphenyl. Biphenyl is a impurity. So to avoid this the bromobenzene should be added to the reaction mixture so that it wil react with the magnesium and not be present in high concentration to react with previously formed Grignard reagent.

The formation of Biphenyl reduce the formation rate of phenylmagnesium bromide because the rate determining step is reaction of bomobenzene with magnesium. If bromobenzene is in large amount the most of our phenylmagnesium will converted into Biphenyl and thus will reduce the amont of phenylmagnesium ( Grignard reagent). Hence it will reduce the yield of the reaction.

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