If the dry ice employed in the synthesis of benzoic acid is allowed to stand exp

Question

If the dry ice employed in the synthesis of benzoic acid is allowed to stand exposed to air, it will absorb water and develop a coating of ice on the surface of the solid carbon dioxide. Given that the Grignard reagent reacts with water (ice) much faster than it does with carbon dioxide and that a ml of water is about 20 drops from a disposable pipet, how many drops of water are required to prevent the formation of benzoic acid? Write a mechanism for the formation of the product that results when Grignard reagent reacts with water.

Explanation / Answer

The Grignard reagent PhMgBr will react with traces of water or other protic solvents to produce benzene:

C6H5MgBr + H2O --> C6H6 + MgBrOH

If x is no. of moles of PHMgBr then no of droplets = x*(20*18)

The straight-forward synthesis of benzoic acid from benzene would utilise a Grignard reaction:

benzene --- Br2 / FeBr3 ---> bromobenzene --- Mg in dry ether --->
phenyl magnesium bromide --- 1. CO2(s) 2. H+/H2O ---> benzoic acid

C6H6 --- Br2 / FeBr3 ---> C6H5Br --- Mg in dry ether ---> C6H5MgBr
--- 1. CO2(s) 2. H+/H2O ---> C6H5COOH

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