Question

Make a flow chart for the synthesis of Benzoic Acid from phenyl magnesium bromide and carbon dioxide. Using your knowledge of the physical properties of the solvents, reactants, and products, show how the products can be purified. Indicate which layer should contain the product in the liquid/liquid extraction steps.

Answer 1

FLOWCHART:

PURIFICATION AND ISOLATION OF BENZOIC ACID:

Phenyl magnesium bromide and ether are added to the beaker containing dry ice (source of CO₂) which results in the formation of benzoate ion. This solution is then made acidic using mineral acids like HCl. The benzoate ion gets protonated to benzoic acid (less polar so soluble in hot water or organic solvents) which dissolves in ether layer while the rest of the products like unreacted magnesium ions, OH^-, Cl^- ,bromomagnesium ion remain in the aqueous layer. Ether layer is seperated from aqueous layer and extracted several times with aqueous NaOH solution. NaOH converts benzoic acid to its sodium salt which being highly polar goes to the aqueous layer (several extrations ensure complete removal of benzoic acid from ether). Both the layers are seperated and aqueous layer is boiled to ensure complete removal of ether. Resulting solution is cooled and made acidic by HCl to precipitate out benzoic acid.