Question

Why is presence of acetic anhydride necessary in the oxidation of toluene

Answer 1

Mix Toluene (3 ml), Potassium Permanganate (10 grams) and dilute solution of sodium hydroxide in water (20 ml) in round bottom flask and set up the reflux. Allow mixture to reflux for 3 to 4 hours until oily toluene disappears. Then cool down the solution and filter the manganese oxide from the solution. Now addition of concentrated HCl to the filtrate will give you the precipitate of benzoic acid . You can either recrystallize it with hot H2O, or can extract the benzoic acid with ether.

The reaction is simple oxidation where Methyl group gets converted to carboxylic group by the use of potassium permanganate as an oxidizing agent. Potassium hydroxide is used to provide alkaline medium because strong oxidizing agent Mn+7 is stable only in alkaline medium. Under neutral conditions Mn+7 is reduced to Mn+4 and under acidic conditions it is reduced to Mn+2; both of which are less powerful oxidizing agents. Added HCl first neutralizes the added KOH and after that converts potassium benzoate ( present in soluble form in filtrate after reflux) into benzoic acid.

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