Question

Though alkaline potassium permanganate and acidic potassium permanganate both are used as oxidants, yet in the manufacture of benzoic acid from toluene we use alcoholic potassium permanganate as an oxidant. Why? Write the balanced redox equation for the reaction.

Answer 1

Hint: Homogeneous reactions proceed at a faster rate than heterogeneous reactions.Heterogeneous chemical reactions involve two or more phases.

Complete step by step answer:
Potassium permanganate is an oxidizing agent. If the oxidation reaction of potassium permanganate is carried out in acidic or alkaline medium, then there is a cost of adding acid or alkali. However, in alcoholic neutral medium, the hydroxide ions are produced in the oxidation reaction and there is no need for incurring extra cost for addition of acid or base.
Also, polar potassium permanganate is soluble in polar alcohol. Similarly toluene and alcohol are miscible with each other. Thus, the reaction mixture containing potassium permanganate, toluene and alcohol is a single phase reaction. It is a homogeneous mixture. Homogeneous chemical reactions proceed faster than heterogeneous chemical reactions. Heterogeneous chemical reactions involve two or more phases. If aqueous acidic or alkaline potassium permanganate was used instead of alcoholic potassium permanganate, then it will be a heterogeneous reaction mixture which will proceed at a slower rate. The reaction mixture is heterogeneous as toluene and water are immiscible.
The balanced redox reaction is as follows:
\[\begin{array}{l}
 {{\rm{C}}_6}{{\rm{H}}_5} - {\rm{C}}{{\rm{H}}_3}\left( l \right){\rm{ + 2 MnO}}_4^ - {\rm{ }}\left( {{\rm{alcoholic}}} \right){\rm{ }} \to {\rm{ }}{{\rm{C}}_6}{{\rm{H}}_5}{\rm{CO}}{{\rm{O}}^ - }\left( {{\rm{alcoholic}}} \right){\rm{ }} \\
 {\rm{ + 2 Mn}}{{\rm{O}}_2}\left( s \right){\rm{ + }}{{\rm{H}}_2}{\rm{O}}\left( l \right){\rm{ + O}}{{\rm{H}}^ - }\left( {aq} \right){\rm{ }} \\
 \end{array}\]

Note:
Since hydroxide ions are produced in the reaction, write benzoic acid as benzoate anion. During the reaction in neutral medium, potassium permanganate is converted into manganese dioxide and not in potassium manganate.