How do you count the following observations?
a) 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 balanced redox equation for the reaction.
b) When concentrated sulphuric acid is added to inorganic mixture containing chloride, we get colourless pungent smelling gas HCl, but if the mixture contains bromide then we get red vapour of bromine. Why ?

Explanation

a) Acidified permanganate oxidises organic compounds to carbon dioxide and water. But alkaline permanganate oxidises the organic compounds to aldehydes and acids. So for the manufacture of benzoic acid from toluene alkaline permanganate is used instead of acidified permanganate.
`2KMnO_(4)+H_(2)Oto2MnO_(2)+2KOH+3(O)`
`C_(6)H_(5)CH_(3)+3(O)toC_(6)H_(5)COOH+H_(2)O`
`2KMnO_(4)+C_(6)H_(5)CH_(3)toC_(6)H_(5)COOH+2MnO_(2)+2KOH`
b) Less volatile acids substitute more volatile acids from the salts. Concentrated sulphuric acid is less volatile and can substitute more volatile HCl and HBr from chlorides and bromides. But HBr is a reducing agent while HCl cannot act as reducing agent. So sulphuric acid can oxidise the colourless HBr to red vapour of bromine.
`2NaCl+H_(2)SO_(4)toNa_(2)SO_(4)+2HCl`
`2KBr+H_(2)SO_(4)toK_(2)SO_(4)+2HBr`
`2HBr+H_(2)SO_(4)to2H_(2)O+SO_(2)+Br_(2)`