Question

An unknown hydrocarbon was found to be insoluble in concentrated sulfuric acid. When added to aqueous potassium permanganate solution, a brown precipitate was observed and the solution became colorless almost immediately. Explain the results observed and decide whether the unknown is a saturated or an unsaturated hydrocarbon

Answer 1

Given that an unknown hydrocarbon is insoluble in sulphuric acid (H2SO4) which is a highly protic polar solvent. This indicates the absence of any polar functionality (e.g. –OH, -C=O) in an unknown compound and is a pure hydrocarbon containing C and H only.

An Unknown compound when added to aq.KMnO4 solution (purple color solution), brown ppt. settles down at bottom, a purple color disappears and a solution become clear. This observation is indication of the presence of unsaturation in an unknown.

The reaction between olefinic compound and aq.KMnO4 solution given in image file. It’s clear that brown ppt. is of MnO2 and purple disappears as KMnO4 in consumed and Gem-diol formed from olefin.