Question

Please write chemical reaction for:

3,4-Methylenedioxybenzaldehyde (piperonal) in Chromic Acid Test: Alternative Test (Jones Test)

Answer 1

Jones test involves subjecting an organic molecule to Jones reagent and noting the colour change. Jones reagent consists of chromium trioxide and sulphuric acid in a polar, aprotic solvent. The combination of the two forms chromic acid. Instead of the oxide, potassium dichromate can also be used. The chromic acid formed acts as the oxidant and like all metal oxide oxidations, it forms a metal ester with the alcohol and then oxidises it while having the metal center reduced.

The use of Jones reagent in alcohols can be stopped at the carbonyl stage only when the reaction is monitored and subjected to very specific conditons as it is rapid and quite exothermic. Usually, in oxidation reactions with Jones reagent, there is very little control and the substrate goes all the way to form carboxylic acid. In oxidations of alcohols, the carbonyls formed are only intermediates and yield final products as acids.

When aldehydes are subjected to the reaction, they proceed to form carboxylic acids in all possible conditions. Also, the reagent does not oxidise ether linakges and so the reaction of piperonal with Jones reagent will only convert the aldehyde to an acid giving piperonylic acid as the product. In the redox reaction, the dark orange chromium (VI) gets reduced to blue-green chromium (III). Only primary alcohols, secondary alcohols and aldehydes show a positive Jones test.