Which reagent acts as the oxidizing agent in the oxidation of alcohols with acidified dichromate?

• A. Oxygen gas
• B. Acidified dichromate
• C. Potassium permanganate
• D. Hydrogen peroxide

Answer: (B)

Oxidizing agent is the species that accepts electrons and is reduced while the alcohol is oxidized. In the oxidation of alcohols with acidified dichromate, the acidified dichromate ion (Cr2O7^2− in acidic solution) does the accepting of electrons and is reduced from Cr(VI) to Cr(III). That is why it is the oxidizing agent here. The alcohol loses electrons and is oxidized to an aldehyde (or to a carboxylic acid under stronger conditions). The characteristic color change from orange dichromate to green Cr^3+ also signals this reduction of the oxidant. Oxygen gas wouldn’t be the oxidant in this setup, and although potassium permanganate or hydrogen peroxide are oxidants in other reactions, they’re not the reagents used in this particular oxidation.