JEE Main · 2005 · 08 May 2005 · Offline · Alcohols Phenols and Ethers

The task is to lift the secondary alcohol pent-3-en-2-ol to the conjugated ketone pent-3-en-2-one while leaving the carbon-carbon double bond untouched, so the reagent has to be a mild, selective oxidant. A harsh oxidant such as acidic permanganate or acidic dichromate would not stop at the ketone; it would attack and cleave the alkene. Chromium(VI) delivered gently, as chromic anhydride ($\mathrm{CrO}{}_3$) in glacial acetic acid, oxidises the alcohol to the ketone while sparing the double bond, which is why it is picked here.