chromium oxidant
/ KROH-mee-um /
For most of the twentieth century, when a chemist wanted to oxidise an alcohol the reagent of choice was a chromium compound. These are the classic, reliable oxidants — chromium in its +6 oxidation state, a hungry electron-acceptor that strips hydrogens off an alcohol and gets reduced (turning orange Cr(VI) into green Cr(III), a handy visible signal that oxidation has happened).
Several forms exist, tuned to different jobs. Chromic acid (H2CrO4, generated from Na2Cr2O7 or CrO3 in aqueous sulfuric acid) and the Jones reagent are strong, watery oxidants that carry a primary alcohol all the way to a carboxylic acid and a secondary alcohol to a ketone. PCC (pyridinium chlorochromate) is the milder, anhydrous workhorse: dissolved in a dry solvent like dichloromethane, it stops a primary alcohol cleanly at the aldehyde because there is no water around to push it on to the acid. All of them take secondary alcohols to ketones and leave tertiary alcohols untouched.
Chromium reagents are powerful and predictable, but they carry a real cost: chromium(VI) is toxic and carcinogenic, and the spent chromium waste is an environmental hazard. That is precisely why modern green-chemistry alternatives — the Swern oxidation, Dess-Martin periodinane, and catalytic TEMPO systems — were developed to do the same selective oxidations without heavy-metal waste. Chromium oxidants remain the textbook reference point and are still used, but with growing caution.
1-hexanol with PCC in dichloromethane gives hexanal (the aldehyde) and stops there; the same alcohol with Jones reagent (CrO3 / H2SO4 / water) gives hexanoic acid (the carboxylic acid).
Anhydrous PCC stops at the aldehyde; watery chromic acid goes on to the acid.
The colour change orange to green signals that Cr(VI) has been reduced to Cr(III) — this is the basis of the old breathalyser test, where breath ethanol turned the chromium green.