Like what is it about chromium atom that make them able to oxidize atoms? Like can’t any other transition metal like manganese, mercury among several others do the same thing? I always see example of chromium in oxidation of alcohol and i ws curious why? Thanx.

  • $\begingroup$ Manganese is also another notable element which has ions that can catalyse oxidation. And strictly speaking, it is the related ions that do the oxidation, not the atoms. As to why this happens, have a look at their half reactions and oxidation states. $\endgroup$ – Linus Choy Feb 3 '17 at 23:47
  • $\begingroup$ Chromium used in the type of chemistry you're talking about is in a +6 oxidation state and can readily accept electrons. Manganese in the form of permanganate (+7 oxidation state) doesn't a great job of oxidizing alcohols though the oxidation is less controlled can leads to by-products that may include some cleavage of adjacent carbon-carbon bonds. Chromium just happens to balance reactivity with selectivity. $\endgroup$ – Zhe Feb 4 '17 at 2:17

Chromium is hardly the only element that is used to oxidise alcohols. Alcohols are really easy to oxidise and apart from chromium, you can also use manganese ($\ce{MnO4-}$ or $\ce{MnO2}$), ruthenium (TPAP), iodine (DMP), sulfur (Swern oxidation and variants), even nitrogen works (TEMPO), and in the Oppenauer oxidation carbon is formally the oxidising agent. The list really does go on, this is just the tip of the iceberg! In all of these reagents the element I named is the one that undergoes reduction.

The only criterion is that, well, it must be an oxidising agent. So, for example, you would probably be hard-pressed to find a calcium-based oxidising agent, since $\ce{Ca^2+}$ is pretty happy being $\ce{Ca^2+}$.

The issue is: saying that "chromium" is involved is a little misleading. Chromium metal is not a very good oxidising agent; neither is Cr(II), and Cr(III) not quite either. You can dump all the $\ce{CrCl3}$ you like into an alcohol, and you probably won't get any ketone. ("Probably" because who knows, you might very well get some ketone if you leave it open to air, since molecular oxygen is a pretty good oxidising agent. There was a slightly infamous incident several years ago where a lab claimed to have discovered a NaH oxidation of an alcohol to a ketone; it turned out that they didn't exclude oxygen thoroughly and that was what was causing the oxidation.)

It's always chromium(VI) that is in the oxidising agents. $\ce{CrO3}$, PDC, PCC, ... all have chromium in the +6 oxidation state. That's strongly oxidising and that's why it sees use as an oxidant. However as said earlier - it's hardly the only element used.

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