Oxidation by Chromic Acid

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One of the reagents that is commonly used for oxidation in organic chemistry is chromic acid. This reagent is straightforward to use once deciphered. However, there are a vast number of different ways that textbooks (and instructors) show it being used in reactions.

chromic-acid-h2cro4-can-be-made-from-k2cr2o7-and-other-chromate-salts-with-acid

Chromic acid, H₂CrO₄, is a strong acid and a reagent for oxidizing alcohols to ketones and carboxylic acids. For fairly mundane reasons owing primarily to safety and convenience, chromic acid tends to be made in the reaction vessel as needed (through addition of acid to a source of chromium), rather than being dispensed from a bottle.

And that’s where the trouble begins. Choosing a source of chromium to make H₂CrO₄ from is a lot like choosing a favorite brand of bottled water. Beyond the packaging, they’re pretty much all the same. Depending on which textbook or instructor you have, however, you might see several different ways to do this, and it can be very confusing.

The key point is that Na₂CrO₄ (sodium chromate), Na₂Cr₂O₇ (sodium dichromate), K₂CrO₄(potassium chromate), K₂Cr₂O₇ (potassium dichromate), and CrO₃ (chromium trioxide) are all alike in one crucial manner: when they are combined with aqueous acid, each of them forms H₂CrO₄, and ultimately it’s H₂CrO₄ which does the important chemistry. Unfortunately I rarely see this point explained in textbooks. I remember this causing some confusion for me when I took the course. The K or Na ions present are just spectators.

Once H₂CrO₄ is formed, its reactions are pretty straightforward: it converts primary alcohols (and aldehydes) to carboxylic acids and secondary alcohols to ketones.

chromic-acid-oxidizes-primary-alcohols-to-carboxylic-acids-and-secondary-alcohols-to-ketones

It does this through addition of the alcohol oxygen to chromium, which makes it a good leaving group; a base (water being the most likely culprit) can then remove a proton from the carbon, forming a new π bond and breaking the O-Cr bond.

mechanism-for-chromic-acid-oxidation-of-primary-alcohols-to-carboxylic-acids

Due to its high toxicity, chromic acid tends to find very little use in the organic chemistry laboratory outside of undergrad labs. There are far more useful reagents out there for performing these transformations.

Contributors

James Ashenhurst (MasterOrganicChemistry.com)