Question

C​omment on the problems of using chromium-based oxidizing agents in practical organic chemistry.

Answer 1

Chromium (VI) compounds are highly toxic for both acute and chronic exposures and can cause cancer, but chromium (III) is an essential element in human health and its toxicity is moderate. Care must be taken when handling chromium (VI) reagents (for example PDC) in finely powdered form, because such compounds have extremely high toxicity from inhalation and oral exposure.

If chromium compounds are used in excess, typically for the oxidation of alcohols to carbonyl compounds and carboxylic acids, any remaining chromium (VI) species can be quenched by the addition of 2-propanol. A color change from a typical orange to deep green indicates the complete quenching.

Another complications are to be expected when the double bond of an unsaturated alcohol is particularly reactive or when the alcohol rearranges readily under strongly acidic conditions. It is possible to avoid the use of strong acid through the combination of chromic oxide with the weak base azabenzene (pyridine). A crystalline solid of composition (C₅H₅N)₂ • Cr0₃ is formed when Cr0₃ is added to excess pyridine at low temperatures. (Addition of pyridine to Cr0₃ is likely to give an uncontrollable reaction resulting in a fire.)

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