Chromium reagents toxicity

Further oxidation of an aldehyde product to the corresponding carboxylic acid does not take place. Moreover, the SM>ern oxidation reaction does not require the use of toxic and pollutant chromium reagents. The activated DMSO species, however, are stable only at low temperature, which might in some cases be a drawback of this method. 

Many of the traditional oxidants are based on chromium) VI) compounds. These chromium reagents are highly toxic, and they are difficult to dispose of properly. Chemists are gradually moving to less toxic oxidants. We will cover the traditional chromium reagents and their uses, and then we will survey the more environmentally friendly alternatives. 

Finally, Cr(VI)-reagents should be mentioned for example the Jones reagent (H2S04/Cr03 in acetone) for the oxidation of primary alcohols to carboxylic acids and the oxidation of secondary alcohols to the corresponding ketones. The main problem with these chromium reagents is their high toxicity. 

Recently, procedures for oxidation of alcohols to aldehydes and ketones have been developed that obviate the toxicity associated with the use of chromium reagents. Because of the greater stability of ketones to most oxidizing conditions, the conversion of secondary alcohols to ketones can be accomplished with a wide variety of reagents and conditions (Table 4.1). 

In recent years, chromium reagents have become less popular. Although inexpensive and effective, they are rather toxic and some are carcinogenic. Disposal of waste products also presents an issue. Many researchers now use the Swern oxidation—this uses dimethylsulfoxide as the oxidant, and the alcohol is activated with oxalyl chloride. The mechanism is shown in Figure 19.19. The advantages of the process are the lack of Cr(VI), anhydrous conditions and... 

Although Cr(VI) oxidants are very versatile and efficient, they have one drawback, which becomes especially serious in larger-scale work the toxicity and environmental hazards associated with chromium compounds. The reagents are used in stoichiometric or excess amount and the Cr(III) by-products must be disposed of safely. 

The deployment of chromium(VI) reagents in the oxidative transformation is compromised due to inherent toxicity, involved preparation of its various complex forms (with pyridine or acetic acid) and cumbersome workup procedures. Chromium trioxide (Cr03) immobilized on premoistened alumina enables efficient oxidation of ben -... 

Hydrogen transfer reactions from an alcohol to a ketone (typically acetone) to produce a carbonyl compound (the so-caUed Oppenauer-type oxidation ) can be performed under mild and low-toxicity conditions, and with high selectivity when compared to conventional methods for oxidation using chromium and manganese reagents. While the traditional Oppenauer oxidation using aluminum alkoxide is accompanied by various side reactions, several transition-metal-catalyzed Oppenauer-type oxidations have been reported recently [27-29]. However, most of these are limited to the oxidation of secondary alcohols to ketones. 

Chromium in the +6 oxidation state, Cr(VI), is a very important and effective oxidant in the organic laboratory. The major drawback to the use of reagents based on this species is that the product, Cr(III), is toxic. Chromium is just one example of a toxic heavy metal that requires quite expensive disposal procedures... 

To overcome the problems of toxicity and work-up associated with many inorganic oxidants, it would be advantageous to develop a catalytic supported oxidant. Towards this aim, chromium(III)-impregnated Nafion 511 (NAFK) has been used as a catalytic oxidant in the presence of t-butyl hydix)peroxide. This reagent gives good yields of ketones (80-100%), but unfortunately oxidation of primary alcohols leads to the formation of complex mixtures. 

The procedure is commendable for its sinq>licity, reduced toxicity (chromium in all its oxidation states is carcinogenic) and achieves good yields of ketones from alcohol, for example, octan-2-ol is oxidized into octan-2-one (92%), cyclohexanol into cyclohexanone (90%) and menthol into menthone (98%). Pyridinium chromate is also a well-known oxidant for allylic oxidations. As a silica gel supported reagent, this is turned into an efficient alcohol oxidant that will leave acid-labile functions unscathed. Another advantage of the reagent is the long shelf-life of more than a year. These solid-supported oxidants also greatly facilitate pr uct work-up, when compared with their solution counterparts. 

Chromium-based oxidants are probably the most widely used of all oxidizing agents. Over the years they have been continually developed and modified to overcome the typical problems that occur during oxidation and to accept wider ranges of substrates with improved selectivities. They have been accepted readily by synthesis chemists since they are easy to handle and are often off the shelf reagents . However, they are not without their problems worit-up can be problematical overoxidation can occur, and, at all times, removal of the product from toxic chromium contaminants is a concern, especially with respect to large scale preparations. In an attempt to circumvent these problems the trend has been to develop the use of catalytic and/or supported reagents. Hiis review is concerned for the most part with the ai lica-tions and limitations of more recent chromium(VI) oxidants. Several other comprehensive reviews have appeared in this area and should be consulted for more detailed descriptions of older methods, chro-mium(V) oxidants, mechanism of oxidation and for typical experimental procedures. 

To the extent possible, all chemistry labs should convert to microscale experiments. This is the single most productive way to avoid the generation of significant wastes and to minimize student exposure. Experiments using toxic substances can often be conducted with alternative reagents [bleach instead of chromium (VI) for oxidation, for example]. 

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