Toxic chromium

Fire Hazards - Flash Point Not flammable Flammable limits in Air (%) Not flammable Fire Extinguishing Agents Not pertinent Fire Extinguishing Agents Not To Be Used Not pertinent Special Hazards cf Combustion Products Toxic chromium oxide fiunes may fom in fires Behavior in Fire Can increase the intensity of fires when in contact with combustible materials Ignition Temperature Not pertinent Electrical Hazard Not pertinent Burning Rate Not pertinent. 

Some heavy metals and semi-metals are quite toxic (chromium, lead, and antimony) and expensive care is needed to prevent them from being dispersed in the environment. Lead in gasoline and paint has been... 

Caution All operations should be conducted in a well-ventilated hood with breathing protection. The chromium carbene complex generally is contaminated with the very volatile and toxic chromium hexacarbonyl, which is also generated as a by-product of the reaction. 

Many of the compounds formed by transition elements appear in various colors. Several are very toxic. Chromium, zinc, cobalt, nickel, and titanium are carcinogenic. 

Chromium hexaca/bonyl Highly Toxic Chromium carbonyl (8) Chromium carbonyl (OC-6-11)- (9) (13007-92-6)... 

This coupling between halides and aldehydes is a chromium-induced redox reaction. A key advantage is the high chemoselectivity toward aldehydes. A disadvantage is the use of excess toxic chromium salts. 

An interesting finding regarding potentially toxic chromium (and cobalt) in the body is elevated blood and urine levels of these metals in patients who have undergone total hip replacement.5 The conclusion of the study was that devices such as prosthetic hips that involve metal-to-metal contact may result in potentially toxic levels of metals in biological fluids. 

The results of animal studies therefore indicate that chromium(VI) compounds are development toxicants. Chromium(III) oxide was not a developmental toxicant in mice by the oral route but chromium(III) chloride was a developmental toxicant. 

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... 

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. 

The search is on for catalysts to replace those containing toxic heavy metals. The addition of hydrogen chloride to acetylene to form vinyl chloride is catalyzed by mercuric chloride. Rhodium (III) chloride on activated carbon works just as well and is much less toxic 97 It should be tried also in other addition reactions of acetylene as well as in trans-esteriflcation reactions of vinyl acetate. The reduction of 2 ethyl-2-hexenal to 2-ethylhexanol can be catalyzed by a mixture of copper, zinc, manganese, and aluminum oxides in 100% yield.98 This is said to be a replacement for carcinogenic copper chromite. In Reaction 4.15, the amount of toxic chromium(II) chloride has been reduced from stoichiometric to catalytic (9-15 mol% chromium(II) chloride) by the addition of manganese metal.99... 

The discovery route utilized the pyridinium chlorochromate (PCC) oxidation of 2-cyclohexylethanol in CH2CI2 in presence of molecular sieves. It is a simple process, as the aldehyde is simply isolated by filtration of the reaction mixture through silica gel. However, this process was proven to be difficult to scale up due to difficulties of filtration of the chromium salts. Furthermore, the environmental issues created by the large amount of toxic chromium salts make this process unsuitable for large-scale synthesis. Two other processes (Scheme 6.7) were therefore developed and tested to prepare the required 2-cyclohexyl acetaldehyde at the pilot-plant scale ... 

Over time, increased quantities of chromium compounds have been used by man and introduced into the environment (Gauglhofer and Bianchi 1991). The danger of environmental contamination depends on the solubility and oxidation state of chromium [International Programme on Chemical Safety (I PCS) 1988], Chromium(III) compounds are generally poorly soluble and show little or no toxicity. Chromium in its hexavalent form is 100 to 1000 times more toxic than the most common trivalent compounds (Katz 1991 Katz and Salem 1993). Hexavalent chromium compounds can reduce plant growth and cause skin and respiratory irritation and ulceration and eventually lung cancer. Hexavalent chromium is recognized... 

Speciation and solubility of chromium in wetlands and aquatic systems is governed by the competition among chromium oxidation states, adsorption/desorption mechanism, and soil/sediment redox-pH conditions. Chromium (VI) is reduced to chromium (HI) at approximately +350 mV in soils and sediment. Reduced Cr(III) can be rapidly oxidized to the tetravalent chromate and dichromate forms by manganese compounds. Cr(III) is much less soluble in natural system than the hexavalent form and has a much lower toxicity. Chromium is less likely to be a problem in wetlands than in nonwetlands because the reducing conditions cause its reduction or conversion to the more insoluble Cr(III) form. This is depicted in Figure 12.15, which shows changes in water-soluble chromium as affected by the soil redox potential. 

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