Chromium salts reduction

Redox titrants (mainly in acetic acid) are bromine, iodine monochloride, chlorine dioxide, iodine (for Karl Fischer reagent based on a methanolic solution of iodine and S02 with pyridine, and the alternatives, methyl-Cellosolve instead of methanol, or sodium acetate instead of pyridine (see pp. 204-205), and other oxidants, mostly compounds of metals of high valency such as potassium permanganate, chromic acid, lead(IV) or mercury(II) acetate or cerium(IV) salts reductants include sodium dithionate, pyrocatechol and oxalic acid, and compounds of metals at low valency such as iron(II) perchlorate, tin(II) chloride, vanadyl acetate, arsenic(IV) or titanium(III) chloride and chromium(II) chloride. 

However, more than stoichiometric use of the chromium salt was problematic, and the toxicity of the salt makes this versatile process inadequate for large-scale synthesis. Truly catalytic use of chromium was achieved in 1996 by using Mn powder as a co-reductant (Equation (20)). In another approach, electrochemical reduction of... 

Another work on the Hquid phase hydrogenation of acetophenone is that of Casagrande et al. The reaction was studied over a series of silica-supported bimetallic catalysts with various Ru/Cr atomic ratios, which were prepared by reduction at room temperature with aqueous sodium tetrahydroborate. The nanostructured catalysts are very active in the low-pressure hydrogenation of acetophenone, although the selectivity towards 1-phenylethanol did not surpass 22% at 90% conversion. The addition of chromium salts to the starting solution gave rise to... 

Divalent chromium salts show very strong reducing properties. They are prepared by reduction of chromium(III) compounds with zinc [187] or a zinc-copper couple and form dark blue solutions extremely sensitive to air. Most frequently used salts are chromous chloride [7SS], chromous sulfate [189], and less often chromous acetate. Reductions of organic compounds are carried out in homogeneous solutions in aqueous methanol [190], acetone [191], acetic acid [192], dimethylformamide [193] or tetrahydrofuran [194] (Procedure 37, p. 214). 

In the tanning process hides are first washed or soaked, hair and keratinous debris are removed, bated (enzymes are used to break down non-collagenous components, which are washed out) and the hide is acid-pickled to prepare for the addition of the chromium salt. Contemporary processes are exclusively based on one-bath procedures and utilize chromium(III). The older two-bath process is now obsolete, mainly because it involved the in situ reduction of chromate, a major environmental and toxicological hazard (cf. chromate toxicity p. 947) to chromium(III) on the hide. A useful review of the history of chromium tannage processes is available.1205... 

Oxidation of Alcohol.—When ethyl alcohol is treated with potassium bichromate in the presence of dilute sulphuric acid a volatile substance with a peculiar sweet odor is given off. At the same time the reduction of the bichromate is indicated by the appearance of the green color characteristic of chromium salts. The volatile product is termed an aldehyde, specifically acetaldehyde, and when analyzed proves to have the composition, C2H4O. As this differs from the alcohol by two hydrogen atoms the action has plainly been one of oxidation by which two hydrogen atoms have been removed. The name is derived from this relation to alcohol, from the two words al- cohol) dehyd-(rogenatum). 

Reduction of the triphenylcyclopropenium cation with zinc, magnesium and chromium salts, or by electrolysis affords hexaphenyl 3,3 -bicyclopropenyl via the triphenylcyclopro-penyl radical With lithium aluminium hydride the triarylcyclopropenium ions are reduced to the corresponding triarylcyclopropene. 

First, a screen based on physicochemical properties should be used. This usually means pH, but also should be extended to materials with high oxidation or reduction potentials (e.g., hexavalent chromium salts). 

In Fig 7.4 the integrated membrane process for the recovery of chromium salts from exhausted baths of the chromium tannage is reported. In the proposed process, the exhausted effluent is submitted to a pre-treatment step by UF, producing a strong reduction of suspended solids and fat substances. Spiral wound UF membranes in polyvinylidenefluoride (PVDF) with a MWCO of 25 kDa was able to reject these compounds at 84% and 98%,respectively. About 40% of organic nitrogen was retained by... 

In order to determine if the black material added to the autoclave was in fact chromium metal or a mixture of Cr(II) salts and potassium, the following experiments were carried out. The black material obtained from the reduction of CrCls BTHF with potassium in benzene was repeatedly extracted with THF to remove and to determine soluble chromium(ll) salts. Also the amount of unreacted potassium was determined by repeated extraction of the black material with alcohol and water and subsequent acidic titration of the extracts. From these two determinations, it was established that a minimum of 58% of the Cr(CO)e formed in the autoclave reaction originated from chromium(O). In a separate experiment, the black material was extracted repeatedly with THF to remove all the soluble chromium salts and was then treated with CO under the same conditions as in the previous reactions. The yield of Cr(CO)6 was 30% based on the material placed in the autoclave. Thus, it is clear that Cr(0) is produced in the reduction, and this Cr(0) will react with CO to yield Cr CO)e- It is difficult to determine the exact amount of Cr(CO)6 which originated from Cr(0) in the black powder due to its instability. Thus, excessive manipulations of the black powder even under argon or very long reduction times in the generation of the black powders seem to deactivate the Cr(0). 

The corrosion products, that are produced in corrosion of higher chromium-alloyed steels, contain chromium salts that lower pH levels considerably in the process of hydrolysis. To investigate the potential influence of this more pronounced pH level reduction, steels with different chromium contents were tested [66]. Table 75 lists the alloying contents of the steels used in order of increasing chromium content. These are four standard commercial types and four test melts. Bending samples were used as described in DIN 50915 [64]. The sample material was adjusted to different hardness values by means of simulated heat treatments. 

The hydrosilylation of terminal alkynes disclosed by Trost can be applied to internal alkynes as well. i Remarkably, the (Z)-isomer is generated in this process, resulting from trans addition during hydrosilylation. The protodesilylation of these sily-lated products in the presence of copper(I) iodide and tetrabuty-lammonium fluoride (TBAF) or silver(I) fluoride (eq 15) leads to internal fraws-olefins. This two-step method is a useful synthetic transformation to access ( j-alkenes from internal alkynes. In contrast, the chemoselective reduction of alkynes to the corresponding ( -alkenes is conventionally accomplished readily with Lindlar s catalyst. The complementary process to afford ( )-olefins has proven much more difficult. Methods involving metal hydrides, dissolving metal reductions, low-valent chromium salts provide the desired chemical conversion, albeit with certain limitations. For example, functional substitution at the propargylic position (alcohols, amines, and carbonyl units) is often necessary to achieve selectivity in these transformations. Conversely, the hydrosilylation/protodesilyla-tion protocol is a mild method for the reduction of alkynes to ( )-alkenes. 

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