Chromate-based oxidants

C. Oxidation with Ruthenium Tetraoxide and Chromate Based Oxidants... 

An alternative tt> the chromium trioxide-pyridine comidex is provided by pyridinium chlorochromate (PCC) and pyridinium dichromate (PDC). These reag ts, now ubiquitous for chromate-based oxidation of alcohols, overcome the hygroscofric nature of the chromium trioxide-i ridine complex and are prepared by a less hazardous procedure both are commercially available as are several other derivative reagents. 

Adsorption inhibitors act by forming a film on the metal surface. The action of traditional oil-based red lead paint formulations presumably involves the formation of soaps and the precipitation of complex ferric salts that reinforce the oxide film. There has been substantial interest in recent years in development of replacements for lead-based and chromate-based inhibitor systems. Adsorption inhibitors based on pol3rmers have been of particular interest. In this volume, Johnson et al. and Eng and Ishida discuss inhibitors for copper 2-undecylimidazole is shown to be effective in acid media, where it suppresses the oxygen reduction reaction almost completely. Polyvlnyllmidazoles are shown to be effective oxidation inhibitors for copper at elevated temperatures. Also in this volume, Chen discusses the use of N-(hydroxyalkyl)acrylamide copolymers in conjunction with phosphate-orthophosphate inhibitor systems for cooling systems. 

Using chromium-based oxidants 2,4-Dimethylpentane-2,4-diol chromate(VI) diester, 122 Trimethylsilyl chlorochromate, 327 Using other oxidizing agents... 

Using chromium-based oxidants 2,4-Dimethylpentane-2,4-diol chromate(VI) diester, 122 Trimethylsilyl chlorochromate, 327 Using other oxidizing agents Bis(tributyltin) oxide, 41 Hydrogen hexachloroplatinate(IV)-Copper(II) chloride, 145 4-Methoxy-2,2,6,6-tetramethyl-1 -oxopiperidinium chloride, 183 Osmium tetroxide, 222 Potassium nitrosodisulfonate, 258 Samarium(II) iodide, 270 From alkenes by addition or cleavage reactions... 

Sometimes, an alcohol via the corresponding chromate ester may direct a chromium-promoted epoxidation of an aJkcne. This side reaction, which can happen with other chromium-based oxidants,83 depends on very exacting stereoelectronic factors to occur. 

Mechanistic evidences show that PDC, similar to other chromium-based oxidants, operates via an intermediate chromate ester that evolves to a carbonyl compound in the rate-determining step.125... 

A useful application of chromium-based oxidants, especially pyridinium chlorochromate, is in the conversion of allylic tertiary alcohols to their transposed a,(3-unsaturated ketones. For example, treatment of the allylic alcohol 24 with PCC gave the a,p-unsaturated ketone 25 (6.23). The reaction is thought to proceed by rearrangement of the chromate ester of the allylic alcohol to give a new allyl chromate ester that is oxidized to the ketone. 

Organic Base oxide/chromate, magnesium oxide Organic Primers and Topcoats Epoxies, epoxy esters, polyesters, alkyds, vinyls, acrylics, oils, urethanes, cellulosics... 

Metal [258], metalloid [259], and nonmetal [260] -based oxidants are very efficient but rather toxic agents thus, vanadium oxytrichloride [258f], 2,6-dicarboxypyridinum chlorochromate (DCPCC) [258a], bis(trimethylsilyl) chromate (BTSC) [258e], chromic acid [261], and selenium... 

The popularity of lead chromate(VI) oxide as a pigment has declined in recent years due to the increased demand of the cheaper and more durable molybdate orange and red (qq.v.) pigments which are based on the composition PbCrO4.PbM04. PbS04. 

Many catalyst chemical formulations and geometric shapes are used to promote oxidation-reduction reactions. Chemical types used for VOC oxidation include platinum, platinum alloys, copper chromate, copper oxide, cobalt oxide, chromium oxide, manganese oxide, and nickel. The catalysts are often categorized as platinum metal group (PMG) and base metal (or metal oxide) types. The active catalyst is often supported on an inert carrier such as gamma alumina. Catalyst forms include metal ribbons, mesh and gauze honeycomb monoliths and small beads or particles that can be used in a fixed, fluidized, or moving bed. 

In Fig. 1.3, the principal ideas of chromate reaction are summarised. Anodic oxidation of iron produces Fe and the cathodic reduction of oxygen gives OH which react to form a passive layer of Fe(OH)2 on the steel substrate, yielding a well adherent oxide layer. In addition, chromates are reported to be attached to the iron oxides and incorporated into the passive layer. In some cases, mixed oxides of Fe(OH)2 and CrOOH are found which can explain the outstanding performance of chromate-based pigments. 

Conversion coatings are not as protective as anodized coatings. In most cases, conversion coated surfaces are subsequently primed or painted. Xraditionally, pretreatment of Al has relied on chromate-based systems. Xhere are two general classes of chromate conversion processes the activated acidic formulations such as the chromium chromate processes that use a sodium fluoride-chromic acid chemistry, and alkaline oxide processes based on a sodium chromate-sodium hydroxide or carbonate chemistry. 

Numerous modifications of chromium-based catalysts have been made through the introduction of various additives, the most effective of which are titanium alkoxides (38,39). These additives apparentiy reduce surface silyl chromate moieties to chromium titanates, which are then oxidized to titanyl chromates. These catalysts offer a better control of the resin molecular weight (39). 

Zinc compounds are generally colorless unless the other component, eg, chromate, is colored. The lack of color of most zinc compounds in visible light is a great advantage in that they do not color paint films, plastics, mbber, cosmetics, etc. However, when excited by various types of radiation and at various temperatures, zinc oxide, sulfide, selenide [1315-09-9], and related compounds exhibit luminescence, ie, they emit colored light (see Luminescent materials). Zinc-based phosphors can be produced in many colors, depending upon the added dopants. They are used in television tubes, luminescent glasses, and various specialty products. 

Zinc yellow is made by a variety of processes, all based on the reaction of 2inc compounds, chromates, and potassium salts in aqueous solution. If products free of chloride and especially sulfate are desired, they are excluded from the system. In one process, for example, 2inc oxide is swollen with potassium hydroxide and the chromates are added as a solution of potassium tetrachromate [12422-53-6] (194). 

Basic lead silicochromate [11113-70-5] (National Lead Co. designation Pigment M-50) is a composite in which basic lead chromate, ie, chrome orange, is precipitated onto a lead siUcate—siUca base. It does not have an appreciable chromate solubiUty and depends on lead oxide for its effectiveness. 

Cathylates are stable to oxidation with potassium chromate in acetic acid and with A-bromoacetamide, acetylation and formylation with 85 % formic acid at room temperature. They are cleaved by bases to give the parent alcohol, carbon dioxide and ethanol. 

When produced by such dry methods it is frequently unreactive but, if precipitated as the hydrous oxide (or hydroxide ) from aqueous chromium(III) solutions it is amphoteric. It dissolves readily in aqueous acids to give an extensive cationic chemistry based on the [Cr(H20)6] ion, and in alkalis to produce complicated, extensively hydrolysed chromate(III) species ( chromites ). 

Aluminium coatings exposed as sprayed show remarkable protective properties and in many cases the only evidence of age is the appearance of small hard nodules of aluminium oxide, and these do not appear to be deleterious. However, the dead white of the sprayed surface soon becomes discoloured by dirt and so a paint covering is usual. A chromate inhibitor is not essential and two coats of a simple vinyl-based sealing paint have given remarkable results in the tests of the American Welding Society over periods of 12 and 15 years. 

A large number of electrolytic treatments of magnesium, anodic or a.c., have been developed, in which adherent white or grey films consisting of fluoride, oxide, hydroxide, aluminate or basic carbonate are deposited from alkaline solutions containing caustic alkali, alkali carbonates, phosphates, pyrophosphates, cyanides, aluminates, oxalates, silicates, borates, etc. Some films are thin, and some are relatively thick. All are more or less absorbent and act as good bases for paint, though none contributes appreciable inhibition. All can, however, absorb chromates with consequent improvement of protective efficiency. 

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