Preparation of Anhydrous Chromium III Chloride

Preparation of Anhydrous Chromium(III) Chloride. Perform the experiment in a fume cupboard Grind 5 g of charcoal into a fine powder in a mortar, mix it with 12.5 g of chromium(III) oxide, add a thick starch size or a dextrin solution in cold water, and make beads about 5 mm in diameter from the mixture. Put the beads onto a clay dish and dry them in a drying cabinet at 110-120 °C. Next put them into an iron crucible, cover them with the charcoal powder and a lid, and roast them. 

Assemble an apparatus (see Fig. 61). Place the dry beads into a porcelain or quartz tube and perform chlorination at 750-800 °C during one hour. Ghromium(III) chloride can sublime, therefore see that the tube outlet is not clogged by the product. Cool the apparatus in a stream of chlorine. Extract the beads from the tube and mechanically separate the chromium chloride formed on their surface from the unreacted charcoal. In what other ways can anhydrous chromium(III) chloride be obtained  

Put several chromium(III) chloride crystals into a test tube with water. Does the salt dissolve Add a little of a solution of a chro-mium(II) or iron(II) salt. What do you observe  

Preparation of Chromium(III) Nitride. Work in a fume cupboard Assemble an apparatus for preparing nitrides (see Fig. 84). Put 0.5-1 g of anhydrous chromium(III) chloride into a boat. Put the latter into a tubular furnace. Displace the air from the apparatus with a stream of dry ammonia and then heat the furnace to 600 °C. Continue the heating in an ammonia stream for one hour, next switch off the furnace and cool the apparatus without stopping the stream of gas. Extract the boat and weigh the product. Write the equation of the reaction. Calculate the yield in per cent. 

Test how chromium nitride reacts with water and with an alkali solution in the cold and with heating. Roast part of the nitride in a porcelain crucible on the flame of a burner. Identify the product. 

The hydration isomers contain strongly co-ordinated water which cannot be removed by heating. A common method for dehydrating hydrated chlorides is to reflux with SOCI2. The reaction is most suitable for the hexaaqua-complex, whose dehydration can be represented by  

To obtain an adequate amount of this isomer, double the amounts in the preparation in Sec. 11.5.3. Weigh out 5 g of the prepared compound, grind to a fine powder. Distil purest thionyl chloride and add 15 cm of the freshly distilled liquid to the powdered solid in a flask fitted with a reflux condenser. Test the evolved gases with acidified dichromate paper and a drop of cone, ammonia. When the evolution of SO2 and ammonia, as tested, appears to have stopped, reflux the contents under a nitrogen atmosphere for 1-2 hours. Remove the unreacted thionyl chloride by vacuum distillation and store the solid formed in a stoppered bottle in a vacuum desiccator. 

An alternative preparation involves the reaction of the Cr(III) oxide with tetrachloromethane. The oxide is available commercially but it is better to prepare it fl-eshly by heating ammonium dichromate carefully. The reactions involved are  

Because of the high toxicity of C(X i2, the experiment should be carried out by mature students under close supervision. The door of the fume cupboard should be closed throughout and it should be served by a water-fed scrubbing tower. 

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The tris and bis complexes of acetylacetone (2,4-pentanedione) (167) with chromium(III) have been known for many years (168,169).739 The tris compound is generally prepared by the reaction of an aqueous suspension of anhydrous chromium(III) chloride with acetylacetone, in the presence of urea.740 Recently a novel, efficient synthesis of tris(acetylacetonato)chromium-(III) from Cr03 in acetylacetone has been reported.741 The crystal structure of the tris complex has been determined.744 A large anisotropic motion was observed for one of the chelate rings, attributed to thermal motion, rather than a slight disorder in the molecular packing. 

This is prepared by passing dry hydrogen chloride over chromium, or hydrogen over anhydrous chromium(III) chloride. It is a white solid. If pure chromium is dissolved in dilute hydrochloric acid in the absence of air, a blue solution of the hydrated chloride, containing the hexaaquo-ion [Cr(H20)6]2+. is obtained. The same solution is also obtained by reduction of the + 6 oxidation state (through the + 3) using a solution of a dichromate(VI) and reducing with zinc and hydrochloric acid ... 

Aquopentammine- or acidopentamminechromium(III) salts have been prepared by the reaction of liquid ammonia with anhydrous chromium (III) chloride 1 by the oxidation... 

Pfeiffer prepared trichloro(tripyridine)chromium(III) from anhydrous chromium(III) chloride and from dichloro-tetraaquochromium(III) chloride 2-hydrate (the common hydrated chromic chloride of commerce), but neither details of the preparations nor yields were reported. The reaction of [Cr(H20)4Cl2]Cl-2H20 with pyridine gives [Cr(C6H6N)3-CI3] with a yield of about 4%. In the procedure described here, the complex can be prepared in about 90% yield when anhydrous chromium(III) chloride is used as the starting material. 

By the action of liquid ammonia on anhydrous chromium (III) chloride, Christensen prepared a mixture of University of Illinois, Urbana, 111. t University of Kansas, Lawrence, Kans. 

If a solvent is to be chosen for the purpose of preparation of a pure substance by synthesis, clearly the solvent must be one that will not destroy the desired product, or transform it in any undesirable way. Usually it is obvious what must be avoided. For instance, one would not expect to be able to prepare a strictly anhydrous salt using water as the reaction medium. Anhydrous chromium (III) chloride must be prepared by some reaction that involves no water at all, neither in a solvent mixture nor in any of the starting materials, nor as a by-product of reaction. A method that works uses... 

In the preparation of some complexes, particularly organometallic complexes, the presence of water must be avoided. An important example from classical coordination chemistry is that the action of ammonia (either as a gas or in solution) on hydrated chromium(III) salts—those commercially available—leads to the precipitation of insoluble hydroxy complexes and not to the formation of [Cr(NH3)6]. This complex is prepared by reaction between liquid ammonia and anhydrous chromium(III) chloride. 

The condensation is usually carried out by adding a solution containing equimolar amounts of the allyl halide and the aldehyde or ketone to a solution of at least two equivalents of chromium-(II) chloride in THF at 0 5°C. Frequently, the less precious component is used in 50-100% excess. Although commercially available anhydrous chromium(II) chloride can be utilized (Method B), its in situ preparation from chromium(III) chloride and lithium aluminum hydride (Method A) is often preferred. The removal of chromium and aluminum hydroxide, which are formed on aqueous workup, can be accomplished by filtration in the presence of a filtration aid. 

If tri8(ethylenediamine)chromium(III) chloride is prepared by the method of Rollinson and Bailar,7 it is worthy of note that if the dehydration of the hydrated chromium (III) sulfate is performed in a vacuum oven (2-3 mm. Hg, 70-80°C.) the anhydrous sulfate is obtained in a much more reactive form so that the reaction with ethylenediamine, when started by local heating, proceeds by itself and is finished within an hour. Editor s note For an alternative preparation, see Inorganic Syntheses, 10, 33 (1967) also ref. 26. 

Some salts could not be dehydrated in THF. Iron(III) chloride for example, polymerizes THF and must be prepared using neat trimethylchlorosilane. Cobalt(II) chloride dihydrate gave a similar result requiring neat trimethylchlorosilane. In both cases, nearly quantitative yields of the anhydrous salt were obtained. Hydrated zinc chloride, which we prepared by adding 10 wt % water to the anhydrous chloride because well-defined hydrates are not commercially available, was very efficiently dehydrated in neat trimethylchlorosilane to give a 96% yield of zinc chloride. In contrast, the THF/trimethylchlorosilane mixture afforded a comparatively modest 71% yield of the tetrahydrofuranate. Chromium(III) chloride hexahydrate, on the other hand, could not be completely dehydrated in neat trimethylchlorosilane and required THF for an efficient reaction giving 89% yield of chromium(III) chloride tris(tetrahydrofuran). Our results are summarized in Table I. 

Chromium(III) chloride-Lithium aluminum hydride. The reagent is prepared by reduction of 2 equiv. of anhydrous CrC j with 1 equiv. of LiAIH, in THF at 0 and is presumably chromium(ll) chloride. The advantage of this new chromium... 

The reaction provides a valuable method of preparing anhydrous chlorides of metals. It has been used to prepare the anhydrous chlorides of copper(II), zinc, cadmium, chromium(III), iron(III). cobalt(Il) and nickel. 

The anhydrous hahdes, chromium (II) fluoride [10049-10-2], chromium (II) bromide [10049-25-9], CrBr2, chromium (II) chloride [10049-05-5], CrCl2, and chromium (II) iodide [13478-28-9], 03x1, are prepared by reaction of the hydrohaUde and pure Cr metal at high temperatures, or anhydrous chromium (II) acetate [15020-15-2], Cr2(CH2COO)4, atlower temperatures, or by hydrogen reduction of the Cr(III) hahde at about 500—800°C (2,12). 

These have been known for many years.1052-1054 Chromium(III) is approximately octahedral ( ie(f = 3.69-4.1 BM) the compounds have a layer structure. In the chloride, r(Cr—Cl) is 5.76 A between layers and 3.46 A within layers. The iodide is isomorphous with the chloride and the bromide has a similar but distinct structure. All may be prepared by the direct halogenation of the metal. Other methods are available, e.g. CrCl3 may be prepared by heating Cr203-xH20 in CCU vapour at 650 °C.1055 The anhydrous halides are insoluble in water, however reducing agents such as zinc catalyze dissolution. The trichloride reacts with liquid ammonia to form ammine complexes. 

Low-valent metal salts have been used to bring about reductive cleavage of oximes. Corey and Rich-man used chromium(II) acetate to convert O-acetyl ketoximes into imines, which were hydrolyzed to ketones. " Aqueous titanium(III) chloride and vanadium(II) salts also reduce oximes again, the imines are usually hydrolyzed in situ, but some hindered imines, such as compound (37), are isolable." A method of preventing hydrolysis is to carry out the reduction in anhydrous conditions in the presence of an acylating agent. The products of such reactions, when applied to oximes of enolizable ketones, are en-amides. For example, these ketoximes are converted into A/-formylenamines when heated in acetonitrile with anhydrous titanium(III) acetate and acetic formic anhydride cyclohexanone oxime gives the en-amide (38 97% Scheme 22)." This type of reduction has been used by Barton and coworkers to prepare enamides from steroidal oximes. They reported that the reaction could be performed by acetic... 

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