This experiment introduces you to the use of liquid ammonia as a reaction medium, with all of the attendant precautions that must be taken. You will also learn how to measure the magnetic moment of an inorganic complex in solution by NMR spectroscopy. In addition, if all steps of the experiment are carried out, a high-temperature reaction will be carried out in a tube furnace. Ask your laboratory instructor whether you will prepare Cr₂O₃ and anydrous CrCl₃, or whether the latter will be provided.

Place 2.5 grams of (NH₄)₂Cr₂O₇ in a large evaporating dish. Heat a metal spatula until red hot. Quickly insert the spatula to the solid (NH₄)₂Cr₂O₇ until the reaction proceeds under its own exothermicity. CAUTION! Since hot solid pieces might be thrown into the air, be sure that no flammable items are in the area! Continue this process until all of the orange (NH₄)₂Cr₂O₇ is converted to the dull green Cr₂O₃. Wash the Cr₂ O₃ with hot water until the washings are colorless. Dry the Cr₂O₃ at ~100^oC (in the oven). Then determine the yield. Write the equations for the reactions that have taken place in your notebook.

CAUTION! Phosgene, a poisonous gas, and hexachlorobenzene, a carcinogen, are formed as byproducts of this reaction! This experiment should be performed in the hood with the sash closed!

The equipment shown should already be set up in a reserved hood. Unlike most experiments, this one works best if the reaction tube is dirty and contains bits of anhydrous CrCl₃ left over from the previous user (not the green/grey residues). You should remove the green/grey residues to avoid the contamination of your product. Place Cr₂O₃ (1.5 g) in the center of the reaction tube as shown. Add 30 mL of carbon tetrachloride to the addition funnel. Stuff a wad of glass wool into the end of the reaction tube. CAUTION! If too much glass wool is stuffed into the end it can clog during the reaction and cause problems. Set the furnace to a setting of 7 and the variac to a setting of 55. When the temperature of the furnace reaches 450^oC, start the nitrogen flow (slowly), and open the stopcock to the addition funnel such that a drop comes down every 2-3 seconds. Monitor the temperature and turn down the temperature control as needed to keep the temperature below 800^oC. After all of the carbon tetrachloride is added, shut down the nitrogen, the variac, and the tube furnace. Once the furnace has cooled, collect the product using your spatula and/or an aluminum rod to scrape the product. Collect the shiny purple flakes, but not the grey foam that may appear at the front of the tube.

References:

1. Angelici, R. J. Synthesis and Technique in Inorganic Chemistry; University Science Books: N.Y., 1986; Ed. 2, p. 33.



2. Inorganic Synthesis, Vol. 2, p. 193 (1946).

CAUTION! Use a well-ventilated hood! The product is light- sensitive.

Introduce about 8 mL of liquid ammonia into a Schlenk flask which is pre-cooled in an acetone/dry ice bath. Add about 5 mg of clean sodium metal (not necessary to weigh---about the size of a piece of cooked rice) and about 2 mg of ferrous ammonium sulfate. After the blue color of the sodium has disappeared, add (very slowly!) 500 mg of anhydrous chromium(III) chloride in 20-mg portions while the solution is constantly stirred. (Chromium(III) chloride must be added VERY slowly to avoid causing the solution to boil over the side of the flask!) After the addition has been completed (about 2 hours), stop the stirring and allow the brown precipitate to settle and decant the clear, slightly colored supernatant liquid from the beaker.

Transfer the residue to a large crystallizing or evaporating dish and allow it to stand with occasional stirring until the odor of ammonia is gone and a bright yellow, freely flowing powder remains. Dissolve the yellow solid in 5 mL of 0.75 M HCl at 40 ^oC. After filtering the solution, add 4 mL of concentrated (16 M) nitric acid immediately and cool the mixture in an ice bath. It is important that these latter steps be done as quickly as possible to avoid reaction of [Cr(NH₃)₆]³⁺ with Cl^_ to form [Cr(NH₃)₅Cl]²⁺. Filter the yellow product and wash with very dilute nitric acid, then with 95% ethanol and finally with diethyl ether. Allow to air-dry on the filter apparatus. Determine the yield. Since the compound slowly decomposes in light, it should be stored in a container protected from light. Write the chemical equations for the transformations that have taken place during the synthesis, and that might take place if the product is not isolated in a timely manner.

Characterization required: Magnetic susceptibility by NMR (in solution---see handout on magnetic susceptibility), UV-visible spectroscopy, determination of D_o for [Cr(NH₃)₆]³⁺.

References:

1. Angelici, R. J. Synthesis and Technique in Inorganic Chemistry; University Science Books: N.Y., 1986; Ed. 2, p. 39.



2. Inorganic Synthesis, Vol. 2, p. 153 (1946).



3. J. Phys. Chem. 1965, 69, 2201.