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University of research and technological investigation YACHAY TECH

School of Chemical Sciences and Engineering Author: PhD Juan Pablo Saucedo

Department of Chemistry

INORGANIC CHEMISTRY II LABORATORY SEMESTER: 2019-2

PRACTICE 3: Synthesis of the cis and trans isomers of K[Cr(ox)2(H2O)2]3H2O.

Objective

Understand the geometric isomerism in coordination compounds trough the synthesis of the cis and trans-chromium oxalate complexes.

Previous investigation. Isomerism in Coordination Chemistry.

Experimental procedure

Materials 2 Beakers (50 mL) 1 Spatula 1 Watch glass 1 Crystallizer 1 Hot plate with stirring 2 Magnetic bars 1 Hirsch funnel 1 Kitasato flask 1 Rubber tubings 1 Vacuum pump 1 Universal stand 2 Utility clamps Filter paper 1 container for water bath Reactants K2Cr2O7 (2 g) Sodium oxalate (7 g) Distilled H2O Ethanol Acetone Experimental procedure Synthesis of the trans-K[Cr(ox)2(H2O)2]3H2O In 50 ml beaker, 1,75 g of oxalic acid are dissolved in the minimum amount of boiling water. A solution of 0,5 g of potassium dichromate dissolved in the minimum amount of hot water is added drop to drop (very vigorous reaction), covering the beaker with a watch glass each time the solution is added and while the reaction takes place. The solution is evaporated to a half of the volume (at which point a high surface tension is observed) and poured into a crystallizer allowing to evaporate at room temperature until the trans isomer (two or three days) has crystallized. The

crystals are collected by filtration, washed first with cold water and then with ethanol. Crystallized compound is dried, weighed and the yield is calculated as a function of chromium. Note that there is an equilibrium between the cis form and the trans form but due to the low solubility of the trans isomer this is initially deposited. Store the product and label the vial with the compound number and your laboratory team number. Synthesis of the cis-K[Cr(ox)2(H2O)2]3H2O A very homogeneous mixture of finely ground potassium dichromate (0,5 g) and oxalic acid (1,75 g) (do not inhale the powder) is prepared in a mortar. The powder is packed in a porcelain capsule. A small hole is made in the center of the mixture and three or four drops of water are added. Immediately cover the capsule with a watch glass. After a short period of induction the reaction begins and becomes vigorous. A viscous liquid should be obtained, then 15 ml of absolute ethanol are poured over the viscous liquid and the mixture is stirred until the reaction product solidifies. If the solidification is slow, then the liquid is decanted and the process is repeated with a second portion of alcohol until the entire product is a crystalline solid. The crystals are collected by filtration, dried and weighed. Store the product and label the vial with the compound number and your team name.

Final Questions and requirements

Please, after a discussion with your team partner, answer next questions in order to plain the next experimental sessions. 1. 2. 3. 4.

Which are the principal reactions? How could we know the structure of the complexes? Please discus about the chromium bonds in the reactant and the product. Propose a candidate for the gas leaving the reaction mixture in the second synthesis.

References 1) Aziza, B. K., Tofiq, D. I. Int. J. Chem. Environ. Eng. 2012, 3(1), 34. 2) Kauffman, G. B., Faoro, D. Inorganic Syntheses, 1977, XVII(39), 147.