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Author: Subject: Ethylenediamine complexes of chromium (III)
benzylchloride1
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[*] posted on 12-2-2009 at 19:55
Ethylenediamine complexes of chromium (III)


Here is an interesting series of syntheses starting from hydrated chromium sulfate that produces several chromium complexes with different colors. I conducted this experiment for my inorganic lab at the university. The safety information is greatly exagerrated; none of the chemicals used are hazardous unless handled improperly. Here is my experiment:

Introduction

This series of preparations involves the synthesis of 3 different ethylenediamine complexes of chromium (III). Anhydrous chromium (III) sulfate is produced by drying hydrated chromium (III) sulfate which is a hydrate of variable constitution. The anhydrous chromium (III) sulfate is needed for the synthesis of tris-(ethylenediamine) chromium (III) sulfate. The tris-(ethylenediamine) chromium (III) sulfate is produced by heating the anhydrous chromium (III) sulfate with anhydrous ethylenediamine. This serves to coordinate the chromium with the ethylenediamine forming a brownish-red colored complex. This complex is paramagnetic and has 3 unpaired electrons.
The tris-(ethylenediamine) chromium (III) sulfate is dissolved in concentrated hydrochloric acid and allowed to crystallize; this forms the yellow colored tris-(ethylenediamine) chromium (III) chloride 3.5 hydrate by the exchange of sulfate for chloride due to the large excess of hydrochloric acid used. The tris-(ethylenediamine) chromium (III) chloride is than heated at 210 degrees Celsius to form the red-violet cis-dichlorobis(ethylenediamine) chromium (III) chloride. This controlled pyrolysis causes one of the ethylenediamine ligands to be expelled and replaced by two chloride ligands to form this complex.

Cr2(SO4)3 + 6 (NH2)C2H4(NH2) = [Cr(en)3]2(SO4)3

[Cr(en)3]2(SO4)3 +6HCl = 2[Cr(en)3]Cl3 + 3H2SO4

[Cr(en)3]Cl3 + heat = [Cr(en)2Cl2]Cl
Safety Information Pertaining to this Experiment.

Heating devices such as ovens and hot plates are used in this experiment; a danger of thermal burns is present if the experimenter is not cautious. A Bunsen burner is not used for heating because flammable liquids are used in this experiment. Laboratory glass ware is also used in this experiment which when broken can present hazards. Wear laboratory safety goggles at all times during this experiment. Always wash hands after handling chemicals. Above all else use common sense.

Safety Information pertaining to chemicals used in this experiment:

Chromium sulfate is a toxic irritant. Do not breathe dust created by this compound, do not ingest this compound. Anhydrous ethylenediamine is very corrosive and smells terrible, ethylenediamine is also toxic if ingested or inhaled. Do not breathe vapors of this compound and use only in a fume hood wearing gloves. Ethylenediamine is also extremely flammable; do not use near sources of ignition such a flames. Dimethyl sulfoxide has a bad smell and is absorbed readily through the skin carrying anything dissolved in it into the body. Wear gloves and handle this compound only in a fume hood. Acetone is extremely flammable and volatile. Do not handle this compound near flames. Hydrochloric acid is very corrosive and can cause burns if spilled on the skin. This compound has a pungent burning odor, so use it only in a fume hood. Always add all acids to water and not the other way around. Above all else, always use common sense.
Procedure
Anhydrous chromium (III) sulfate

20g of chromium (III) sulfate hydrate was weighed out and placed in a 100mL beaker. The chromium salt was green in color. The sulfate was placed in a 110 degree oven for 3 days. The sulfate was then powdered with a mortar and pestle and placed back in the oven for another day. The green colored anhydrous chromium (III) sulfate was than stored in a vacuum dessiccator until needed. The anhydrous sulfate does not dissolve in water.

Tris(ethylenediamine)chromium (III) sulfate

5.0 grams of the anhydrous chromium (III) sulfate was weighed out and placed in a large glass test tube. 10mL of anhydrous ethylenediamine was than added and a stirring rod used to mix the contents of the test tube. Glass wool was than used to plug the mouth of the tube to prevent water vapor from entering the tube. The mixture was heated in a boiling water bath for 1 hour with frequent stirring, the solution turned brown in the process. The contents of the tube soon become solid and make stirring extremely difficult. After 1 hour 50mL of water is then added to tube in portions to dissolve the product.
The solution of the chromium complex in water was a brownish-yellow color. The solution was then filtered to remove unreacted chromium (III) sulfate. The solution was then evaporated to dryness with a rotary evaporator under reduced pressure yielding a brownish orange colored material. 100mL of dimethyl sulfoxide was than added and the mixture was heated with a heat gun in a fume hood to prevent exposure to the vapors of this solvent. The dimethyl sulfoxide caused the product which adhered tenaciously to the wall of the round bottomed flask to soften and form a coarse powder and a slightly orange colored solution. The mixture was then filtered to collect the product. The product was broken up and washed with two 50mL portions of acetone to remove adhering dimethyl sulfoxide. The product was then at room temperature yielding an orange-brown colored tris(ethylenediamine)chromium (III) sulfate. Two batches on this scale were run and the total yield of product was 12.5g. The theoretical yield was 19.2g, for a percentage yield of 65% for the two combined batches.

Tris(ethylenediamine)chromium(III) chloride

A solution of 1mL of concentrated hydrochloric acid in 6mL of water was prepared in a 50mL beaker and was heated to 60 Celsius with an electric hot plate. 6.0 grams of tris(ethylenediamine)chromium (III) sulfate was weighed out and added to the hydrochloric acid solution with stirring. The solution was quickly vacuum filtered and was a reddish-orange color. A solution of 5mL of concentrated hydrochloric acid in 8mL of ethyl alcohol was then added with stirring and the mixture was placed in an ice salt bath to cool for 30 minutes to cause the desired complex to crystallize. The solution was then vacuum filtered, collecting the yellow needle shaped crystals of the tris(ethylenediamine)chromium (III) chloride. The crystals were than washed with two 10mL portions of acetone to dry them. After air drying for ½ hour the product weighed 2.1 grams. The theoretical yield was 6.4 grams for a percentage yield of 33%.

Cis-dichlorobis(ethylenediamine)chromium (III) chloride

A drying oven was heated to 210 degrees Celsius and monitored with a mercury thermometer to ensure that the temperature was constant at this setting. 1.15 grams of the tris(ethylenediamine)chromium (III) chloride was than weighed out and placed in a thin layer on a tared watch glass. The watch glass was then placed in the oven and periodically weighed, after the product had lost 30.6% of its weight, or a loss of .352 grams. The solid turned from yellow to orange and eventually to a reddish-violet color. The product was then pulverized and placed in a tared vial. The product weighed 0.83 grams. The theoretical yield was 0.8 grams and the percentage yield was over 100% indicating some undecomposed product or a weighing error.





Experimental Data:

Compound Weight Theoretical Yield Percentage Yield
Tris(ethylenediamine)chromium (III) sulfate
12.5g 19.2g 65%
Tris(ethylenediamine)chromium(III) chloride
2.1g 6.4g 33%
Cis-dichlorobis(ethylenediamine)chromium (III) chloride
0.83g .8g >100%


Calculations and Results

Theoretical yield of tris(ethylenediamine)chromium (III) sulfate: 19.2 grams

10g Cr2(SO4)3 1 Mole Cr2(SO4)3 1 Mole [Cr(en)3]2(SO4)3 751.99g [Cr(en)3]2(SO4)3
391.99g Cr2(SO4)3 1 Mole Cr2(SO4)3 1 Mole [Cr(en)3]2(SO4)3

Percentage yield: 65% 12.5g [Cr(en)3]2(SO4)3/ 19.2g [Cr(en)3]2(SO4)3

Theoretical yield of tris(ethylenediamine)chromium(III) chloride: 6.4g

6.0g [Cr(en)3]2(SO4)3 1 Mole [Cr(en)3]2(SO4)3 2 Mole [Cr(en)3]Cl3-3.5H2O 401.5g [Cr(en)3]Cl3-3.5H2O
751.99g [Cr(en)3]2(SO4)3 1 Mole[Cr(en)3]2(SO4)3 1 Mole [Cr(en)3]Cl3-3.5H2O

Percentage yield: 33% 2.1g [Cr(en)3]Cl3-3.5H2O/ 6.4g [Cr(en)3]Cl3-3.5H2O

Theoretical yield of Cis-dichlorobis(ethylenediamine)chromium (III) chloride: 0.8g

1.15g [Cr(en)3]Cl3-3.5H2O 1 Mole [Cr(en)3]Cl3-3.5H2O 1 Mole [Cr(en)2Cl2]Cl 278.5g [Cr(en)2Cl2]Cl
401.5g [Cr(en)3]Cl3-3.5H2O 1 Mole [Cr(en)3]Cl3-3.5H2O 1 Mole [Cr(en)2Cl2]Cl

Percentage yield: >100% 0.83g [Cr(en)2Cl2]Cl / 0.8g [Cr(en)2Cl2]Cl

Discussion

This experiment demonstrates the synthesis of several ethylenediamine complexes of chromium (III). Many difficulties were encountered in the experiment. When the ethylenediamine was added to the anhydrous chromium (III) sulfate, the solid part of the mixture solidifies and makes stirring impossible which lowers the yield by not allowing the ethylenediamine to come in contact with the rest of the chromium sulfate. The test tubes containing the mixture where broken up in and the contents placed in new test tubes to enable the full contact of the ethylenediamine with the chromium sulfate. A possible remedy would be to run the reaction in an Erlenmeyer flask, adding the anhydrous chromium (III) sulfate in small portions with continuous stirring. The tris(ethylenediamine)chromium (III) sulfate produced is extremely soluble in water making it hard to crystallize from this solvent.
After evaporating the water, the complex adheres strongly to the walls of the glass vessel that is used to contain it. An attempt was made to recrystallize the complex from acetone, but was unsuccessful due to the complexes extremely low solubility in acetone. Dimethyl sulfoxide was added along with a little water to the product and the mixture was heated. The complex did not dissolve very much, but caused the complex to break up and form a coarse powder which was easy to filter. Since dimethyl sulfoxide has a low volatility, acetone was used to wash the product free from the dimethyl sulfoxide.
The yield of tris(ethylenediamine)chromium(III) chloride was very low due to its high solubility in water from which it crystallized. Adding more ethanol than specified in the procedure did not cause any more of the desired product to crystallize out. An initial trial of the synthesis of cis-dichlorobis(ethylenediamine)chromium (III) chloride failed because the electronic thermometer in the drying oven indicated the wrong temperature and the pyrolysis reaction did not proceed properly. The product from the first attempt at this reaction was recrystallized from a solution of dilute hydrochloric acid; yielding yellow needles of the starting compound. The second attempt produced the desired complex based on it color, but the percentage yield was over 100% due to unreacted starting material. The complex was not recrytallized because only a small amount of product was obtained and it would be difficult to obtain a reasonable amount of complex by this recrystallization method. This procedure produces the desired in compounds in mediocre at best yields.




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[*] posted on 13-2-2009 at 01:24


Was your hydrated chromium(III)sulfate green? I also have this compound, but mine is dark purple and looks very similar to chrome alum, which is potassium chromium sulfate. According to my experience chrome alum and chrome sulfate can be exchanged in almost all experiments.

I heated both chromium sulfate and chrome alum and both chemicals form a green/yellow solid on heating, which is insoluble in water. Good to read about the ethylene diamine complex. I already was thinking that anhydrous chromium(III)sulfate does not dissolve in any solvent. This is something to try for me next weekend ;)




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benzylchloride1
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[*] posted on 13-2-2009 at 19:01


The dehydrated chromium sulfate was green in color and did not dissolve in water. The hydrated salt was also green in color. I would recommend this series of preparations for anyone interested in coordination complexes. Complexes of several different colors are produced. Th only problem is the low yield in the anion exchange reaction. The tris(ethylenediamine)chromium (III) chloride forms very nice yellow needle shaped crystals. Woelen do you have a picture of your hydrated chromium (III) sulfate? I have never seen purple colored chromium (III) sulfate. I have prepared potassium chromium sulfate and it forms very large purple octahedral shaped crystals.

[Edited on 13-2-2009 by benzylchloride1]




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