benzylchloride1
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Mood: Pushing the envelope of synthetic chemistry in one's basement
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cis and trans dichlorobis(ethylenediamine)cobalt(III) chlorides
I recently conducted an experiment at the university that involved the synthesis of these two coordination complexes. My lab write up is posted below.
Introduction
This set of two experiments involves the preparation of the two isomers of dichlorobis(ethylenediamine)cobalt(III) chloride. The trans isomer is
prepared by heating cobalt(II) chloride with an aqueous solution of ethylenediamine. Cobalt(II) is oxidized to cobalt(III) by drawing air through the
mixture. Ordinarily cobalt(III) is highly oxidizing and will oxidized water to oxygen gas. Coordinated cobalt(III) on the other hand is a very stable
species in aqueous solution. Hydrochloric acid is then added to the solution of the cobalt complex. Upon cooling, emerald green crystals of the
hydrochloride salt of transdichlorobis- (ethylenediamine)cobalt(III) chloride form. The next step of the synthesis involving the isomerization of the
trans complex to the cis complex by dissolving the trans complex in water and adding sodium bicarbonate to neutralize the hydrochloric acid still
present in the complex. The isomerization does not occur unless the acid is neutralized. The solution is then evaporated and the cis isomer
crystallizes as purple colored crystals contaminated with sodium chloride. The sodium chloride cannot be removed by recrystallization because of the
great solubility of the complex in water. Alfred Werner prepared these to complexes over 100 years ago Alfred Werner receive the Nobel Prize in
chemistry in 1913 for his extensive work in the field of coordination complexes. Both of the complexes synthesized in this experiment are octahedral.
The octahedral complexes of cobalt (III) were studied extensively and lead to many of the theories of inorganic chemistry regarding octahedral
complexes.
3[Co(H2O)6]Cl2 +4NH2C2H4NH2 + O2 = trans 2[Co(en)2Cl2]Cl
trans [Co(en)2Cl2]Cl = cis [Co(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:
Cobalt (II) chloride hexahydrate is fairly toxic. Handle this compound with gloves and do not inhale any dust produced by this compound. Do not ingest
this compound. 10% ethylenediamine solution in water is very corrosive and smells bad. Wear gloves and handle this compound only in a fume hood.
Hydrochloric acid is highly corrosive as a solution and vapor; handle this compound with gloves in a fume hood. Sodium bicarbonate is an skin a
respiratory irritant, do not breathe the dust of this compound. The coordination complexes produced in this experiment are also toxic. Handle these
with care and wear gloves. Use common sense when handling lab equipment and all chemicals.
Procedure
Trans-dichlorobis(ethylenediamine)cobalt(III) chloride
All steps were carried out in a fume hood. 6.0 grams of cobalt (II) chloride hexahydrate was dissolved in 20mL of distilled water in a 250mL filter
flask. 20mL of a 10% ethylenediamine solution in water was then added. The solution was wine red in color. A rubber stopper bearing a glass tube was
placed in the mouth of the flask to serve as an air inlet for the subsequent air oxidation. Air was pulled through the flask by attaching a piece of
rubber tubing to the side arm of the flask and attaching the tube to a trap chilled in an ice bath, the trap was connected to the laboratory vacuum
source. The trap consisted of a 1000mL Erlenmeyer flask with a 2-hole rubber stopper bearing a piece of glass tubing extending to the bottom of the
flask and another piece of tubing extending 1 inch past the inner opening of the rubber stopper. The flask containing the reaction mixture was than
heated in a boiling water bath for 1 hour. Air was fairly rapidly pulled through the mixture. The solution turned to a purple color as the oxidation
proceded. After 1 hour the flask was removed from the water bath and disconnected from the vacuum. The reaction mixture was than cooled in a cold
water bath to room temperature. 12mL of concentrated hydrochloric acid, 12M, was than slowly added to the mixture with swirling. White fumes of
ethylenediamine hydrochloride were apparent in the flask due to unreacted ethylenediamine. The solution was then concentrated nearly to dryness with a
rotary evaporator. Upon cooling the purple solution upon cooling in a ice bath, green crystals of trans-dichlorobis(ethylenediamine)cobalt(III)
chloride formed. The mixture was allowed to cool in an ice bath for ½ hour. The crystals were then vacuum filtered using a glass fritted funnel;
funnel with filter paper cannot be used due to the corrosiveness of the solution. The product was partially dried by pulling air through the filter
for several minutes. An additional crop of crystals can be obtained by concentrating the mother liquer with a rotary evaporator and then cooling in an
ice bath and filtering again. The product was then dried in an oven for 45 minutes and then placed in a tared vial. The product is then weighed and
the weight recorded. 2.18 grams of product was obtained with a percentage yield of 31%. 3 hours should be designated for this part of the experiment.
Cis-dichlorobis(ethylenediamine)cobalt(III) chloride
.196 grams of the Trans-dichlorobis(ethylenediamine)cobalt(III) chloride from the last experiment was placed in a 50mL beaker with a small magnetic
stir bar. 3mL of a saturated sodium bicarbonate solution was then added. The green solution then turned purple. 5mL of water was than added. The
mixture was then heated on a hot plate with magnetic stirring at a medium temperature. Once the water had evaporated, a purple colored solid remained.
The solid was broken up and placed in a tared vial. The product was contaminated with sodium ions. No attempt was made at recrystallizing the product
because of its high solubility in water. 0.19 grams of the product was obtained. The percentage yield is meaningless because of the large amount of
sodium present in the product.
Experimental Data
Compound Grams Obtained Theoretical Yield Percentage Yield
Trans-dichlorobis(ethylenediamine)cobalt(III) chloride 2.18g 7.10g 31%
Cis-dichlorobis(ethylenediamine)cobalt(III) chloride 0.196g 0.196g 97%
Calculations and Results
Theoretical yield of Trans-dichlorobis(ethylenediamine)cobalt(III) chloride: 7.10g
6.0g [Co(H2O)6]Cl2 1 Mole [Co(H2O)6]Cl2 1Mole [Co(en)2Cl2]Cl 281.5g [Co(en)2Cl2]Cl
237.85g [Co(H2O)6]Cl2 1Mole [Co(H2O)6]Cl2 1Mole [Co(en)2Cl2]Cl
Percentage Yield: 31%
Theoretical Yield of Cis-dichlorobis(ethylenediamine)cobalt(III) chloride: 0.196g
Percentage Yield: 97%
Discussion
This experiment demonstrates the synthesis of the two isomers of dichlorobis(ethylenediamine)cobalt(III) chloride. The two isomers have different
colors; the trans isomer is green, while the cis isomer is purple in color. When the ethylenediamine is added to the solution of cobalt (II) chloride,
the solution darkens in color due to a ligand exchange reaction. The color of the solution changes from a red color to a purple color when air is
drawn through the solution. The color change is due to the oxidation of cobalt (II) to Cobalt (III) and the formation of the coordination complex
which is later isolated. The purpose of adding the hydrochloric acid is to provide an excess of chloride ion which by the common ion effect causes the
complex to crystallize out of the solution. The low yield of the complex is due to the very high solubility of the complex in water even after the
addition of the hydrochloric acid. Concentrating the solution yields more product, but the yield is still very low after obtaining two crops of
crystals.
2 attempts at synthesizing the cis isomer were made; neither were satisfactory. The first attempt involved dissolving the product in water and
allowing the mixture to stand in a watch glass for ten minutes and than heating the watch glass on a boiling water while directing a light stream of
compressed air on the solution of the complex to speed the evaporation. This procedure produced a green colored mass which had the appearance of the
starting material; this method was a complete failure. This was the procedure that was described in Microscale Inorganic Chemistry. The isomerization
reaction is inhibited by a trace of acid, the trans dichlorobis(ethylenediamine)cobalt(III) chloride is actually a hydrochloride salt. The acid from
this salt inhibits the reaction. An attempt at promoting this reaction was made by adding sodium bicarbonate to neutralize the acid. After evaporating
the purple solution obtained, a purple solid which contains the cis isomer was obtained. The solid still contains a large amount of sodium
bicarbonate. No attempt was made at recrystallizing the complex because of its great solubility in water and its low solubility in organic solids.
Neither procedure for the synthesis of the cis isomer works very well. A possible solution would be to use ethylenediamine as the base to neutralize
the acid and to extract the dry product with a solvent that the ethylenediamine hydrochloride is soluble in, but in which the complex is not soluble
in. Coordination complexes are often difficult to purify due to their high solubility in water which makes it difficult to separate the complexes from
simple inorganic salts that are often present in the reaction from which the complex must be isolated from in pure state.
Amateur NMR spectroscopist
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JohnWW
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I think I remember doing the same experiment in an inorganic chemistry lab class, about 40 years ago. The existence of the green trans isomer and
purple cis isomer of that Co(III) compound has been known for a long time.
The Co(III) depends on hexacoordination involving two molecules of a strong chelating ligand, ethylenediamine, for its stability. As an addition to
the experiment, you could measure the UV/visible spectra of the two isomers, in solutions of suitable concentration, using a scanning spectrometer,
and if possible provide an explanation of the different positions of the UV/visible absorption bands in terms of ligand field stabilization energy
(LFSE).
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Jor
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I have done this myself 6 weeks ago, at home. It is a nice experiment, crystals of the trans-isomer:
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