ISOMERISM AND KINETICS IN COORDINATION CHEMISTRY
1. To prepare and study the kinetics of the interconversion of some of the isomers of a typical coordination compound.
Isomerism played a central role to establish the basic concept in coordination chemistry while reaction kinetics of coordination compound is known as the area of continuing intense research activity. The mechanism of the reactions of coordination compounds relevance to enzyme reactions where the active site is being involve in coordination to a metal ion.
(A) Preparation of Trans-Dichlorobis(ethylenediamine)cobalt(III) chloride
1. About 16g of cobalt(II)chloride hexahydrate is dissolved in 50ml of distilled water in 250ml of Erlenmeyer flask. 2. 60ml of 10% solution of ethylenediamine is added into the flask. 3. A vigorous stream of air is being drawn through the solution for about 1-2 hours 4. 35ml of concentrated hydrochloric acid is added
5. The solution is then being evaporated on a steam bath until a crust forms over the surface. 6. The solution is cooled and stand overnight before the bright green plates of the hydrochloride, trans-[Coen2Cl2]Cl.HCl is filtered 7. During filtered process, the product is washed with ethanol and ether before dried at 110 °C. 8. The dull green powder is obtained.
(B) Preparation of the cis isomer
1. About 5g of product obtained in A is being dissolved in distilled water. 2. The solution is then being heated in steam bath until purple or pink solid is obtained. 3. Step 1 and 2 are repeated for three times to make sure the purple or pink solid is obtained.
(C) The cis to trans isomerisation
1. A solution of (0.02M; 14mg in 25ml) of the trans isomer and cis isomer is prepared in methanol and its UV-visible spectrum is recorded. 2. The solution is placed quickly and tightly stoppered into a constant temperature bath at 35 °C 3. The most appropriate wavelength is selected.
4. The optical density, D at that wavelength is measure at 15 minutes intervals for about 90 minutes by withdrawing samples from the thermostated solution.
(a) Preparation of Trans-Dichlorobis(ethylenediamine)cobalt(III) chloride.
Mass of product obtained, trans (Coen2Cl2)Cl
Mass of cobalt(II) chloride hexahydrate used,CoCl2.6H2O
Mass of ethylenediamine used,CH3CH(NH2)2
Molecular weight of cobalt(II) chloride hexahydrate
Molecular weight of ethylenediamine
No. of mole cobalt(II) chloride hexahydrate used,CoCl2.6H2O,
= (16.002g / 237.930g/mol)
= 0.067 mol
No. of mole of ethylenediamine used,CH3CH(NH2)2
= 10% x (60.000g/60.000g/mol)
= 0.100 mol
Reaction equation for the preparation is :
4CoCl2.6H2O + 8CH3CH(NH2)2 + 8HCl + O2 4trans (Coen2Cl2)Cl.HCl …(1) trans (Coen2Cl2)Cl.HCl 110Ctrans (Coen2Cl2)Cl + HCl ... (2)
From the equation,
1 mol of CoCl2.6H2O reacted with 2 mol of CH3CH(NH2)2 to formed 1 mol of trans (Coen2Cl2)Cl The limiting reactant is ethylenediamine. Thus,
2 mol CH3CH(NH2)2 ≡ 1 mol trans (Coen2Cl2)Cl
0.100 mol CH3CH(NH2)2 = 0.100 x 1
= 0.05 mol trans (Coen2Cl2)Cl
The theorical yield of trans (Coen2Cl2)Cl = 0.05 mol x 285.4 g/mol
The percentage yield = x 100%
(b) Preparation of the cis isomer.
Mass of product obtained, cis (Coen2Cl2)Cl
Mass of of trans (Coen2Cl2)Cl used
Reaction equation for the preparation is,
trans (Coen2Cl2)Cl cis (Coen2Cl2)Cl ….(3) From the equation,
1 mol of trans (Coen2Cl2)Cl produce 1 mol of cis (Coen2Cl2)Cl
Thus, 1.492g of trans (Coen2Cl2)Cl produce 1.492g of cis (Coen2Cl2)Cl
The percentage yield = 0.523g x 100%
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