Prac Report

Nerissa Govender
209539579
CHEM 310
Experiment 1- Synthesis of Co (acac)3
Date of experiment performed: 03 August 2011
Date of hand in: 10 August 2011
Aim
To prepare cobalt (III) acetylacetonate in the laboratory using cobalt (II) carbonate and acetylacetone at room temperature and pressure.
Introduction
The anion acetylacetone (acac) used in this practical functions as a ligand which forms a complex with the cobalt metal cation in an octahedral array where there is electrostatic bonds between the cobalt metal and both the oxygen’s of the acetylacetone to form a six membered ring1.

Figure1: The 3-D molecular structure of cobalt (III) acetylacetonate 2 Figure2(a): Water formed as a by-product2 Figure2(b): Carbon dioxide formed as a by-product2

The hydrogen peroxide used in this practical as an oxidizing agent to produce the cobalt (III) acetylacetonate from cobalt (II) carbonate and a stoichiometrically calculated amount of acetylacetone3.
2CoCO3 + 6CH 3COCH 2COCH3 + H2O2 2Co(CH3COCHCOCH3 ) + 2CO2 + 4H2O 4 (1)

Equation (1) shows stoichiometrically the reaction that occurs between the reagents. The transition of Co2+ to Co3+ is apparent by the colour change of the new complex formed which is a shiny greenish black colour from a dull brown colour.

Experimental Procedure
A slurry of cobalt (II) carbonate and acetylacetone was made using (2.53g, 0.021 mole) of cobalt (II) carbonate and approximately (20ml, 0.2mole) of acetylacetone, these were added to an 100ml Erlenmeyer flask and were stirred with a glass stirring rod to produce the slurry. The mixture was then heated to 100°C on a hot plate the temperature was measured using a thermometer placed directly into the slurry. The mixture was removed from the heat and hydrogen peroxide was added directly to the mixture at a rate of 2ml/min ensuring that the solution did not bubble over from the hydrogen peroxide being added in too fast. The mixture was then placed back on the hotplate where it was heated to incipient boiling i.e. when the boiling of the mixture had just become apparent. The mixture was removed from the hotplate and another 15ml of hydrogen peroxide was added ensuring no bubbling over of the solution. The mixture was placed back onto the hotplate where it was heated to boiling. The mixture was removed from the hotplate and placed in an ice bath laced with salt to decrease the temperature of the ice bath as a result of decreasing the freezing point of the water. The mixture was left in the ice bath to cool for twenty minutes where a green-black sludge was noted. The sludge was filtered off under suction using a Buchner funnel. The sludge was washed with water first and then cold ethanol in roughly 5ml amounts. The material was removed from the filter paper and placed on a glass watch-glass where it was heated in an oven at 110°C. The compound was allowed to dry and the mass was determined using a 2-decimal place mass balance and weigh boat. The resulting compound produced was Cobalt (III) acetylacetone which weighed (2.96g,0.021 mole, m.p. 212°C,lit5 210-214°C ) and was in the form of a shiny green black powder.
Calculations
The percentage yield was calculated as follows;
2CoCO3 + 6CH 3COCH 2COCH3 + H2O2 2Co(CH3COCHCOCH3 ) + 2CO2 + 4H2O 4 (1)

Mass of Co(acac)3 obtained: 2.96g
Mole ratio of Co(acac)3 and CoCO3:
1:1
Molar mass Co(acac)3 = 58.933 + 3[(5 x 12.011) + (7 x 1.0079) + (2 x 15.999)] = 356.26g/mol
Theoretical mass of Co(acac)3 = n x MM = 0.021mol x 356.26g/mol = 7.48g

Percentage yield = (actual yield)/(theoretical yield) x 100 = 2.96g/7.48 x 100 = 39.57%

Discussion
The low percentage indicates that product was lost or the reaction did not go to completion. The reaction did go to completion as a large amount of product was seen before filtration was performed, most of the product was lost due to adding a larger amount of ethanol to the product than was necessary, since the resulting product was soluble ethanol this resulted in a large amount of product becoming soluble and which was filtered off as a result. Loss of product also occurred due to transferal from glassware from Buchner funnel to watch-glass due to scraping difficulties from scraping product from filter paper and the walls of the Buchner funnel.
An NMR spectrum can be obtained as a d6 complex is formed of low spin, we deduced this as Co3+ being of high oxidation state than its Co2+ state it has a higher splitting energy and therefore is strong field forcing all six electrons into the t2g orbitals, the electrons are all paired (diamagnetic) and does not affect the NMR spectrum as there is no interfering magnetic moment from unpaired electrons. The ligand acetylacetonate is a strong field ligand which also increases the splitting energy and is thus also strong field; therefore Co3+ will always form d6 low spin complexes.
References
1. http://www.molchem.science.ru.nl/molmat/mm-web/srm4.doc, (Accessed: 07/08/2011)
2. http://www.lookchem.com/cas-216/21679-46-9.html, (Accessed: 08/08/2011)
3. http://www.docstoc.com/docs/44965705/Process-For-The-Preparation-Of-Cobalt-(III)-Acetylacetonate---Patent-4338254, (Accessed: 08/08/2011)
4. http://course1.winona.edu/cmiertschin/450/Lab/Acac_lab.pdf, (Accessed: 08/08/2011)
5. Dr G Maguire, Inorganic Chemistry 310 practical manual, University of KwaZulu-Natal, 2011, page 2

References: 1. http://www.molchem.science.ru.nl/molmat/mm-web/srm4.doc, (Accessed: 07/08/2011) 2. http://www.lookchem.com/cas-216/21679-46-9.html, (Accessed: 08/08/2011) 3. http://www.docstoc.com/docs/44965705/Process-For-The-Preparation-Of-Cobalt-(III)-Acetylacetonate---Patent-4338254, (Accessed: 08/08/2011) 4. http://course1.winona.edu/cmiertschin/450/Lab/Acac_lab.pdf, (Accessed: 08/08/2011) 5. Dr G Maguire, Inorganic Chemistry 310 practical manual, University of KwaZulu-Natal, 2011, page 2