Complex will hold ligand that will organize coordination bond to the metal centre by donating solitary braces of negatrons to the empty d-orbital of the metal which is similar to the Lewis acid-base reaction.
In this experiment, Acetylacetone and pyridine is the ligand will move as Lewis base and will be coordinated to the metal centre of Vanadium, V, which is the Lewis acid.
Acetylacetone is a ?-diketone that contains two alpha H atoms next to the two carbonyl groups. As carbonyl-group is electron retreating group, these two carbonyl functional group will make an inductive consequence which cause the alpha H to be more acidic and be easy deprotonated.
Resonance stableness of delocalising the anion formed from the deprotonation under basic status besides increases the chance of the deprotonation as shown in Figure 1:
Figure 1: Mechanism of the deprotonation and delocalization of the alpha H
Acetylacetone exist in both keto and enol isomer signifier as shown in Figure 2:
Figure 2: Keto-enol tautomerisation of the Acetylacetone
The enolate anion is able to move as a bidentate ligand as it can chelate onto the oxovanadium metal ion with the two O atoms as the binding sites to organize the VO ( acac ) 2 composites.
Two acetylacetonate ligands are able to adhere to the oxovanadium metal and organize a square pyramidal composite. Addition of the pyridine compound will organize an octahedral geometry composite. These can be farther proven utilizing IR spectrometry where the different strengths, lengths and nature as harmonizing to Hooke ‘s jurisprudence, IR soaking up spectra is able to be alone for each composite.
Preparation of VO ( acac ) 2
The readying of VO ( acac ) 2 started with 5ml of concentrated sulphuric acid ( H2SO4 ) easy added to 5 milliliter of deionized H2O in a 50 milliliter round-bottomed flask. Then, 18 milliliter of ethyl alcohol and followed by 2 g of V pentaoxide ( V2O5 ) was added into the unit of ammunition underside flask. The solution was refluxed in an oil bath for an hr. The mixture was so cooled and filtered utilizing cotton wool. Subsequently, 6 milliliter of Acetylacetone was added bead wise to the filtrate. The reaction mixture was carefully added part by part to the solution that contained 7.4g of Na2CO3 in 120 milliliter of deionsed H2O in a 500 milliliter conelike flask. The mixture was so cooled in ice H2O and filtered to obtain the dark green merchandise. Merchandise was dried under IR lamp. The merchandise was weighted and per centum output was calculated.
Preparation of VO ( acac ) 2py
The VO ( acac ) 2py was prepared foremost by fade outing 0.5 g of the VO ( acac ) 2 merchandises obtained in 20 milliliter of ethyl alcohol in a 50ml of unit of ammunition underside flask. Then, the mixture was refluxed smartly with 2 milliliters of pyridine for one hr which was so concentrated utilizing rotary evaporator. Crystal of petroleum VO ( acac ) 2py formed upon chilling in ice H2O which was so filtered and wash with 5 milliliters of quintessence. Merchandise was dried under IR lamp. The petroleum dry merchandise was weighted and per centum output was calculated. The IR spectrum of merchandise was so determined.
Data Treatment and Analysis
Calculation of Percentage Yield of VO ( acac ) 2
The per centum output was calculated utilizing the undermentioned equation:
V2O5 + 4H+ i? 2 ( VO ) 2+ + 2H2O + O2 — — — — — — — — — — — — -Reaction 1
2 ( VO ) 2+ + 2 acacH i? 2H+ + VO ( acac ) 2 — — — — — — — — — — — — Chemical reaction 2
Mole of H2SO4 used =
Sum of H+ present = 0.0938 mol x 2 = 0.1876 mol
Mole of acac used =
Mole of V2O5 used = ( restricting agent )
The ratio of V2O5 and VO2+ is 1:2,
Mole of VO2+ used = 0.0111 mols x 2 = 0.0222 mols
As ratio of VO2+ and VO ( acac ) 2 is 1:1, so theoretical mole of VO ( acac ) 2 = 0.0222 mols
Theoretical mass of VO ( acac ) 2 = mole of VO ( acac ) 2 ten molar mass of VO ( acac ) 2
= 0.0222 mols X 265.16 g/mol = 5.890 g
Percentage output of VO ( acac ) 2 =
Calculation of Percentage Yield of VO ( acac ) 2py
Mole of pyridine used =
Mole of VO ( acac ) 2 used = ( restricting agent )
VO ( acac ) 2 is the confining agent. The ratio of VO ( acac ) 2 and VO ( acac ) 2py is 1:1.
Mole of VO ( acac ) 2py = 0.00194 mols
Hence, theoretical mass of VO ( acac ) 2py = mole of VO ( acac ) 2py X molar mass of VO ( acac ) 2py = 0.00194 mols x 344.26 g/mol = 0.667 g
Percentage of VO ( acac ) 2py output =
Table 1: Summarized Data from IR spectrum of VO ( acac ) 2py
Important extremums observed in the spectrum of VO ( acac ) 2 /cm-1
Important extremums observed in the spectrum of VO ( acac ) 2py /cm-1
Geometry of VO ( acac ) 2 and VO ( acac ) 2py
Vanadium [ IV ] metal exists as vanadyl, VO2+ in aqueous solution. Before the reaction of the VO2+ with the acetylacetonate, VO2+ is coordinated to five H2O molecules and existed as octahedral geometry. However, acetylacetonate is a stronger ligand that displaces the H2O molecules. Since Vanadium is bonded to oxygen with a V=O bond, merely 2 acetylacetonate will merely be able to organize organize bond with the vanadium cardinal metal ion to organize the VO ( acac ) 2 complex with a square pyramidic geometry as shown in Figure 3.
Figure 3: Chemical reaction diagram of the coordination geometry of VO ( acac ) 2 and VO ( acac ) 2py
The geometry of this composite is in the square pyramidic geometry alternatively of the rhombohedral pyramidal geometry is due to the steric consequence caused by the acetylacetonate ligands. The geometry of the square pyramidal is to cut down the steric repulsive force to the lower limit with the optimum distance apart of the two ligands.
Besides, from Figure 3, it can be seen that there is a vacant coordination site on the rule z-axis available which allow another more coordination site if there is the presence of strong ligand such as pyridine in this experiment. Therefore, complexation occurred between the pyridine molecules and the VO ( acac ) 2 where N lone brace in the pyridine molecules act as a ?- donor-ligand and strong Lewis base that donate the lone brace of negatron to the VO ( acac ) 2 composites at the vacant coordination site. With the new VO ( acac ) 2py composite formed holding a deformed octahedral geometry due to the unequal bond length between the V=O bond and the V=N bond at the axial place of the V metal Centre which lead to asymmetry. Besides, the octahedral geometry of the VO ( acac ) 2py is a 6-coordinate composite that is of high thermodynamic stableness.
Switching of the V=O stretching set
The composite of the VO ( acac ) 2 has an oxidization composite of +4 which the V ( IV ) ion exists as a d1. Using the Crystal Field Theory, there is electronic repulsive force between the negatrons of the acac ligands and the negatron on the V metal ion which causes the ab initio debauched 5 d-orbitals of V metal ion to divide into two different degrees. The degree with the lower energy has a t2g symmetricalness and consists of the dxy, dxz, and dyz atomic orbitals. The degree with the higher energy has eg symmetricalness and consists of the dx2-y2 and dx2 orbitals. The difference between the energy degrees is the ligand field dividing parametric quantity, a?†o. In the octahedral geometry, eg orbitals are on the axial way and are the most affected by the electrostatic repulsive force which destabilize the orbitals and excite them to higher energy degree. The t2g orbital is non on the axial place and therefore, non much affected by repulsive force and therefore, stabilized and travel to take down energy as shown in Figure 4:
Figure 4: Molecular diagram illustration of Crystal Field Theory
As mentioned in the old paragraph, the pyridine is a ? giver ligand that binds to the VO ( acac ) 2 composite at the vacant site at the axial place of the V ( IV ) ion. The add-on of the pyridine to the complex reduces a?†0 which this decrease of the splitting parametric quantity was observed utilizing the comparing of the IR spectrum of the VO ( acac ) 2 and VO ( acac ) 2py.
From the spectrum found in Appendix 2, the spectrum of the VO ( acac ) 2py complex can be seen that the V=O stretching frequence of VO ( acac ) 2py complex is 32 cm-1 lower than the VO ( acac ) 2 complex when the V=O stretching frequence lessening from 998.04 cm-1 to 966.04 cm-1. These reduces of the stretching frequence is relative to the stretching energy of the V=O bond of the composite. The add-on of the pyridine will ensue in the decrease of the stretching energy of the V=O bond. This shows that the V=O bond of the VO ( acac ) 2py is destabilized upon the add-on of pyridine to VO ( acac ) 2. This can be shown utilizing Hooke ‘s Law as shown:
where V is the stretching frequence in cm-1, K is the force invariable, ? is the decreased mass.
The binding of the pyridine has decrease the force invariable and the lone brace of the pyridine ligand is electron-donating which increases the mass of V ion that consequence in larger decreased mass, ? , which harmonizing to the Hooke ‘s jurisprudence, consequence in lower stretching frequence wavenumber. The moving ridge figure is relative to the sum of energy that is needed for passage where it extremely depends on the ligand field dividing parametric quantity. The shifting of the stretching frequence indicate that the ligand field dividing parametric quantity has decreased in magnitude which the add-on of the pyridine ligand has caused destabilization of VO ( acac ) 2py complex.
Similarly, the lone brace of negatrons of the N in pyridine is added onto the anti-bonding of the molecular orbitals of VO ( acac ) 2 complex. This causes the pyridine to be trans to the V=O bond. Besides, this bonding of the negatrons donated will increase the electron denseness of the V ‘s vitamin D orbitals which will diminish the P ? > vitamin D ? contribution from O atom to the V atom in the V=O bond. This consequence in the falls of the bond order of the composite and the lowering of the bond strength which besides lengthen the V=O bond length. Overall, there is lessening of energy demand to stretch the V=O bond and therefore lead to the shifting of the V=O stretching quiver to take down frequence in the spectrum. This besides farther proven that the VO ( acac ) 2py has an octahedral construction.
Besides, there is the delocalization of the ? negatrons from the aromatic ring of the pyridine which able to stabilise the composite more and diminish the P ? > vitamin D ? contribution from O to V in the V=O bond. This make the vanadium-pyridine complexation more executable and stable that consequence in the stronger V-N stretching bond which gain partial dual bond character. As a consequence of the inter-electron repulsive force, the negatron denseness of the V=O bond is so shifted towards the O atom and do the weakening of the V=O dual bond that cause the bond to loses the feature of the dual bond nature. Therefore, lesser energy is needed to stretch the diminished and lengthened V=O bond and lead to the shifting of the frequence of the spectrum.
There is besides the ground of the trans influence where the trans influence is the consequence of pyridine ligand that weakens the bond that is trans to itself in the composite. The negatrons of the oxo-ligand in the axial place of the complex occupies the dz2 orbital that will do new adhering ligand to organize organize bond with the V ion to be trans place to the oxo ligand. Pyridine ligand have donate negatrons to the VO ( acac ) 2 complex metal centre and reside in the 3dz2 orbital from the site trans to the O atom which causes repulsive force. Therefore, the pyridine ligand and the oxo-ligand are in the trans place to each other which uses the same orbitals on the metal V ion for adhering. However, the O atom can non donate the negatrons to the metal every bit good as the pyridine ligand which causes the V=O bond to be weaker. Therefore, both axial bonds to the V are weakened and lengthened which decrease the force invariable ( K ) and diminish the stretching frequence. The displacement besides helps to corroborate the presence of VO ( acac ) 2py.
On the other manus, the VO ( acac ) 2py spectrum found in Appendix 2 have showed extremum of 998.04 cm-1 which has a little different to the 997.7cm-1 extremum found in the VO ( acac ) 2 spectrum. This shows that there is still presence of VO ( acac ) 2 in the merchandise. Besides the literature value of the V=O stretching bond of the VO ( acac ) 2 is found out to be 995 cm-1 which is near to the 997.7 cm-1 in the merchandise spectrum. These farther prove that the VO ( acac ) 2 is present. This can explicate for the comparatively low output of 58.5 % of the VO ( acac ) 2py synthesis as the reaction has non gone into completion. However, comparing the strength of the extremum of 997.7 cm-1 and 966.04 cm-1 which indicate V=O bond in VO ( acac ) 2 and VO ( acac ) 2py complex severally, there is higher strength of the 966.04cm-1 which show that more pyridine adduct is formed in merchandise. This besides shows that the reaction may be an equilibrium reaction where both merchandise and reactant can be present. This consequence can besides be interpreted as the reaction is non complete and more reflux clip or higher temperature for reflux is needed to transport out to guarantee more merchandises formed.
Last, there is a wide low strength extremum at 3449.72 at the VO ( acac ) 2py spectrum found in Appendix 2 which was due to the presence of H2O that may be caused by deficient drying of the VO ( acac ) 2py merchandise.
Observation of Color alterations
During the synthesis of the complex, there is a series of observation of colour changing of the compound in the solution. The colour changes observed during the synthesis of the VO ( acac ) 2 is due to the alteration of the oxidization province of V2O5 ( oxidization province of V = +5 ) . The orange pulverization of V2O2 is being reacted with the H2O and H2SO4 to cut down to a dark green solution of [ V ( H2O ) 5 ] 3+ ( oxidization province of V= +3 ) composite during center of the vigorous reflux. When the reflux is complete, the solution was in dark blue colour which is due to the decrease of the staying V2O5 to dark bluish [ VO ( SO4 ) ( H2O ) 5 ] composite ( oxidization province of V = +4 ) . During the forming of the pyridine adduct, the dark blue VO ( acac ) 2 is changed to dark green VO ( acac ) 2py complex.
This colour altering phenomenon can besides be explained utilizing the Crystal Field Theory mentioned. Electrons of the two different energy degree orbitals can be excited with a?†o energy. The higher the oxidization province, there will be more different between the two dividing energy degrees which hence lead to higher a?†0. For V2O5 ( oxidization province of V = +5 ) will hold a higher a?†0 and hence will absorbs visible radiation of the electromagnetic spectrum of higher energy with higher frequence and reflect low energy visible radiation. Therefore, V2O5 will absorb the bluish green visible radiation of higher energy and reflect orange of lower energy visible radiation which is shown off as orange with bare eyes. When there is a decrease of the compound to VO ( acac ) 2 or VO ( acac ) 2py compound, the light absorbed was at lower frequence and visible radiation reflected was of higher frequence such as bluish green and therefore, there is the alteration of orange pulverization to bluish green merchandise. Therefore, VO ( acac ) 2 and VO ( acac ) 2py appeared as bluish green compounds.
Possible Restrictions and Beginnings of Mistakes in Experiment
There are several restrictions of the experiment that consequence in non holding 100 % output of the merchandise of both VO ( acac ) 2 and VO ( acac ) 2py composites.
First, there may be loss of merchandise due to the cotton wool filtration that causes some of the filtrate to be trapped in the cotton wool and unable to go through through.
Second, harmonizing to the Le-Chatelier ‘s rule, for the Reaction 2, if there is an addition in the H+ concentration, reaction will switch the concentration of the reaction to the left which consequence in lesser merchandise green goods. When the reaction mixture is added easy to the alkaline Na carbonate solution, uncomplete neutralisation may happen where non all H+ are removed, and therefore lesser merchandise is produced. Therefore, longer chilling clip will be needed for re-crystallization so as to let more crystallisation of VO ( acac ) 2 and force the reaction to the right.
Another restriction is of utilizing of the sulfuric acid, H2SO4 may ensue in the decrease of the VO2+ ions to V3+ ions which decrease VO2+ ions available for merchandise of VO ( acac ) 2 formations.
The lower per centum output of the VO ( acac ) 2py may be due to incomplete refluxing and therefore take to low output obtained. Besides, drosss may be present in the VO ( acac ) 2 merchandise produced that observed at the underside of the flack catchers which lead to false weight measured and inaccurate per centum calculated. The unit of ammunition underside flask should be swirled every 15 proceedingss during the reflux which can forestall the subsiding of the V2O5 solid at the underside of the flask and placing drosss.
The output of the VO ( acac ) 2py can be improved with add-on of extra pyridine and increase the reflux clip.
Possible Precaution in Experiment
For the synthesis of the VO ( acac ) 2 composites, there is a measure of the adding of ethyl alcohol into the mixture of solution for reflux.
Ethanol was used as dissolver. The metal V atom is surrounded by non-polar ligands of acetylacetonate or pyridine in 2nd portion of the experiment. Ethanol is more non-polar than H2O and will organize solvent-solute interactions with the composites and dissolve those ligands for better interaction in the solution. The usage of the ethyl alcohol besides helps to divide the VO ( acac ) 2 from the indissoluble V2O5 residue that will be filtered and remain in the cotton wool during filtration. Besides, ethyl alcohol was used to forestall side reaction that will originate so as to increase the output of the merchandise. From reaction 1, there is the reaction of the V2O5 and the H2SO4 to bring forth the intermediate VO2+ ion that besides responding with the acetylacetonate in reaction 2, VO2+ can respond with the H2O and O molecules by-product formed from reaction 1. It is as shown in Reaction 3. The side reaction will diminish the sum of VO2+ available for reaction with the acetylacetonate in reaction 3 to organize merchandise of VO ( acac ) 2. This will diminish the sum of output of the merchandise produced.
4VO2+ + 2H2O + O2 i‚® 4VO2+ + 4H+ — — — — — — — — — — — — — -Reaction 3
However, this side reaction can be reduced with the add-on of ethyl alcohol where ethyl alcohol will respond with O and bring forth ethanal as shown:
C2H5OH + ?O2 i‚® CH3CHO + H2O — — — — — — — — — — — — — -Reaction 4
Besides, utilizing vacuity to take the O and H2O formed can be a good manner to cut down job of side reaction with VO2+ .
Another safeguard taken was adding bead wise of the acetylacetonate to guarantee sufficient reaction clip and prevent inflow of the H+ concentration and switch the equilibrium to the left and diminish the output of the merchandise formed.
Anhydrous Na carbonate, Na2CO3, was dissolved in H2O so as to organize alkalic solution to neutralize H+ ions so as to switch the acetylacetonate toward the enol signifier. This will increase the formation of the VO ( acac ) 2 merchandises. Besides, the mixture solution was added easy to the Na carbonate solution so as to forestall the rapid foaming caused by the merchandise of CO2 and lose of merchandise due to overruning. Besides, easy seting the pH with more alkalic environment and less mixture solution allows the reaction to respond at steady gait and cut down any side reaction to happen.
The per centum output of the merchandise of VO ( acac ) 2 and VO ( acac ) 2py complex were 77.8 % and 58.5 % severally. From the IR spectrum of VO ( acac ) 2py, it is shown that the V=O bond have shifted from 9998.04 cm-1 to 966.04 cm-1. This shifting of the extremum indicate the formation of the pyridine adduct, VO ( acac ) 2py, with the lessening in the wavenumber of the V=O stretching set.
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