Synthesis and Recrystallization of Tris(acetylacetonato)iron (III)

Abstract

The purpose of this laboratory experiment was to explore inorganic synthesis techniques by synthesizing the metal complex tris(acetylacetonato)iron (III), also known as Fe(acac)3, and subsequently recrystallizing the product to determine its melting point and compare the percent yields between the crude and recrystallized forms. In the synthesis, Fe(acac)3 was formed through the coordination reaction between acetylacetonate ligands and iron (III) ions. Recrystallization was employed to purify the product. The percent yield of the crude product was found to be 90.

05%, while the percent yield of the pure, recrystallized product was 48.1%. Melting point data indicated that the recrystallized product had a slightly higher melting point than the crude product, likely due to the presence of impurities affecting the crystalline structure.

Introduction

In the first part of this lab, the complex Fe(acac)3 is synthesized through the coordination reaction between acetylacetonate ligands and the iron (III) ion. Metal ions can bind with neutral molecules or anions, ligands, to form a coordination complex, in this experiment it is Fe(acac)3.

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The ligand acts as a Lewis base, which is a compound that can donate an electron pair to an acceptor compound, and it forms a coordinate bond with the Lewis acid, or the metal ion, which accepts the electron pair. Acetylacetone can be deprotonated to donate the anion, acetylacetonate.

Together, these two things form metal acetylacetonate complexes, and the bond between the two, through the electrons, is a coordinate bond. In this experiment, acetylacetonate is deprotonated to produce an acac anion, which with iron(III) can form Fe(acac)3.

This can happen as the oxygen atoms create a six-membered ring with the metal ion, creating an octahedral structure.

Figure 1: Molecular Structure of Fe(acac)3

    O
     
      Fe
     / 
    O   O
      /
      C
       
        CH3

Recrystallization in the second part of the experiment is done to purify the product, as the crystals formed in the first part may contain impurities. Recrystallization involves adjusting the solution conditions near the saturation point and shifting the equilibrium point by cooling the mixture. Properly aligned molecules will form crystals, pushing impurities into the solution.

Procedure

Synthesis of Fe(acac)3

1. Put on gloves and keep them on for the entire experiment.
2. Set up two ice water baths, placing a 150 mL beaker of RO water in one of them.
3. Measure approximately 1.00 g of FeCl3•6H2O into an Erlenmeyer flask.
4. Add 15 mL of distilled water to the flask and place it on a stirrer/hot plate without applying heat.
5. Add a magnetic stir bar and add 2 mL of acetylacetone to the iron chloride solution dropwise.
6. Obtain ~2.5 g of sodium acetate in a 50 mL beaker, add 10 mL of RO water, and stir to dissolve completely.
7. Add the sodium acetate solution to the iron chloride solution dropwise.
8. Heat the solution at ~70 °C for 10 minutes on the hot plate, then allow it to cool on the lab bench for ~10 minutes, and remove the stir bar.
9. Filter the final product using a vacuum filtration system and wash it with ice-cold distilled water from the first step.
10. Let it dry for about 5 minutes, then determine the mass of the dry product.

Recrystallization of Fe(acac)3

1. Set up two hot water baths, one in a 600 mL beaker and one in a 150 mL beaker.
2. Obtain ~0.3 g of the product from the first part of the experiment in a 50 mL beaker and place it in the small hot water bath.
3. Collect 10 mL of methanol in a 150 mL beaker and heat it up for a minute or two.
4. Add methanol dropwise to dissolve the product until the solution turns deep red.
5. Place this beaker in the hot water bath and stir continuously to evaporate some of the methanol.
6. Continue until small crystals form at the bottom of the beaker.
7. Remove from heat and cool on the lab bench, then transfer to an ice-water bath and further cool for about 10 minutes.
8. Measure 5 mL of methanol into another 50 mL beaker and chill it in another small ice bath.
9. Filter the product using vacuum filtration, wash it with the cooled methanol, and let it air dry on the filtration unit for about 5 minutes.
10. Determine the mass of the final recrystallized product.
11. Using a Mel-temp apparatus, determine the two melting points of the dry recrystallized product and crude product (around 180 to 182 °C).

Observations

Part 1:

• Some of the FeCl3 resisted breaking up into smaller powder-form in the initial step.
• FeCl3 turned a very dark red color immediately upon contact with acetylacetone.
• The mixture had a chemical odor, but no specific smell could be identified.
• The dissolution of sodium acetate in water was not immediate, requiring stirring.
• The mixture became thicker and small solids appeared in the liquid while heating.
• The filtration/vacuuming process was quick, and the filtered liquid was thin and red, while the powder product left on the filter paper was a lighter red and sparkly.

Part 2:

• Upon adding methanol to the crude product, it immediately dissolved.
• The solution turned deep red.
• Tiny, dark red crystals began to form at the bottom during cooling.
• The final vacuumed product was finer than the crude product and still red but less sparkly.
• Comparing the crude product and the crystallized product, the crystallized form had a deeper red color.
• The crude product melted completely at 172 °C, while the crystallized product melted at 175 °C.

Data Sheet

Experimental Observations:

Calculations

For Data Sheet

Mass of Crude Fe(acac)3 = 35.514 g - 34.315 g = 1.199 g

Mass of Recrystallized Fe(acac)3 = 34.454 g - 34.210 g = 0.244 g

Questions

From Part I

1. What is the limiting reagent?

Reactants:

FeCl3•6H2O + 3 CH3COCH2COCH3 → Fe(CH3COCHCOCH3)3 + 3 HCl + 6 H2O

FeCl3•6H2O:

1.02 g / 270.295 g/mol = 3.77 x 10^-3 mol

CH3COCH2COCH3:

2.0 g / 100.03 g/mol = 1.99 x 10^-2 mol

Limiting reagent: FeCl3•6H2O (in excess)

2. What is the percent yield of the crude product?

% Yield = (Actual Yield / Theoretical Yield) x 100%

Actual Yield: 1.199 g (from experimental data)

Theoretical Yield = m = Mn

Theoretical Yield = 353.169 g/mol x 3.77 x 10^-3 mol = 1.331 g

% Yield = (1.199 g / 1.331 g) x 100% = 90.05%

3. Give three reasons why you did not get 100% yield.

1. Some of the crude product may have been left in the Buchner funnel when transferring the product to the watch glass for weighing.
2. During vacuum filtration, some of the crude product might have been filtered through the paper and into the system if it was still dissolved in the solvent (acetylacetone).
3. The crude product contains impurities before the second part of the lab, which could have increased the overall mass, making the percent yield appear higher than it actually is.

From Part II

4. What is the percent yield of the pure product?

(2.26 g pure product / 0.30 g crude product) = (x g pure product / 1.37 g crude product)

x = 0.962 g pure product

% Yield = (Actual Yield / Theoretical Yield) x 100%

% Yield = (0.962 g / 2.0 g) x 100% = 48.1%

5. Compare your crude vs. pure product yield. List three reasons why they may be different/same.

1. The crude sample has impurities, providing an inaccurate mass value and resulting in a higher percent yield compared to the recrystallized product.
2. Both the crude and recrystallized product yields were affected by some product being left behind in the Buchner funnel during transfer, contributing to lower percent yields in both parts of the experiment.
3. The recrystallized product may have a lower percent yield due to differences in the recrystallization process, where some product might have dissolved with the methanol, reducing the final mass volume.

6. What can you say about the difference, if any, between the crude and the recrystallized product in your observed melting point data?

We observed that the recrystallized product had a slightly higher melting point than the crude product. This difference can be attributed to the presence of impurities in the crude product. Impurities disrupt the crystalline structure of a substance, affecting the strength of intermolecular forces that hold the solid together. The stronger these forces, the higher the melting point. Impurities alter the specific, repeating pattern of forces in the solid, reducing the energy required to melt the portion of the solid containing impurities. Therefore, the presence of impurities in the crude product likely contributed to the lower melting point observed.