Experiment 5

Synthesis of Tris(2,4-pentanedionato)iron(III) & Tris(ethylenediamine) nickel(II) chloride

Outline of Experiment:

• Day 1
• Prepare Tris(2, 4-pentanedionato)iron(III)
• Day 2
• Recrystalize the Tris(2, 4-pentanedionato)iron(III) (optional, at instructor discretion)
• Prepare Tris(ethylenediamine) nickel(II) chloride

Part A: Stoichiometry and Synthesis of Tris(2, 4-pentanedionato)iron(III)

For this experiment, you will synthesize a new compound, or set of compounds, from the starting reactants.  Use only the quantities of material listed in the handout.  Determine the limiting reactant which will be used to determine your theoretical yield.  Be certain that you can identify the correct limiting reactant, and disregard compounds that are used as solvent and which are not present in the product.

Hydrates are compounds which contain water molecules as an integral part of their composition.  The molar mass of the compound we will be using, Iron(III) chloride hexahydrate (FeCl₃·6H₂O), is 270.2952 g/mol, which includes the FeCl₃ (162.204 g/mol, which is the molar mass of anhydrous FeCl₃) but also the six (6) moles of water (total mass is 108.0912 g/mol compound) which are an integral part of the complex, shown below.  

You should calculate the molar mass of the compounds used in this experiment yourself to verify these values.  When you calculate the number of moles of a compound which exists as a hydrate, you must include the total molar mass, including the water, not just the anhydrous (without water) part.

For this experiment, the Iron(III) chloride hexahydrate will be reacted with 2,4-pentanedione, which is shown below.

Follow the published experimental procedures.  When requested to obtain about 1.3 g FeCl₃·6H₂O, you should weigh out about this amount, but it does not need to be exactly 1.300 g (about means close, but necessarily exactly this amount, meaning that 1.250 g is okay, as well as 1.345 g).  Record your mass to three (3) decimal places.  The reactants for this experiment are the FeCl₃·6H₂O and 2,4-pentanedione (2.00 mL; density is 0.957 g/mL).  The other chemicals included in the reaction are either solvent (methanol) or buffer (sodium acetate), and are not included in limiting reactant calculations.  When the two reactants are mixed, there will be an intense color change, turning blood red.  In fact, this reaction is similar to the interaction of iron with oxygen in blood, which, of course, is blood red.  Our new compound is Tris(2,4-pentanedionato)iron(III), which is shown below.

The reaction is:

FeCl3•6H2O (aq) + 3 C5H8O2 (MeOH)  + 3 NaC2H3O2•3H2O (aq) →  Fe(C5H7O2)3 (s) + 3 NaCl (aq/MeOH) + 3 HC2H3O2 (aq/MeOH) + 9 H2O (l)
Reactants: FeCl3•6H2O (aq) - Iron(III) chloride hexahydrate (1.300 g) dissolved in water C5H8O2 (MeOH) - 2,4-pentanedione (2.00 mL) dissolved in methyl alcohol NaC2H3O2•3H2O (aq) - Sodium acetate trihydrate (2.5 g) dissolved in water	Products: Fe(C5H7O2)3 (s) - solid product NaCl (aq/MeOH) - NaCl dissolved in water & methyl alcohol HC2H3O2 (aq/MeOH) - Acetic acid in water-methyl alcohol H2O (l) - liquid water

After you have made your product, and collected it via vacuum filtration, you should dessicate (dry) your solid until the next lab period for quantitation (a recrystallization may be performed on the second day, if your instructor requests it).  It is helpful to pre-weigh the dry filter paper prior to doing the filtration, as well as the dry plastic container, so that when you have dried your sample, to determine the amount of product, you will weight the plastic container, with the solid product and filter paper, and subtract the mass of the container and filter paper. After you obtain a mass for the crystallized material, you should calculate a percent yield based on the theoretical yield determined from the mass (moles) of the limiting reactant.  

Label your container by putting your Name and "DLR110-Exp5A" on the label (use labelling tape available on the front bench).  Be sure to mass the container and transfer all of the solid product, along with the pre-weighed filter paper, into the container.  Do not put the lid on the container, because the lid, if attached, will prevent evaporation and and your sample will not dry.  Place the sample (container, filter paper, and reaction product) in the dessicator (drying chamber) to dry until the next lab period.

At the beginning of the next lab period, retrieve your container from the dessicator and weigh the entire contents. You can calculate the actual yield by subtracting the mass of the dry container and the dry filter paper from the total mass just determined.  You will compare the mass of this crude product, with the mass you recover as recrystallized material (if required to do a recrystallization) using the recrystallization procedure described below, during the second day of this lab. A recrystallization step will yield a purer product, but will affect your yield, especially if you do not follow the instructions carefully.  After this recrystallization step, you will dessicate your product until the next lab period and quantitate your yield at that time.

Optional Recrystallization (from original protocol written by Mark Yeager) - perform only if requested by your instructor.

To understand the purpose of recrystallization, see the article about the recrystallization of sugar at the end of this experiment.

Gently scrape the solid product off of the filter paper into a 150-mL beaker.  Try not to scrape bits of paper along with the product, as these insoluble particles will be collected by filtration and contribute to the mass of your recovered product.  Dispose of the filter paper in the "Filter Paper" waste container (or the container marked as solid waste).  Dissolve the solid product in the beaker by adding about 1-2 mL of methanol, and stirring with a glass stirring rod.  If the solid does not all dissolve, add another 1-2 mL of methanol (you may have to keep adding methanol and stirring to get all the solid to dissolve, but be sure to add no more than 2 mL of methanol at a time.  It may take as much as 15-20 mL of methanol to dissolve all of the product.

Add 25 mL of water to the product/methanol solution.  The product will recrystalize from the solution, since it is soluble in methanol but not in water.  As it recrystallizes, a lot of the impurities that were present in the original product stay dissolved in the methanol/water solvent.  However, some of the product will also stay in solution, which means that some yield is sacrificed in order to get a pure product.

Suction filter the recrystallization mixture.  Label your container by putting your Name and "DLR110-Exp5A" on the label (use labelling tape available on the front bench).  Mass the container and transfer the solid product, along with the pre-weighed filter paper, into the container.  Do not put the lid on the container, because the lid, if attached, will prevent evaporation and and your sample will not dry.  Place the sample (container, filter paper, and reaction product) in the dessicator (drying chamber) to dry until the next lab period.  At the beginning of the next lab period, retrieve your container from the dessicator and weigh the entire contents. You can calculate the actual yield by subtracting the mass of the dry container and the dry filter paper from the total mass just determined.   Percent yield is calculated using the formula at the end of this protocol.

Part B: Synthesis of tris(ethylenediamine) nickel(II) chloride

You will prepare tris(ethylenediamine) nickel(II) chloride in this part of the experiment.  This compound is colored, but not red like the experiment in Part A.  Like the experiment in Part A, however, you must do a limiting reactant determination to find out which of the starting reagents (which appear in the product) is limiting.  The reactants for Part B are shown below.

Follow the protocol shown below, which is not included in your experimental packet.  Be sure to determine the limiting reactant, theoretical yield (based on the limiting reactant), and percent yield.  Answer all of the questions at the end of this section in your lab notebook.

Synthesis of tris(ethylenediamine) nickel(II) chloride (Protocol by Mark Yeager; copyright by Mark Yeager)

The equation for the synthetic reaction you will carry out is:

NiCl₂•6H₂O (aq) + 3 C₂H₈N₂ (aq) → [Ni(C₂H₈N₂)₃]Cl₂ (s) + 6H₂O (l)

The procedure for making  [Ni(C₂H₈N₂)₃]Cl₂ (tris(ethylenediamine)nickel(II) chloride) is given below.  It will be up to each group to determine what measurements and calculations must be made to determine the limiting reagent, the theoretical yield and percent yield of the solid product, and, in the written report, to account for the difference between theoretical and percent yield (lost product, unreacted starting material, etc.). Note that the ethylenediamine is provided as an aqueous solution (C₂H₈N₂ (aq)), not as a pure compound. You must take this into account in your limiting reactant calculations.

Safety: In this part of the experiment you will be using acetone, which is flammable, so there will be no flames in the lab during this experiment. Ethylenediamine is very corrosive, and it will be stored in the fume hood to reduce the hazard of any spills. You should wear gloves when handling this solution, since it is corrosive.

Make sure all glassware is very clean -- any residue of sodium acetate from Part A will interfere with this reaction.

(Use the quantities and procedure shown below, or follow the instructions included in the published protocols, if desired.)

Measure about 2 to 2.5 grams of nickel(II) chloride hexahydrate, and put it in a 150-mL beaker. Dissolve it in as little DI water as possible (5 mL maximum). Use a glass-stirring rod to mix the solution. Slowly add 10.00 mL (carefully measured) of 25.0% (m/m; mass percent) ethylenediamine-water solution (density of the solution is 0.950 g/mL), again using the stirring rod to mix. Add about 25 mL of acetone (extremely flammable) to the reaction mixture, in 5 increments of about 5 mL each (thoroughly mix after each 5-mL addition). Continue stirring until precipitation begins.

Cool the beaker on ice for about 10-15 minutes to maximize precipitation (if the beaker is warm to the touch after the addition of acetone, wait for it to cool to near room temperature before putting it in the ice bath).

Suction filter the product mixture, collecting the solid on the filter paper. If any particles of product remain in the reaction container, use a rubber policeman to scrape them into the funnel. If this is not helpful, use a small amount of the filtrate (do not use water!) to wash the rest of the product into the funnel.

After the liquid has been drawn through the filter paper, carefully break up the solid with a spatula, making sure that you don't tear the filter paper. Dry the crystals by leaving the vacuum on for about 5 minutes.

Remove the funnel from the flask. Carefully transfer the product from the filter paper to a pre-massed vial labeled with the product name and the names of the people in your group. Put the vial in the desiccator. Use the next lab period to determine the mass of the dry product. Be sure to dispose of all waters in the proper containers.

Simply dessicate all of your product until the next lab period and quantitate your yield at that time.  Label your container by putting your Name and "DLR110-Exp5B" on the label.  Do not put a lid on the container, because the lid will prevent evaporation and and your sample will not dry.

Results

As is always the case, your Results section must include the important observations and data from the Experimental section, and any calculations you perform. In this case, be sure to include those observations that confirm that the product you collect was the compound you were trying to synthesize. You should also include calculations to show how you determine which reactant is the limiting reactant, and the theoretical yield and the percent yield of the products.

Discussion

Specific points that you should address in this section (in addition to the usual discussion and explanations) are:

1. How do you know that a chemical reaction took place?
2. How do you know that the compound you collected as a product is the compound you were trying to synthesize?
3. How did you determine which reactant was the limiting reactant?
4. What was your percent yield, and does this reflect a good yield of product?
5. What specific observations show where in the procedure you lost some of the product?

Recrystallization (a description)

Anyone who has been to Hawaii is probably aware of how sugar (sucrose) is prepared from sugar cane.  The sugar cane is grown for up to two years before it is ready to be harvested.  When the crop is ready, the entire field is burned, killing the plants and burning the leaves, leaving only the sugar cane stalks.  The very dirty stalks are collected, taken to the processing plant and then crushed with large amounts of water being added.  The water dissolves the sugar, leaving the dirt, ash and plant solids behind.  The liquid mixture (a heterogeneous mixture), which contains the dissolved sugar, but also is still muddy and dirty, is allowed to crystallize, producing still impure sucrose.  After several dissolving and recrystallization steps, pure, white sucrose is produced.  Overall, the process of recrystallization has been used to remove impurities (which remain in the liquid, called "mother liquor") and produce pure crystalline sucrose.