Molecularity: Synthesis of Bis(dithiocarbamato)nickel; Stoichiometry of Reaction

• To improve your skill in observation
• To synthesize a chemical substance and purify it by recrystallization
• To discover the stoichiometry of a chemical reaction

Chemical synthesis--the art and practice of the design and creation of molecules--is the essence of chemistry. In no other field of science does the practitioner have the ability to create the objects of his/her interest. It is this single fact that sets chemistry apart from physics, geology, and biology, in which the scientist is restricted to examining what s/he finds, rather than what s/he creates. Chemistry has an extra dimension that combines vision and art in the process of creating new substances with potentially new and exciting uses.

This experiment provides you with an opportunity to experience chemical synthesis first hand, to determine the composition of the molecule that you create, and to spectroscopically characterize the molecule (i.e., to discover what types and energies of light the molecule interacts with). The reaction that you will carry out is shown (sans stoichiometry) in equation (1).

The reaction will be carried out in water solvent. C₅H₁₀NS₂^1- is the formula for the diethyldithiocarbamate anion (click here to see the structure). Hereafter this anion will be abbreviated to dtc^-.

This reaction is an example of a class of reaction that permeates chemistry: the donation of an electron pair by a Lewis base to a Lewis acid to form an adduct in which acid and base are joined by a covalent bond. The donation/acceptance of electron pairs is involved in a huge number of chemical reactions ranging from simple laboratory processes to the complex reactions in biological systems.

The adduct of Ni²⁺ with the diethyldithiocarbamate anion is called a transition metal complex to indicate that it is a complex molecule involving an electron pair donor and a transition metal ion (Ni²⁺).

1. Based on your measurements of experiments done by your section, construct a plot of the height of the column of product versus the volume of nickel solution used in the reaction. Why does the amount of product increase up to a point, then decrease?
2. Use your plot to determine the stoichiometry of the reaction between nickel chloride hexahydrate and sodium diethyldithiocarbamate trihydrate. Write a balanced chemical equation to represent the reaction.
3. Which reagent limited the extent of reaction in test tube 1? In test tube 2? In either case, did you have any visual indication of this?

• Labkit
• Kimwipes
• aspirator with trap
• Bunsen burner
• Ring stand/ring/wire gauze
• filter paper
• ruler
  
  
• 0.3 M aqueous stock solution of NiCl₂.6H₂O
• 0.3 M aqueous stock solution of Na(C₅H₁₀NS₂).3H₂O (sodium diethyldithiocarbamate)
• dichloromethane, CH₂Cl₂
• petroleum ether, C_nH_2n+2

Safety goggles must be worn at all times in the laboratory. Dichloromethane and petroleum ether are volatile and flammable. Avoid inhaling the fumes by working with these solvents in the fume hood. Keep these substances away from heat sources, particularly open flames.

The class as a whole will carry out a Job's Method study of the stoichiometry of reaction (1). Each pair of students will contribute a portion of the study by examining the amount of product obtained by mixing two specified volumes of nickel solution with corresponding volumes of a solution containing diethyldithiocarbamate.

NOTE: THIS EXPERIMENT WILL PRODUCE EXCELLENT DATA IF OPERATIONS AND MEASUREMENTS ARE PERFORMED CAREFULLY; OTHERWISE, IT WILL PRODUCE POOR DATA.

Record your observations in your notebook.

Synthesis and Stoichiometry. Obtain your Labkit and other required equipment. Find 50-mL Erlenmeyer flasks, a 100-mL beaker, 2 small test tubes, and a test tube rack in the Labkit. If necessary, wash the Erlenmeyer flasks and test tubes using brushes and Alconox detergent, then thoroughly rinse with water to remove all traces of Alconox. Dry the outsides of the tubes and mark them 1 and 2 using your lab marker.

Put about 2-inches of distilled water into the 100-mL beaker, and heat the water to boiling using a Bunsen burner. Once boiling has been achieved, turn off the flame.

Prepare 2 Pasteur pipet filters as follows. Roll a 4cmx4cm piece of Kimwipe between your thumb and index finger to form a thin strip, and insert the strip into the large end of a 5.75" Pasteur pipet. Push the Kimwipe firmly into the narrowing portion of the pipet using a "ramrod." Label the pipet filters 1 and 2 using the lab marker.

At this point, please see your instructor for your volume assignments. After receiving your assignments, obtain 5 mL of a 0.3 M stock solution of NiCl₂.6H₂O and transfer it to one of your small Erlenmeyer flasks. Obtain 5 mL of a 0.3 M stock solution of Nadtc.3H₂O and transfer it to the other small Erlenmeyer. Write down the concentrations of the stock solutions in your notebook.

Using the syringe pump and graduated pipet, transfer your first assigned volume of the nickel chloride solution into test tube 1 and your second assigned volume into test tube 2. Rinse the pipet with distilled water and aspirate dry.

Again using the syringe pump and graduated pipet, transfer the first assigned volume of the dtc solution to tube 1 and the second assigned volume to tube 2. The total final volume in each tube should be 2.0 mL. Describe in detail in your notebook what happens and what you observe. Mix the contents of each tube by flicking the bottom of the tube with your index finger while holding the top of the tube between your other thumb and index finger. Do both the tubes look the same? What is similar? What is different?

Place the test tubes into the hot water bath for 5 minutes to coagulate the solid. Then remove the tubes (careful--hot!) and place them back in the rack.

Filter the contents of each tube using the following technique:

1. Lightly clamp the open end of the aspirator vacuum hose vertically using a ring stand and clamp
2. Turn on the aspirator to full water pressure;
3. Insert the narrow end of the appropriately-numbered pipet filter into the open end of the aspirator vacuum tube;
4. Squirt about 1 mL of distilled water into the pipet filter from your wash bottle. This "seats" the filter paper;
5. Using your lab marker, mark a line on the outside of the pipet level with the top of the filter paper;
6. Using a Pasteur pipet with bulb, transfer the contents of the test tube as quantitatively as possible to the pipet filter. Quantitatively is the chemist's way of saying "get as much as you can."
7. Add about 1 mL of distilled water to the test tube, repeatedly draw up and expel the water using the Pasteur pipet, then again quantitatively transfer the contents to the pipet filter;
8. Using a small amount of distilled water and a Pasteur pipet, attempt to move all of the solid to the dense-packed column of solid at the bottom of the filter;
9. Leave the pipet filter in the vacuum hose JUST UNTIL all of the water has been suctioned from the filter, then remove it. Make a mark level with the top of the column of solid, and place the pipet on a paper towel strip on your lab bench.
   
   Note: It is not necessary to turn off the aspirator between filtrations. Return to the third step and repeat for the second test tube.

You should now have two Pasteur pipet filters containing solid product. The instructor will collect the filters from you and others in the class and will arrange them in order of increasing volume of NiCl₂ solution used. Proceed with the synthesis and purification procedure below.

Once all filters have been collected, examine the lineup of filters and try to visually determine the value of x. Using a ruler graduated in mm, measure the height of the column of product in each pipet filter by measuring the distance between the marks. PLEASE MAKE THESE MEASUREMENTS CAREFULLY! RECORD THE MEASUREMENTS IN UNITS OF mm IN YOUR NOTEBOOK. This gives you a measure of the relative amounts of product formed for a large range of relative volumes of reactants.

Synthesis. Using your remaining supplies of NiCl₂ and Nadtc stock solutions, mix equal volumes of the two solutions in a small test tube. Make sure the reaction solution is well-mixed, then filter it through a Hirsch funnel. Dry the solid on the filter by suctioning for 10 minutes. Be sure that you have the atmosphere vent completely clamped off; otherwise, suction will be weak and drying will be slow.

NOTE: We would like to save any left over aqueous nickel chloride and aqueous Na(Et₂dtc). Please return your excess to the containers provided by the instructor.

Purification. Place your Hirsch funnel in the mouth of a third CLEAN AND DRY 25-mL Erlenmeyer flask. In a fume hood, add about 2 mL of dichloromethane to the funnel, and carefully stir with the "ramrod" to loosen the solid and facilitate its dissolution. Allow the solution to drain through the filter into the Erlenmeyer. Examine the filtered solution with a flashlight to make sure that it is clear. What do you observe about this solution? While swirling the dichloromethane solution of your product, add petroleum ether dropwise from a Pasteur pipet, 1-2 mL at a time. Reswirl after each 5-10 drops. After each pipetful, examine the solution with a flashlite to detect the presence of crystals. When crystals are detected, stopper the flask and place it in an ice bath for 15 minutes. Isolate the product by filtration on a (cleaned!) Hirsch funnel, and suction dry. What do you observe about your product. How is it similar to the originally formed product? How is it different?

Characterization. If desired, you may characterize your product using infrared, uv-visible, and nmr spectroscopies. See your instructor for assistance.

Clean-up. When you have finished all of your work:

• Rinse all glassware exposed to Ni(dtc)x with acetone in the hood, and pour the acetone into the non-chlorinated solvent waste.
• Clean Labkit glassware by recommended procedures, shake off excess water, and return to the Labkit.
• Clean Pasteur pipets, graduated pipets, and vials by recommended procedures, shake off excess water, and place in the drying oven.
• Return all borrowed equipment to the instructor before leaving lab.
• Clean up your work area before leaving lab.

Dispose of dichloromethane/petroleum ether mixtures in the chlorinated solvent waste bottle. Discard bis(dithiocarbamato)nickel in the heavy metal waste jar. Dispose of pipet filters in the broken glass receptacle.

Preparation Questions