Laboratory Tutorials - Synthesis

Since each chemical synthesis is unique, there is no absolutely formulaic procedure by which all compounds can be produced. Most synthesis procedures do share a number of commonalities which will be discussed on this page.

1. Mixing of Reagents

   If the reagents that are being used in the reaction are not properly mixed (they are not homogeneously incorporated) the rate of reaction will be drastically slowed.

2. Selecting Solvent for Reaction

   The solvent that is being used not only has to be able to support the reaction without interference, it must also dissolve the starting materials. The product may or may no be soluble in the solvent. If the product is insoluble in the solvent it will precipitate and therefore be easily separated from the residual starting materials.

3. Production of Heat (Exothermic Reactions)

   Many reactions produce heat; care must be taken when dealing with such situations. Frequently, stirring can be utilized as a means of dispersing the heat. Also, if the heating problem is anticipated, the reagents can be combined slowly to avoid excessive heating. Also keep in mind that although a reaction may be exothermic, it may be necessary to heat it to increase the rate at which the reaction occurs. For example, the synthesis of SnI4 from tin and I2 requires heat to begin the reaction. Once begun, the reaction produces heat and the external source of heat must be removed.

4. Isolating and Purifying the Product

   After the synthetic procedure has been carried out, the product must be separated from byproducts and the reagents that were used to make the product.  In some cases, the product will precipitate from solution and can be isolated simply by filtration. This is the case for most of the syntheses shown below.  If the product is soluble in the solvent, it will be necessary to remove some or all of the solvent.  Unfortunately, removal of the solvent usually leaves behind not only the product but also any unreacted starting materials or byproducts.  These undesired compounds must then be separated from the product.

   If the product is a solid, recrystallization can be used to purify it.  A solvent can be chosen that will dissolve the product but will not dissolve the unwanted starting materials and byproducts, or, the solvent may dissolve the impurities but not the product.  If the product is a liquid, it can be distilled to remove impurities that have boiling points different from that of the product.  There are also chromatographic techniques that can be used for both solids and liquids.

5. Identification of the Product

   After the substance has been isolated it must be identified. Even though the synthetic procedure was devised to allow you to prepare a certain compound, the substance obtained in the reaction may not actually be that compound.  In order to identify the substance you generally need to perform several  measurements of physical and chemical properties.  For example, if you are trying to prepare SnI₄, you can measure the melting point - it should be 144°C, according to the literature.  Usually the comparison of several measurements with those reported in the literature are needed in order to be sure that you have indeed prepared the correct compound.

Sample Syntheses

• Preparation of Tin Tetraiodide

  Procedure
  Sn + 2 I₂ → SnI₄

  1. Weigh to +/- 0.01 g about 2.5 g of iodine and 1 g of tin. Place the two substances together in a 50-mL Erlenmeyer flask.
  2. Under the hood, add 10 mL of toluene to the flask.
  3. Cover the flask with a small watch glass and set it on a hot plate at medium heat. Continue to heat the flask gently, swirling the contents from time to time, until the purple vapors of iodine are no longer visible. The solution should then be red-brown in color. If the solution begins to boil, turn down the heat 9the reaction is exothermic).
  4. Filter the hot solution through a filter paper in a stemless funnel. Collect the filtrate in a clean, dry, 50-mL beaker. The residue in the filter paper may be discarded.
  5. Allow the filtrate to cool to room temperature. Then cool the beaker in an ice water bath to complete the crystallization of the product.
  6. When crystallization is complete, filter the mixture through a Buchner funnel; use a rubber policeman to transfer the product. Suction for a few minutes to get the crystals as dry as possible, then transfer the product to a beaker.
  7. Recrystallize the crude product from toluene.

  Points to remember

  • All work for this synthesis must be done in a fume hood.
  • Do not use water in this procedure because the product reacts with water.
  • During recrystallization, it is recommended that you begin with the crude product and 5 mL of toluene, and that you end by washing the recrystallized product with ice-cold toluene. Execution of the intervening steps of the recrystallization process is left to your judgment.
• Preparation of Alum from Aluminum Metal

  Procedure

  1. Weigh to +/- 0.01 g 0.5 g of aluminum powder and 0.2 g of KOH into a 400-mL beaker. Under the hood, add 25 mL of distilled water and stir. The addition of water should cause hydrogen gas to be given off. The reaction is complete when no additional hydrogen gas is being generated (i.e. no more bubbles are forming in the beaker). This reaction should take approximately 10 minutes to reach completion.
  2. NOTE: The reaction that commences with the addition of distilled water is exothermic. The beaker being used will become very hot. This heat can be dissipated by stirring the solution as the reaction is ongoing.
  3. Filter the solution by gravity using a short stem funnel. The residue on the filter paper should be washed twice, using 3 mL of distilled water in each wash. The washings should be collected in the beaker containing the filtrate.
  4. Pour about 10 mL of 9 M sulfuric acid into the filtrate slowly while stirring. A gelatinous precipitate of aluminum hydroxide will form but this will redssolve as the solution becomes acidic. Warming the solution gently on a hot plate may be necessary to dissolve the precipitate. If solid material remains, add 1mL of sulfuric acid. If this does not eliminate all solid matter, the solution should be filtered so these particles are removed. Allow the solution to cool to room temperature before placing the beaker in an ice bath for 20-30 minutes.
  5. The crystals should be removed from the mother liquor through vacuum filtration. An attempt at a quantitative transfer of material should be made at this point. Prepare a wash by combining equal amounts of 95% ethanol and distilled water and wash the alum crystals that have been collected. Air dry the crystals in a recrystallization dish.
• Preparation of Copper Ammonia Complex

  Tetrafluoroborate Salt
  Cu(NH₃)₄(BF₄)₂

  1. Weigh 2.0 g of NaBF₄ and 2.3 g CuSO₄ into separate 250 mL beakers.
  2. Dissolve the CuSO₄ in about 20 mL distilled water and add 10 mL of 6 M NH₃ until the solution is deep blue and clear.
  3. Add the NaBF4, stirring continuously to aid dissolution, and then add 20 mL of 95% ethanol to the solution.
  4. Place the beaker into an ice bath for about two hours, then use a Buchner funnel to vacuum filter the crystals that have formed and allow them to air dry overnight.

  Sulfate Salt
  Cu(NH₃)₄SO₄

  1. In a 250 mL beaker, dissolve about 2 g CuSO₄ 5H₂O in 20 mL distilled water.
  2. Add 6M NH₃ until the solution is dark blue and clear, then add 20 mL of 95% ethanol before placing the beaker into an ice bath for about 2 hours.
  3. Filter the crystals that form in a Buchner funnel using suction and allow the crystals to air dry overnight.
• Preparation of Cuprous Iodide

  Procedure
  Cu + 1/2 I₂ → CuI

  1. Weigh to +/- 0.01 g about 3.2 g of copper and 7.6 g of iodine. Place the two substances together in a 125-mL Erlenmeyer flask containing a magnetic stir bar.
  2. Under the hood, add 70 mL of toluene to the flask.
  3. Cover the flask with a small watch glass and set it on a hot plate and heat to 95 °C (about 4 hours).
  4. Filter the hot solution through filter paper in a stemless funnel. Collect the filtrate in a clean, dry, 50-mL beaker. The residue in the filter paper may be discarded.
  5. Allow the filtrate to cool to room temperature. Then cool the beaker in an ice water bath to complete the crystallization of the product.
  6. When crystallization is complete, filter the solution through a Buchner funnel; use a rubber policeman to transfer the product. Suction for a few minutes to get the crystals as dry as possible, then transfer the produce to a beaker.

  Points to remember

  • Although toluene is not extremely toxic, all reagents should be handled in the hood.
• Preparation of Libethenite

  Procedure

  1. Weigh about1.0 g of CuSO₄ 5H₂O into a 250 mL beaker and dissolve in 20 mL of water.
  2. Weigh 0.41 g of (NH₄)₂HPO₄ into a 150 mL beaker and dissolve in 20 mL of water.
  3. Add the (NH₄)₂HPO₄ solution to the 250 mL beaker containing copper sulfate, stirring continuously.
  4. Add 6 M NH₃ dropwise until the pH of the solution is 7, as determined with pH paper.
  5. Heat the beaker on a hot plate for one hour at 70 °C.
  6. After the solution has cooled, filter using a Buchner funnel and wash with distilled water.
  7. Allow the precipitate to dry in a crystallizing dish for one hour, then place the sample in a 120 °C oven for two hours.

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