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JoVE Lab Manual
Lab: Chemistry

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Education
Proper Lab Notebook Keeping
 

Procedure

Source: Lara Al Hariri and Ahmed Basabrain at the University of Massachusetts Amherst, MA, USA

  1. Nitration of Methyl Benzoate

    From this lab onward, you'll keep a record of every experiment in your lab notebook. This starts with preparing your lab notebook before you come to the lab. This preparation, which is often called a pre-lab, typically includes balanced chemical equations for the reactions, important properties of the reagents and products, and a step-by-step procedure for the lab in your own words. In this lab, you will perform the nitration of methyl benzoate, an aromatic ester that will undergo an electrophilic aromatic substitution to make methyl 3-nitrobenzoate.

    First, you'll mix aqueous sulfuric acid and nitric acid to make positive nitronium ions. In a separate container, you'll mix sulfuric acid with methyl benzoate to protonate the ester group. The way the charge is distributed makes the ring most reactive towards positive ions at the carbons one step away from the ester. This is known as the meta position in di-substituted benzene rings. When you combine the solutions, nitronium adds to methyl benzoate at one of those carbons. Then, the benzene ring and the ester group lose their extra protons to make methyl 3-nitrobenzoate.

    After the reaction, you'll dilute the mixture with water from melted ice and collect the insoluble product by vacuum filtration. At the end, you'll estimate the yield of your reaction based on the starting volume of methyl benzoate and the mass of the product.

    During the experiment, make sure to write down what you did and your observations as they happen. An accurate record is essential for reproducing your results and identifying sources of experimental error. Always catch up on your lab notebook before starting the next step.

    • Before you start the experiment, put on a lab coat, safety glasses, and nitrile gloves. Make sure to review the safety information in your pre-lab.
    • Fill a 100-mL beaker about halfway with crushed ice. Note: The reactions in this lab are exothermic, so use ice baths to keep the solutions cool.
    • Prepare the mixture of nitric acid and sulfuric acid. In your fume hood, clamp a 20-mL test tube upright in the ice bath. Label two 10-mL graduated cylinders ‘nitric acid’ and ‘sulfuric acid’.
    • Measure 2 mL of concentrated sulfuric acid. If you add too much, use a pipette to transfer the excess to a small beaker labeled ‘sulfuric acid waste’.
    • Record the volume of sulfuric acid in your lab notebook as the volume used in the mixture of acids (Step 1). Pour the sulfuric acid into the test tube.

      Table 1: Nitration of methyl benzoate

      Step 1: Make positive nitronium ions                                   
      Volume of sulfuric acid (mL)
      Volume of nitiric acid (mL)
      Time of stirring (min)
      Step 2: Protonate ester on methyl benzoate
      Volume of sulfuric acid (mL)
      Volume of methyl benzoate (mL)
      Step 3: Combine solutions to form 3-methyl benzoate
      Time of addition (min)
      Reaction time on ice (min)
      Reaction time at RT (min)
      Yield of methyl 3-nitrobenzoate (g)
      Click Here to download Table 1
    • Use the other graduated cylinder to measure 2 mL of concentrated nitric acid and pipette any extra nitric acid into a separate waste beaker. Record the volume of nitric acid in the graduated cylinder before pouring it into the test tube.
    • Stir the solution for 2 min with a glass rod. Don't remove the solution from the ice bath. When you're done, record for how long you stirred the solution.
    • Fill a 400-mL beaker about 3/4 full with ice and place it by an available lab stand in your fume hood. Clamp a clean 25-mL Erlenmeyer flask in this ice bath.
    • Prepare the mixture of sulfuric acid and methyl benzoate (Step 2). Measure another 2 mL of sulfuric acid with the corresponding graduated cylinder. Record the volume and carefully pour it into the Erlenmeyer flask.
    • Bring a clean 10-mL graduated cylinder, a pipette, and a small beaker to the solvent hood.
    • Carefully measure 1.1 mL of methyl benzoate and pipette any excess into the beaker for disposal in the organic waste. Remember to record the volume of methyl benzoate before pouring it into the flask of sulfuric acid.
    • Place a thermometer in the flask and wait for the solution to cool to between 10 °C and 15 °C.
    • Once the methyl benzoate solution is below 15 °C, get a clean Pasteur pipette. Note the time in your lab notebook as the start of the solution addition and transfer the glass rod from the test tube to the Erlenmeyer flask (Step 3).
    • At a rate of 2 - 3 drops per min, add the mixed acids to the methyl benzoate solution and stir the mixture with the glass rod. Monitor the temperature of the solution throughout the addition process. Note: If the solution reaches 15 °C, wait until the temperature has dropped below 15 °C before adding more of the acid mixture. The addition process will take about 10 to 15 min.
    • Once you've finished, note the time in your lab notebook and leave the flask in the ice bath for another 10 min while the mixture reacts.
    • Obtain about 20 mL of deionized water in a graduated cylinder. Fill a 600-mL beaker with ice and water and place the graduated cylinder in it to chill.
    • Once the solution has reacted for 10 min on ice, remove the flask from the ice bath, record the time in your lab notebook, and let the solution continue reacting at room temperature for another 10 to 15 min.
    • While you wait, fill a 100-mL beaker with about 25 mL of fresh ice. When the solution has reacted at room temperature for at least 10 min, record the time as the end of the reaction and pour the mixture into the 100-mL beaker containing ice. Note: Using ice lets you dilute the solution without heating it up too much because the heat from the reaction of sulfuric acid and water is expended on melting the ice.
    • Wait for the ice to melt, which usually takes about 20 min. Remember to write down your observations, such as how long it took for the solid product to appear and what the precipitate looks like.
    • Once the ice has melted completely, set up for vacuum filtration using a 250-mL filter flask. Place a filter paper in the funnel and use a clean pipette to wet the filter paper with cold water from the graduated cylinder.
    • Open the flask to vacuum and pour your product mixture into the funnel. Rinse the remaining solid into the funnel with cold water. Then, pour the rest of the cold water into the funnel to wash the solid.
    • Once no more liquid is dripping from the funnel into the flask, close the vacuum and break the vacuum seal. Carefully place the funnel in a beaker so that your product can dry overnight.
    • Dispose of the filtrate in the aqueous acidic waste container. Pour the ice baths into the sink and wash your glassware using your usual methods. Dispose of used Pasteur pipettes in the glass waste container, neutralize and clean up any spilled acid, and throw out paper waste in the lab trash.
    • Calculate the theoretical yield of methyl 3-nitrobenzoate in g.

      Table 2: Product yield of methyl 3-methyl benzoate

      Volume of methyl benzoate (mL)                                   
      Density of methyl benzoate (g/cm3) 1.08
      Molecular weight of methyl benzoate (g/mol) 136.15
      Amount of methyl benzoate (mol)
      Molecular weight of methyl 3-nitrobenzoate (g/mol) 181.15
      Theoretical yield of methyl 3-nitrobenzoate (g)
      Actual yield of methyl 3-nitrobenzoate (g)
      Actual yield of methyl 3-nitrobenzoate (mol)
      Percent yield
      Click Here to download Table 2
    • The next day, check on your product. If it seems dry, bring your funnel to a balance to weigh your product.Note: If it seems wet, leave it to dry some more.
    • Tare a weighing boat and use a small spatula to carefully transfer the product from the filter paper to the weighing boat.
    • Compare the mass of your product to the theoretical yield. If the mass reading is stable and less than the theoretical yield, you can assume that the product is dry. Record this mass in your lab notebook.
    • Throw out the solid product and the filter paper in the lab trash and wash the Büchner funnel.
  2. Results
    • Calculate the yield of your reaction. From the theoretical maximum yield, convert the mass of your product to moles.
    • Divide that amount by the maximum yield and multiply it by 100. The total nitrated product yield, which includes minor products, is usually about 85%.

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