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Lab: Chemistry

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Basic Organic Chemistry Techniques


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

  1. Synthesis of Benzoic Acid

    In this lab, you'll learn several important techniques that you'll use throughout this class. This includes measuring the mass or volume of reactants and products, methods of heating and cooling reaction mixtures, proper selection and handling of glassware, and separating solids and liquids by vacuum filtration. First, you'll synthesize benzoic acid by protonating sodium benzoate with hydrochloric acid, or HCl, using water as the solvent. Sodium benzoate is very soluble in water, but benzoic acid is not, so you can filter the solid product out of the solution.

    You'll use vacuum filtration, which is used for powdery solids, to filter out the benzoic acid. Benzoic acid is less soluble in water at lower temperatures, so you'll collect more product if you cool the reaction solution before you filter it. When you do this lab, remember that HCl is toxic and a strong acid. It's always best to work with strong acids and organic solvents in a fume hood. If you spill any HCl, follow your lab's procedures for neutralizing and cleaning up strong acids.

    • Put on a lab coat, safety glasses, and nitrile gloves. Change your gloves if you get acid or organic solvent on them.
    • Bring a clean 50-mL beaker and your lab notebook to the balance, and then tare a clean empty weighing boat on the balance.
    • Use a clean spatula to measure out ~2 g of sodium benzoate. Note: Stay between 1.9 and 2.1 g.
    • Dispose of any extra sodium benzoate into the appropriate waste container. Close the bottle of sodium benzoate when you're done with it.
    • Write down the mass of sodium benzoate and pour it into your beaker. Clean the spatula with a lab wipe so that it can be used again and return to your fume hood.

      Table 1: Reaction yield of benzoic acid

      Mass of sodium
      benzoate (g)
      Moles of sodium
      Volume of
      HCl (mL)
      Moles of
      Moles of benzoic
      Mass of benzoic
      acid (g)
      Theoretical yield
      - - - - Percent yield
      Click Here to download Table 1
    • Get a clean 10-mL graduated cylinder and measure 10 mL of deionized water. Pour the water into the beaker of sodium benzoate. Stir the solution with a glass rod until the sodium benzoate has completely dissolved.
    • Obtain a clean 5-mL graduated cylinder and measure 5 mL of 3 M HCl. Note: If you pour too much, use a Pasteur pipette to transfer the excess to a small beaker labeled ‘acid waste’. Close the bottle and put it back when you're done.
    • Use a clean Pasteur pipette to draw up a small amount of HCl from your graduated cylinder. Start adding the HCl to the sodium benzoate solution at a rate of about one drop every 6 seconds or 10 drops/min. Stir the solution frequently with the glass rod. This should take about 10 min. Note: As you add more HCl, you'll start to see benzoic acid precipitating from the solution.
    • When you are done, fill a 600-mL beaker about halfway with crushed ice and add tap water to just fill the spaces between the ice to make an ice bath.
    • Place the ice bath by a lab stand in your fume hood and use a medium clamp to hold the reaction beaker in the bath. Make sure that the surface of the solution is below the ice.
    • Clamp a thermometer in the reaction solution.
    • While the reaction cools, obtain 25 mL of deionized water in a 50-mL graduated cylinder. Place the graduated cylinder in a 600-mL beaker and pack ice around it to cool the water.
    • Once the reaction solution reaches 10 °C, it's ready to be filtered. Set up another lab stand and obtain a clean 250-mL filter flask. Clamp the flask upright on the lab stand.
    • Ensure that your vacuum tubing is connected to the vacuum source, and then carefully push the free end of the tubing onto the barbed arm of the flask.
    • Place a rubber adapter and a Büchner funnel in the mouth of the flask. Confirm that the reaction solution has reached 10 °C and remove the thermometer from the beaker.
    • Obtain a piece of circular filter paper and place it in the Büchner funnel so that the small holes are completely covered.
    • Get a new Pasteur pipette, and use a few drops of cold deionized water to wet the filter paper. Note: This helps keep the filter paper in place when you pour the solution into the funnel.
    • Open the vacuum line part way to reduce the pressure in the flask.
    • Remove the beaker from the ice bath, stir the solution well with the glass rod until the solid is swirling around in the liquid, and then slowly pour it onto the filter paper. Note: If the liquid does not move quickly through the funnel, make sure that there are no gaps between the funnel, adapter, and flask, and then gradually open the vacuum until the liquid rapidly flows into the flask.
    • Use a clean Pasteur pipette to rinse the inner walls of the beaker with ~10 mL of cold deionized water and pour the rinse into the funnel. Pour the rest of the cold deionized water over the benzoic acid on the filter to wash it. Note: This is an important step that removes impurities, side products, and unreacted compounds.
    • Leave the benzoic acid in the funnel with the vacuum running until it is completely dry. Note: This usually takes ~2 h, so let's continue to the second half of the lab in the meantime.
  2. Refluxing Tetrahydrofuran (THF) and Measuring Product Mass

    While your benzoic acid dries, you'll practice refluxing tetrahydrofuran, or THF, which has a boiling point of 66 °C. THF is volatile, flammable, and an irritant, so avoid letting it touch your skin and always work with it in a fume hood. Refluxing is a technique that constantly condenses the vapor and returns it to the flask. This allows reactions to be performed at high temperatures, such as at the solvent's boiling point, without losing the solvent to evaporation.

    Refluxing is achieved with a special piece of glassware called a condenser, which is attached directly to your flask. The condenser is cooled with water flowing through the outer chamber. As vapor rises through the condenser, it loses heat to the cool walls, condenses, and drips back into the flask.

    Refluxing is usually performed in a round-bottom flask. These flasks must be heated in a heating bath or with specialized equipment, but they provide more even heating than an Erlenmeyer flask or beaker as a result. Water baths are typically used for experiments performed below 80 – 100 °C.

    • Set up the glassware and equipment for reflux. Place a lab jack by your available lab stand and set a stirring hotplate on top of it. Clamp a 50-mL round-bottom flask at least 12 cm above the hotplate.
    • Bring a 50-mL graduated cylinder to the solvent hood and measure 25 mL of THF. If you pour too much, label a small beaker ‘organic waste’ and pipette the excess into that. Remember to cap the bottle when you're done.
    • Carefully pour the THF into the round-bottom flask, clean up any spills with a lab wipe or paper towel, and add a small oblong stir bar to the flask.
    • Position a medium clamp ~20 cm above the flask. Obtain a condenser and apply vacuum grease to the lower ground-glass joint.
    • Fit the condenser into the flask and rotate them in opposite directions until the vacuum grease is evenly spread between the two ground-glass surfaces.
    • Clamp the condenser in place and use a lab wipe to remove excess grease from the connection between the two pieces of glassware.
    • Attach rubber tubing to each of the ports on the condenser. Place the free end of the tubing connected to the top port, which is the outlet, in the drain. Connect the free end of the tubing from the bottom port, which is the inlet, to the waterline.
    • Fill a 250-mL beaker with ~200 mL of tap water and place this water bath on the hotplate. Use the lab jack to raise the bath until the water level is about 2/3 of the way up the spherical part of the flask. This will ensure the THF heats evenly.
    • Secure a thermometer (range 0 – 100 °C) in the bath using the thermometer clamp.
    • Slowly open the water tap, watch the condenser for leaks as it fills, and confirm that the water is flowing into the drain.
    • Slowly turn up the magnetic stirrer until the THF is stirring well. If the stir bar bounces around, turn down the stir motor and try again more slowly.
    • Start heating the water bath to about 70 °C until the THF starts to boil. Note: You'll see vapor condensing in the lower part of the condenser).
    • Monitor the refluxing THF for 10 min. Note: Turn down the heat if the bath goes above 70 °C or if vapor leaves the condenser.
    • After 10 min, remove the thermometer and turn off the heat and the stir motor. Carefully lower the lab jack until the flask is clear of the water bath. Wait at least 10 more min for the setup to cool, then unplug and remove the hotplate.
    • Turn off the water tap, unclamp the condenser, and carefully detach it from the flask. Tilt the condenser until most of the water has flowed through the outlet tubing into the drain. Then, disconnect the inlet tubing from the condenser and let the remaining water flow into the drain. Do the same for the outlet tubing.
    • Use lab wipes to remove the vacuum grease from the condenser and the flask, then retrieve the stir bar from the flask.
    • Bring the flask to the waste hood and empty it into the non-halogenated organic waste container. Pour any excess HCl or THF into the acid and organic wastes, respectively. Empty the ice and water baths into the sink and rinse the beakers with tap water.
    • Clean most of your other glassware while the benzoic acid from earlier finishes drying according to your lab’s standard practices. Usually, you'll rinse your glassware both with an organic solvent, like acetone, and with detergent and deionized water. Note: Your lab may have specific procedures for glassware used with acids. If so, follow that procedure to clean the graduated cylinder used with HCl and your acid waste beaker, if applicable.
    • Remove the reusable pipette bulbs and dispose of your pipettes in the glass waste container. Throw out used lab wipes and paper towels in the lab trash.
    • Check on your benzoic acid. If it is dry, turn off the vacuum and break the vacuum seal by disconnecting the tubing from the filter flask. Gently place the funnel in it 250-mL beaker and bring it and your lab notebook to the balance.
    • Tare a weighing boat and use a clean spatula to transfer the product to the weighing boat. Record the mass of your product in your lab notebook.
    • Then, put the product in the lab trash and wipe off the spatula. Dispose of the filtrate in the aqueous waste container.
    • Clean the rest of your glassware, throw out the filter paper and other waste in the lab trash, and put away your lab equipment. Lastly, clean the floor of your fume hood with a damp paper towel.
  3. Results
    • Calculate the reaction yield for the protonation of sodium benzoate. Identify the reagent that the reaction runs out of first, which is called the limiting reagent.
    • Calculate the number of moles of sodium benzoate and HCl that you used. Note: You used about 0.014 moles of sodium benzoate and 0.015 moles of HCl. Thus, the maximum yield of benzoic acid is equal to the starting amount of sodium benzoate in moles.
    • Convert the mass of your benzoic acid to moles, divide that by the moles of sodium benzoate that you used, and multiply it by 100 to get the percent yield. Note: The recovered yield for this reaction is usually between 80% and 98% because a small amount of benzoic acid stays in solution. If the yield is above 100%, then it's likely that your product was still wet when you weighed it. Impurities, unreacted compounds, and products of side reactions can also affect the accuracy of yield calculations.

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