How to Prepare Benzocaine (Fischer Esterification)

Step-by-Step Guide

1. 1

   Step 1: Add a magnetic stir bar

   A solid will precipitate out of the solution.
   
   Remember, sulfuric acid is very corrosive and can cause serious chemical burns if allowed to come into contact with your skin.
   
   Use extreme caution when handling this chemical. , The water-in tube attaches to the bottom spigot of the condenser and water-out tube attaches to the top spigot of the condenser. , Set the heat to 2 initially and the stir to 6-7.
   
   You may need to adjust the heat to keep the solution refluxing, but not boiling violently.
   
   You know the solution is refluxing when liquid droplets form at the tip of the condenser where the condenser is attached to the round bottom flask. , Cool the solution at room temperature for 2-3 minutes. , Reattach the condenser to the round bottom flask, and resume heating under gentle reflux until all of the solid has dissolved.
   
   It is very important for this experiment to succeed that ALL of the solids dissolve during this period. ,, This should take between 7-10 minutes. ,, To do this, calculate the approximate volume of 10% aqueous sodium carbonate required to neutralize the total amount of sulfuric acid you used (Usually between 18-24 mL). , Be sure to add the sodium carbonate slowly as frothing occurs during neutralization.
   
   A solid should precipitate.
   
   This is crude benzocaine. ,,,, Heat the benzocaine-water mixture to about 60 °C (140 °F).
   
   You can measure the temperature by placing the thermometer adapter onto the top of condenser and sliding the thermometer through the condenser until the thermometer’s tip rests 1 to 2 centimeters (0.4 to
   0.8 in) above the liquid in the round bottom flask. , This step should only require 5-10mL of methanol.
   
   Do not add more methanol than is necessary to dissolve the solid. , This should take 7-10 minutes. ,,,,
2. 2

   Step 2: 1.0g of p-aminobenzoic acid

3. 3

   Step 3: and 10mL of absolute ethanol to a round bottom flask.

4. 4

   Step 4: Swirl the mixture until the solid is completely dissolved.

5. 5

   Step 5: Add 1mL of concentrated sulfuric acid drop wise with a Pasteur pipette to the ethanol-aminobenzoic solution.

6. 6

   Step 6: Attach a condenser to the round bottom flask containing the ethanol solution

7. 7

   Step 7: and set up the apparatus for heating under reflux.

8. 8

   Step 8: Heat the solution on a hot plate under gentle reflux for approximately 30 minutes.

9. 9

   Step 9: If any solid precipitate remains after heating

10. 10

    Step 10: take the condenser off of the round bottom flask and remove the round bottom flask from the hot plate.

11. 11

    Step 11: Add 3mL of absolute ethanol and 0.5mL of sulfuric acid (by drop) to the round bottom flask containing the solution and precipitate.

12. 12

    Step 12: Once the solid is completely dissolved

13. 13

    Step 13: continue heating the solution under gentle reflux for another 30 minutes.

14. 14

    Step 14: Remove the round bottom flask from the heat source and allow the reaction mixture to cool to room temperature.

15. 15

    Step 15: Fill a 100mL beaker with 30mL of water and pour the cooled reaction mixture into the beaker containing the 30mL of water.

16. 16

    Step 16: Basify the reaction mixture and water solution to a pH of about 8.

17. 17

    Step 17: Slowly pour the calculated amount of 10% aqueous sodium carbonate into the beaker containing the reaction mixture and water solution.

18. 18

    Step 18: Assemble your vacuum filtration apparatus and collect the crude benzocaine via vacuum filtration.

19. 19

    Step 19: Weigh the crude benzocaine collected during vacuum filtration.

20. 20

    Step 20: Transfer the crude benzocaine to a 50mL Erlenmeyer flask and add a magnetic stir bar and 20mL of water

21. 21

    Step 21: Place the Erlenmeyer flask on a hot plate and set the heat to 2-3 and the stir to 6-7.

22. 22

    Step 22: Add enough methanol to the heated solution to dissolve the benzocaine solid.

23. 23

    Step 23: Remove the Erlenmeyer flask from the heat source once all the solid dissolves

24. 24

    Step 24: and allow the solution to cool to room temperature.

25. 25

    Step 25: Cool the solution in an ice water bath for 15 minutes to complete crystallization of the benzocaine.

26. 26

    Step 26: Use vacuum filtration to isolate the crystallized benzocaine

27. 27

    Step 27: Weigh the final product and calculate the percent yield.

28. 28

    Step 28: Obtain an IR spectra of your final product for comparison against an authentic sample.

Detailed Guide

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